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XPath 4.0 is an expression language that allows the processing of values conforming to the data model defined in [XQuery and XPath Data Model (XDM) 4.0]. The name of the language derives from its most distinctive feature, the path expression, which provides a means of hierarchic addressing of the nodes in an XML tree. As well as modeling the tree structure of XML, the data model also includes atomic values, function items, maps, arrays, and sequences. This version of XPath supports JSON as well as XML, and adds many new functions in [XQuery and XPath Functions and Operators 4.0].
XPath 4.0 is a superset of XPath 3.1. A detailed list of changes made since XPath 3.1 can be found in J Change Log.
This is a draft prepared by the QT4CG (officially registered in W3C as the XSLT Extensions Community Group). Comments are invited.
XPath was originally designed as an expression language to address the nodes of XML trees; it has since been extended to address a variety of non-XML data sources. XPath gets its name from its use of a path notation for navigating through the hierarchical structure of an XML document; similar capabilities for navigating JSON structures were added in versions 3.0 and 3.1. XPath uses a compact, non-XML syntax, allowing XPath expressions to be embedded within URIs and to be used as XML attribute values. XPath is designed to be embedded in (or invoked from) other host languages, including both languages specialized towards XML processing (such as [XSL Transformations (XSLT) Version 4.0] and XSD), and general purpose programming languages such as Java, C#, Python, and Javascript. The interface between XPath and its host language is formalized with an abstract definition of a static and dynamic context, made available by the host language to the XPath processor.
[Definition: A host language for XPath is any environment that provides capabilities for XPath expressions to be defined and evaluated, and that supplies a static and dynamic context for their evaluation. ]
[Definition: XPath 4.0 operates on the abstract, logical structure of an XML document or JSON object rather than its surface syntax. This logical structure, known as the data model, is defined in [XQuery and XPath Data Model (XDM) 4.0].]
XPath 4.0 is a subset of XQuery 4.0. In general, any expression that is syntactically valid and executes successfully in both XPath 4.0 and XQuery 4.0 will return the same result in both languages. There are a few exceptions to this rule:
Because XQuery expands
predefined entity references and character references
and XPath does not, expressions containing these produce different
results in the two languages. For instance, the value of the string literal
"&"
is &
in XQuery,
and &
in XPath. (A host language may expand predefined entity references or character references
before the XPath expression is evaluated.)
If XPath 1.0 compatibility mode is enabled, XPath behaves differently from XQuery in a number of ways, which are noted throughout this document, and listed in I.4.2 Incompatibilities when Compatibility Mode is false .
Because these languages are so closely related, their grammars and language descriptions are generated from a common source to ensure consistency.
XPath 4.0 also depends on and is closely related to the following specifications:
[XQuery and XPath Data Model (XDM) 4.0] defines the data model that underlies all XPath 4.0 expressions.
The type system of XPath 4.0 is based on XML Schema. It is implementation-defined whether the type system is based on [XML Schema 1.0] or [XML Schema 1.1].
The system function library and the operators supported by XPath 4.0 are defined in [XQuery and XPath Functions and Operators 4.0].
Note:
The XML-based syntax for XQuery known as XQueryX is no longer maintained.
This document specifies a grammar for XPath 4.0, using the same basic EBNF notation used in [XML 1.0]. Unless otherwise noted (see A.3 Lexical structure), whitespace is not significant in expressions. Grammar productions are introduced together with the features that they describe, and a complete grammar is also presented in the appendix [A XPath 4.0 Grammar]. The appendix is the normative version.
In the grammar productions in this document, named symbols are underlined and literal text is enclosed in double quotes. For example, the following productions describe the syntax of a static function call:
[93] | FunctionCall |
::= |
EQName
ArgumentList
|
/* xgc: reserved-function-names */ |
/* gn: parens */ | ||||
[73] | ArgumentList |
::= | "(" ((PositionalArguments ("," KeywordArguments)?) | KeywordArguments)? ")" |
The productions should be read as follows: A function call consists of an EQName followed by an ArgumentList. The argument list consists of an opening parenthesis, an optional list of one or more arguments (separated by commas), and a closing parenthesis.
This document normatively defines the static and dynamic semantics of XPath 4.0. In this document, examples and material labeled as “Note” are provided for explanatory purposes and are not normative.
Certain aspects of language processing are described in this specification as implementation-defined or implementation-dependent.
[Definition: Implementation-defined indicates an aspect that may differ between implementations, but must be specified by the implementor for each particular implementation.]
[Definition: Implementation-dependent indicates an aspect that may differ between implementations, is not specified by this or any W3C specification, and is not required to be specified by the implementor for any particular implementation.]
A language aspect described in this specification as implementation-defined or implementation dependent may be further constrained by the specifications of a host language in which XPath is embedded.
The basic building block of XPath 4.0 is the expression, which is a string of [Unicode] characters; the version of Unicode to be used is implementation-defined. The language provides several kinds of expressions which may be constructed from keywords, symbols, and operands. In general, the operands of an expression are other expressions. XPath 4.0 allows expressions to be nested with full generality.
Note:
This specification contains no assumptions or requirements regarding the character set encoding of strings of [Unicode] characters.
Like XML, XPath 4.0 is a case-sensitive language. Keywords in XPath 4.0 use lower-case characters and are not reserved—that is, names in XPath 4.0 expressions are allowed to be the same as language keywords, except for certain unprefixed function-names listed in A.4 Reserved Function Names.
[Definition: In the data model, a value is always a sequence.]
[Definition: A sequence is an ordered collection of zero or more items.]
[Definition: An item is either an atomic value, a node, or a function item.]
[Definition: An atomic value is a value in the value space of an atomic type, as defined in [XML Schema 1.0] or [XML Schema 1.1].]
[Definition: A node is an instance of one of the
node kinds defined in Section 5 NodesDM40.]
Each node has a unique node identity, a typed value, and a string value. In addition, some nodes have a name. The typed value of a node is a sequence
of zero or more atomic values. The string value of a node is a
value of type xs:string
. The name of a node is a value of type xs:QName
.
[Definition: A function item is an item that can be called using a dynamic function call.]
Maps (see 4.15.1 Maps) and arrays (see 4.15.2 Arrays) are specific kinds of function items.
[Definition: A sequence containing exactly one item is called a singleton.] An item is identical to a singleton sequence containing that item. Sequences are never nested—for example, combining the values 1, (2, 3), and ( ) into a single sequence results in the sequence (1, 2, 3). [Definition: A sequence containing zero items is called an empty sequence.]
[Definition: The term XDM instance is used, synonymously with the term value, to denote an unconstrained sequence of items.]
Element nodes have a property called in-scope namespaces. [Definition: The in-scope namespaces property of an element node is a set of namespace bindings, each of which associates a namespace prefix with a URI.] For a given element, one namespace binding may have an empty prefix; the URI of this namespace binding is the default namespace within the scope of the element.
In [XML Path Language (XPath) Version 1.0], the in-scope namespaces of an element node are represented by a collection of namespace nodes arranged on a namespace axis. As of XPath 2.0, the namespace axis is deprecated and need not be supported by a host language. A host language that does not support the namespace axis need not represent namespace bindings in the form of nodes.
[Definition: An expanded QName is a triple: its components are a prefix, a local name, and a namespace URI. In the case of a name in no namespace, the namespace URI and prefix are both absent. In the case of a name in the default namespace, the prefix is absent.] When comparing two expanded QNames, the prefixes are ignored: the local name parts must be equal under the Unicode codepoint collation (Section 5.3.1 CollationsFO40), and the namespace URI parts must either both be absent, or must be equal under the Unicode codepoint collation.
In the XPath 4.0 grammar, QNames representing the names of elements, attributes, functions, variables, types, or other such constructs are written as instances of the grammatical production EQName.
[152] | EQName |
::= |
QName | URIQualifiedName
|
|
[168] | QName |
::= |
[http://www.w3.org/TR/REC-xml-names/#NT-QName]Names
|
/* xgc: xml-version */ |
[161] | URIQualifiedName |
::= |
BracedURILiteral
NCName
|
/* ws: explicit */ |
[162] | BracedURILiteral |
::= | "Q" "{" [^{}]* "}" |
/* ws: explicit */ |
[169] | NCName |
::= |
[http://www.w3.org/TR/REC-xml-names/#NT-NCName]Names
|
/* xgc: xml-version */ |
The EQName production allows a QName to be written in one of three ways:
local-name only (for example, invoice
).
A name written in this form has no prefix, and the rules for determining the namespace depend on the context in which the name appears. This form is a lexical QName.
prefix plus local-name (for example, my:invoice
).
In this case the prefix and local name of the QName are as written, and the namespace URI is inferred from the prefix by examining the in-scope namespaces in the static context where the QName appears; the context must include a binding for the prefix. This form is a lexical QName.
URI plus local-name (for example,
Q{http://example.com/ns}invoice
).
In this case the local name and namespace URI are as
written, and the prefix is absent. This way of writing a QName
is context-free, which makes it particularly suitable for use
in
expressions
that are generated by software. This
form is a URIQualifiedName.
If the
BracedURILiteral has no content (for example, Q{}invoice
)
then the namespace URI of the QName is absent.
[Definition: A lexical QName is a name that conforms to the syntax of the QName production].
The namespace URI value in a URIQualifiedName is whitespace normalized according
to the rules for the xs:anyURI
type in
Section
3.2.17 anyURI
XS1-2 or
Section
3.3.17 anyURI
XS11-2.
It is a static
error
[err:XQST0070] if the
namespace URI for an EQName is
http://www.w3.org/2000/xmlns/
.
Here are some examples of EQNames:
pi
is a lexical QName without a namespace prefix.
math:pi
is a lexical QName with a namespace prefix.
Q{http://www.w3.org/2005/xpath-functions/math}pi
specifies the namespace URI using a BracedURILiteral; it is not a lexical QName.
This document uses the following namespace prefixes to represent the namespace URIs with which they are listed. Although these prefixes are used within this specification to refer to the corresponding namespaces, not all of these bindings will necessarily be present in the static context of every expression, and authors are free to use different prefixes for these namespaces, or to bind these prefixes to different namespaces.
xs
: http://www.w3.org/2001/XMLSchema
fn
: http://www.w3.org/2005/xpath-functions
array
: http://www.w3.org/2005/xpath-functions/array
map
: http://www.w3.org/2005/xpath-functions/map
math
: http://www.w3.org/2005/xpath-functions/math
err
: http://www.w3.org/2005/xqt-errors
(see 2.4.2 Identifying and Reporting Errors).
output
: http://www.w3.org/2010/xslt-xquery-serialization
[Definition: Within this specification, the term URI refers to a Universal Resource Identifier as defined in [RFC3986] and extended in [RFC3987] with the new name IRI.] The term URI has been retained in preference to IRI to avoid introducing new names for concepts such as “Base URI” that are defined or referenced across the whole family of XML specifications.
Note:
In most contexts, processors are not required to raise errors if a URI is not lexically valid according to [RFC3986] and [RFC3987]. See 2.5.5 URI Literals for details.
[Definition: The expression context for a given expression consists of all the information that can affect the result of the expression.]
This information is organized into two categories called the static context and the dynamic context.
[Definition: The static context of an expression is the information that is available during static analysis of the expression, prior to its evaluation.] This information can be used to decide whether the expression contains a static error.
The individual components of the static context are described below. A default initial value for each component must be specified by the host language. The scope of each component is specified in C.1 Static Context Components.
[Definition:
XPath 1.0 compatibility
mode.
This value is true
if rules for backward compatibility with XPath Version 1.0 are in effect; otherwise it is false
.
]
[Definition: Statically known namespaces. This is a mapping from prefix to namespace URI that defines all the namespaces that are known during static processing of a given expression.]
The URI value is whitespace normalized according to the rules for the
xs:anyURI
type in Section
3.2.17 anyURI
XS1-2 or
Section
3.3.17 anyURI
XS11-2.
The statically known namespaces may include a binding for the zero-length prefix; however, this is used only in limited circumstances because the rules for resolving unprefixed QNames depend on how such a name is used.
Note the difference between in-scope namespaces, which is a dynamic property of an element node, and statically known namespaces, which is a static property of an expression.
[Definition: Default namespace for elements and types. This is a namespace URI, or absentDM40. This indicates how unprefixed QNames are interpreted when they appear in a position where an element name or type name is expected.]
If the value is set to a namespace URI,
this namespace is used for any such unprefixed QName. The URI value is
whitespace-normalized according to the rules for the xs:anyURI
type in Section
3.2.17 anyURI
XS1-2 or Section
3.3.17 anyURI
XS11-2.
If the value is absentDM40, an unprefixed QName representing an element or type name is interpreted as being in no namespace.
[Definition:
Default function namespace. This is either a namespace URI, or absentDM40. The namespace URI, if present, is used for any unprefixed QName appearing
in a position where a function name is expected.] The URI value is whitespace-normalized according
to the rules for the xs:anyURI
type in Section
3.2.17 anyURI
XS1-2 or Section
3.3.17 anyURI
XS11-2
In its simplest form its value is simply a whitespace-normalized xs:anyURI
value (most commonly, the URI http://www.w3.org/2005/xpath-functions
)
to be used as the default namespace for unprefixed function names. However, the use of a more
complex algorithm is not precluded, for example an algorithm which searches multiple namespaces for
a matching name.
[Definition: In-scope schema definitions is a generic term for all the element declarations, attribute declarations, and schema type definitions that are in scope during static analysis of an expression.] It includes the following three parts:
[Definition: In-scope schema types. Each schema type definition is identified either by an expanded QName (for a named type) or by an implementation-dependent type identifier (for an anonymous type). The in-scope schema types include the predefined schema types described in 3.1 Predefined Schema Types. ]
[Definition: In-scope element declarations. Each element declaration is identified either by an expanded QName (for a top-level element declaration) or by an implementation-dependent element identifier (for a local element declaration). ] An element declaration includes information about the element’s substitution group affiliation.
[Definition: Substitution groups are defined in Section 2.2.2.2 Element Substitution Group XS1-1 and Section 2.2.2.2 Element Substitution Group XS11-1. Informally, the substitution group headed by a given element (called the head element) consists of the set of elements that can be substituted for the head element without affecting the outcome of schema validation.]
[Definition: In-scope attribute declarations. Each attribute declaration is identified either by an expanded QName (for a top-level attribute declaration) or by an implementation-dependent attribute identifier (for a local attribute declaration). ]
[Definition: In-scope variables. This is a mapping from expanded QName to type. It defines the set of variables that are available for reference within an expression. The expanded QName is the name of the variable, and the type is the static type of the variable.]
An expression that binds a variable extends the in-scope variables, within the scope of the variable, with the variable and its type. Within the body of an inline function expression, the in-scope variables are extended by the names and types of the function parameters.
[Definition: Context value static type. This is a sequence type; it defines the static type of the context value within the scope of a given expression.]
[Definition: In-scope named item types. This is a mapping from expanded QName to named item types.]
[Definition: A named item type
is an ItemType
identified by an expanded QName.]
Named item types serve two purposes:
They allow frequently used item types, especially complex item types such as record types, to be given simple names, to avoid repeating the definition every time it is used.
They allow the definition of recursive types, which are useful for describing recursive data structures such as lists and trees. For details see 3.6.4.4 Recursive Record Tests.
Note:
Named item types can be defined in a host language such as XQuery 4.0 and in XSLT 4.0, but not in XPath 4.0 itself. They are available in XPath only if the host language provides the ability to define them.
[Definition: Statically known function definitions. This is a set of function definitions.]
[Definition: A function definition contains information used to evaluate a static function call, including the name, parameters, and return type of the function.]
The properties of a function definition include:
The function category, which is one of application, system, or external:
[Definition: Application functions are function definitions written in a host language such as XQuery or XSLT whose syntax and semantics are defined in this family of specifications. Their behavior (including the rules determining the static and dynamic context) follows the rules for such functions in the relevant host language specification.] The most common application functions are functions written by users in XQuery or XSLT.
[Definition: System functions include the functions defined in [XQuery and XPath Functions and Operators 4.0], functions defined by the specifications of a host language, constructor functions for atomic types, and any additional functions provided by the implementation. System functions are sometimes called built-in functions.]
The behavior of system functions follows the rules given for the individual function in this family of specifications, or in the specification of the particular processor implementation. A system function may have behavior that depends on the static or dynamic context of the caller (for example, comparing strings using the default collation from the dynamic context of the caller). Such functions are said to be context dependent.
[Definition: External functions can be characterized as functions that are neither part of the processor implementation, nor written in a language whose semantics are under the control of this family of specifications. The semantics of external functions, including any context dependencies, are entirely implementation-defined. In XSLT, external functions are called Section 24.1 Extension Functions XT30. ] For example, an implementation might provide a set of implementation-defined external functions in addition to the core function library described in [XQuery and XPath Functions and Operators 4.0] or a host language.
[Definition: A
function definition is said to be context dependent
if its result depends on the static or dynamic context of its caller.
A function definition may
be context-dependent for some arities in its arity range, and context-independent
for others: for example fn:name#0
is context-dependent
while fn:name#1
is context-independent.]
Note:
Some system functions, such as fn:position
, fn:last
,
and fn:static-base-uri
, exist for the sole purpose of providing information
about the static or dynamic context of their caller.
Note:
Application functions are context dependent only to the extent that they define optional parameters with default values that are context dependent.
The function name, which is an expanded QName.
A (possibly empty) list of required parameters, each having:
a parameter name (an expanded QName)
a required type (a sequence type)
A (possibly empty) list of optional parameters, each having:
a parameter name (an expanded QName)
a required type (a sequence type)
a default value expression (an expression: see 4 Expressions)
A return type (a sequence type)
A (possibly empty) set of function annotations
A body. The function body contains the logic that enables the function result to be computed from the supplied arguments and information in the static and dynamic context.
The names of the parameters must be distinct.
[Definition: A function definition has an arity range, which is a range of consecutive non-negative integers. If the function definition has M required parameters and N optional parameters, then its arity range is from M to M+N inclusive.]
The static context may contain several function definitions with the same name, but the arity ranges of two such function definitions must not overlap. For example, if two function definitions A and B have the same function name, then:
It is acceptable for A to have two required parameters and no optional parameters, while B has three required parameters and one optional parameter.
It is not acceptable for A to have one required parameter while B has three optional parameters.
Note:
Implementations must ensure that no two function definitions have the same expanded QName and overlapping arity ranges (even if the signatures are consistent).
XQuery and XSLT enforce this rule by defining a static error if the rule is violated; but further constraints may be needed if an API allows external functions to be added to the static context.
System functions (also commonly called built-in functions) are function definitions that are always present in the static context by virtue of rules in the host language; they will typically include the functions specified in [XQuery and XPath Functions and Operators 4.0].
The function definitions are available for reference from a static function call, or from a named function reference.
[Definition: Statically known collations. This is an implementation-defined mapping from URI to collation. It defines the names of the collations that are available for use in processing expressions.] [Definition: A collation is a specification of the manner in which strings and URIs are compared and, by extension, ordered. For a more complete definition of collation, see Section 5.3 Comparison of stringsFO40.]
[Definition:
Static Base URI.
This is an absolute URI, used to resolve relative URIs during static analysis.
]
For example, it is used to resolve module location URIs in XQuery, and
the URIs in xsl:import
and xsl:include
in XSLT.
If E is a subexpression of F then the Static
Base URI of E is the same as the Static Base URI of F.
There are no constructs in XPath that require resolution of relative URI references
during static analysis.
Relative URI references are resolved as described in 2.5.6 Resolving a Relative URI Reference.
At execution time,
relative URIs supplied to functions such as fn:doc
are resolved against the Executable Base URI,
which may or may not be the same as the Static Base URI.
[Definition:
Statically known documents. This is a mapping
from strings to types. The string represents the absolute URI of a
resource that is potentially available using the fn:doc
function. The type is the static type of a call to fn:doc
with the given URI as its
literal argument. ]
If the argument to fn:doc
is a
string literal that is not present in statically known documents, then the
static type of
fn:doc
is document-node()?
.
Note:
The purpose of the statically known
documents is to provide static type information, not to determine
which documents are available. A URI need not be found in the
statically known documents to be accessed using
fn:doc
.
[Definition:
Statically known collections. This is a
mapping from strings to types. The string represents the absolute
URI of a resource that is potentially available using the
fn:collection
function. The type is the type of the
sequence of items that would result from calling the
fn:collection
function with this URI as its
argument.] If the argument to
fn:collection
is a string literal that is not present in
statically known collections, then the static type of
fn:collection
is item()*
.
Note:
The purpose of the statically known
collections is to provide static type information, not to determine
which collections are available. A URI need not be found in the
statically known collections to be accessed using
fn:collection
.
[Definition:
Statically known default collection type. This is the type of the sequence of
items that would result from calling the fn:collection
function with no arguments.] Unless initialized to some other value by an implementation, the value of statically known default collection type is item()*
.
[Definition:
Statically known decimal
formats. This is a mapping from QNames to decimal formats, with one default format that has no visible name,
referred to as the unnamed decimal format. Each
format is available for use when formatting numbers using the fn:format-number
function.]
Each decimal format defines a set of properties, which control the interpretation of characters
in the picture string supplied to the fn:format-number
function, and also specify characters to be used in the result
of formatting the number.
The following properties specify characters used both in the picture string, and in the formatted number. In each case the value is a single character:
[Definition: decimal-separator is the character used to separate the integer part of the number from the fractional part, both in the picture string and in the formatted number; the default value is the period character (.)]
[Definition: exponent-separator is the character used to separate the mantissa from the exponent in scientific notation both in the picture string and in the formatted number; the default value is the Unicode Latin small letter e character (#x65).]
[Definition: grouping-separator is the character typically used as a thousands separator, both in the picture string and in the formatted number; the default value is the comma character (,)]
[Definition: percent is the character used both in the picture string and in the formatted number to indicate that the number is written as a per-hundred fraction; the default value is the percent character (%)]
[Definition: per-mille is the character used both in the picture string and in the formatted number to indicate that the number is written as a per-thousand fraction; the default value is the Unicode per mille sign character (#x2030)]
[Definition: zero-digit is the character used to represent the digit zero; the default value is the Unicode digit zero (#x30). This character must be a digit (category Nd in the Unicode property database), and it must have the numeric value zero. This property implicitly defines the ten Unicode characters that are used to represent the values 0 to 9: Unicode is organized so that each set of decimal digits forms a contiguous block of characters in numerical sequence. Within the picture string any of these ten character can be used (interchangeably) as a place-holder for a mandatory digit. Within the final result string, these ten characters are used to represent the digits zero to nine.]
The following properties specify
characters to be used in the picture string supplied to the fn:format-number
function, but not in the formatted number. In each case the value must be a single character.
[Definition: digit is a character used in the picture string to represent an optional digit; the default value is the number sign character (#)]
[Definition: pattern-separator is a character used to separate positive and negative sub-pictures in a picture string; the default value is the semicolon character (;)]
The following properties specify characters or strings that may appear in the result of formatting the number, but not in the picture string:
[Definition:
infinity is the string used to represent the double value infinity (INF
); the
default value is the string "Infinity"
]
[Definition:
NaN is the string used to
represent the double value NaN
(not a number); the default value is the string "NaN"
]
[Definition: minus-sign is the single character used to mark negative numbers; the default value is the hyphen-minus character (#x2D). ]
[Definition: The dynamic context of an expression is defined as information that is needed for the dynamic evaluation of an expression.] If evaluation of an expression relies on some part of the dynamic context that is absentDM40, a type error is raised [err:XPDY0002].
Note:
In previous versions of the specification, this was classified as a
dynamic error. The change allows the error to be raised during
static analysis when possible; for example a function written as
fn($x){@code}
can now be reported as an error whether or not
the function is actually evaluated. The actual error code remains unchanged
for backwards compatibility reasons.
There are other cases where static detection of the error is not possible.
The individual components of the dynamic context are described below. Further rules governing the semantics of these components can be found in C.2 Dynamic Context Components.
The dynamic context consists of all the components of the static context, and the additional components listed below.
[Definition: The first three components of the dynamic context (context value, context position, and context size) are called the focus of the expression. ] The focus enables the processor to keep track of which items are being processed by the expression. If any component in the focus is defined, both the context value and context position are known.
Note:
If any component in the focus is defined, context size is usually defined as well. However, when streaming,
the context size cannot be determined without lookahead, so it may be undefined. If so, expressions like last()
will raise a dynamic error because the context size is undefined.
[Definition: A fixed focus is a focus for an expression that is evaluated once, rather than being applied to a series of values; in a fixed focus, the context value is set to one specific value, the context position is 1, and the context size is 1.]
[Definition: A singleton focus is a fixed focus in which the context value is a singleton item.]. With a singleton focus, the context value is a single item, the context position is 1, and the context size is 1.
Certain language constructs, notably the path operator
E1/E2
, the simple map operator
E1!E2
, and the predicate
E1[E2]
, create a new focus
for the evaluation of a sub-expression. In these constructs, E2
is evaluated once for each item in the
sequence that results from evaluating E1
. Each time E2
is evaluated, it is evaluated with a
different focus. The focus for evaluating E2
is referred to below as the inner
focus, while the focus for evaluating E1
is referred to as the outer
focus. The inner focus is used only for the evaluation of E2
. Evaluation of E1 continues with its original focus unchanged.
[Definition: The context value
is the value currently being processed.]
In many cases (but not always), the context value will be a single item.
[Definition: When the context value is a single item, it can also be referred
to as the context item; when it is a single node,
it can also be referred to as the context
node.] The context value is returned by an expression
consisting of a single dot (.
). When an expression E1/E2
or E1[E2]
is evaluated, each item in the
sequence obtained by evaluating E1
becomes the context value in the inner focus for an evaluation of E2
.
[Definition: The context
position is the position of the context value within the
series of values currently being processed.] It changes whenever the context value
changes. When the focus is defined, the value of the context position is an integer greater than zero. The context
position is returned by the expression fn:position()
. When an expression E1/E2
or E1[E2]
is evaluated, the context position in
the inner focus for an evaluation of E2
is the position of the context value in the sequence obtained by
evaluating E1
. The position of the
first item in a sequence is always 1 (one). The context position is
always less than or equal to the context size.
[Definition: The context
size is the number of values in the series of values currently
being processed.] Its value is always an
integer greater than zero. The context size is returned by the
expression fn:last()
. When an expression
E1/E2
or E1[E2]
is evaluated, the context size in the
inner focus for an evaluation of E2
is
the number of items in the sequence obtained by evaluating E1
.
[Definition: Variable values. This is a mapping from expanded QName to value. It contains the same expanded QNames as the in-scope variables in the static context for the expression. The expanded QName is the name of the variable and the value is the dynamic value of the variable, which includes its dynamic type.]
[Definition: Dynamically known function definitions. This is a set of function definitions. It includes the statically known function definitions as a subset, but may include other function definitions that are not known statically. ]
The function definitions in the dynamic context are used primarily by the fn:function-lookup
function.
If two function definitions in the dynamically known function definitions have the same name, then their arity ranges must not overlap.
Note:
The reason for allowing named functions to be available dynamically beyond those that are
available statically is primarily to allow for cases where the run-time execution
environment is significantly different from the compile-time environment. This could happen, for example,
if a stylesheet or query is compiled within a web server and then executed in the web browser.
The fn:function-lookup
function allows dynamic discovery of resources that were not
available statically.
[Definition:
Current dateTime. This information represents
an implementation-dependent point in time during the processing of
an expression, and includes an explicit timezone. It can be retrieved by the fn:current-dateTime
function.
If called multiple times during the execution of
an expression,
this function always returns the same result.]
[Definition:
Implicit timezone. This is the timezone to be used when a date,
time, or dateTime value that does not have a timezone is used in a
comparison or arithmetic operation. The implicit timezone is an implementation-defined value of type
xs:dayTimeDuration
. See Section
3.2.7.3 Timezones
XS1-2 or
Section
3.3.7 dateTime
XS11-2 for the range of valid values of a timezone.]
[Definition:
Executable Base URI. This is an absolute URI used
to resolve relative URIs during the evaluation of expressions;
it is used, for example, to resolve a relative URI supplied
to the fn:doc
or fn:unparsed-text
functions.
]
URIs are resolved as described in 2.5.6 Resolving a Relative URI Reference.
The function fn:static-base-uri
, despite its name, returns the
value of the Executable Base URI.
In many straightforward processing scenarios, the Executable Base URI in the dynamic context will be the same as the Static Base URI for the corresponding expression in the static context. There are situations, however, where they may differ:
Some processors may allow the static analysis of a query or stylesheet to take place on a development machine, while execution of the query or stylesheet happens on a test or production server. In this situation, resources needed during static analysis (such as other modules of the query or stylesheet) will be located on the development machine, by reference to the Static Base URI, while resources needed during execution (such as reference data files) will be located on the production machine, accessed via the Executable Base URI.
When the fn:static-base-uri
function is called within
the initializing expression of an optional parameter in a function declaration,
it returns the executable base URI of the relevant function call. This allows a user-written
function to accept two parameters: a required parameter containing a relative URI, and an
optional parameter containing a base URI. The optional parameter can be given a default
value of fn:static-base-uri()
, allowing the code in the function body
to resolve the relative URI against the executable base URI of the caller.
[Definition:
Default
collation. This identifies one of the collations in statically known collations as the collation to be
used by functions and operators for comparing and ordering values of type xs:string
and xs:anyURI
(and types derived from them) when no
explicit collation is
specified.]
Note:
Although the default collation is defined (in 4.0) as a property of the
dynamic context, its value will in nearly all cases be known statically. The reason it is defined in the
dynamic context is to allow a call on the fn:default-collation
function to be used when defining
the default value of an optional parameter to a user-defined function. In this situation,
the actual value supplied for the parameter is taken from the dynamic context of the relevant function call.
[Definition:
Default language.
This is the natural language used when creating human-readable output
(for example, by the functions fn:format-date
and fn:format-integer
)
if no other language is requested.
The value is a language code as defined by the type xs:language
.]
[Definition:
Default calendar.
This is the calendar used when formatting dates in human-readable output
(for example, by the functions fn:format-date
and fn:format-dateTime
)
if no other calendar is requested.
The value is a string.]
[Definition:
Default place.
This is a geographical location used to identify the place where events happened (or will happen) when
formatting dates and times using functions such as fn:format-date
and fn:format-dateTime
,
if no other place is specified. It is used when translating timezone offsets to civil timezone names,
and when using calendars where the translation from ISO dates/times to a local representation is dependent
on geographical location. Possible representations of this information are an ISO country code or an
Olson timezone name, but implementations are free to use other representations from which the above
information can be derived.]
[Definition:
Available
documents. This is a mapping of strings to document nodes. Each string
represents the absolute URI of a resource. The document node is the root of a tree that represents that resource
using the data model. The document node is returned by the fn:doc
function when applied to that URI.] The set of available documents is not limited to the set of
statically known documents, and it may be empty.
If there are one or more
URIs in available documents that map to a document
node D
, then the document-uri property of D
must either be absent, or must
be one of these URIs.
Note:
This means that given a document node $N
, the result of
fn:doc(fn:document-uri($N)) is $N
will always be true
, unless
fn:document-uri($N)
is an empty sequence.
[Definition:
Available text resources.
This is a mapping of strings to text resources. Each string
represents the absolute URI of a resource. The resource is returned
by the fn:unparsed-text
function when applied to that
URI.] The set of available text resources is not limited to
the set of statically known
documents, and it may be empty.
[Definition:
Available
collections. This is a mapping of
strings to sequences of items. Each string
represents the absolute URI of a
resource. The sequence of items represents
the result of the fn:collection
function when that URI is supplied as the
argument. ] The set of available
collections is not limited to the set of statically known
collections, and it may be empty.
For every document node D
that is in the target of a mapping in available collections, or that is the root of a tree containing such a node, the document-uri property of D
must either be absent, or must be a
URI U
such that available documents contains a mapping from U
to D
.
Note:
This means that for any document node $N
retrieved using the
fn:collection
function, either directly or by navigating to the root of a
node that was returned, the result of fn:doc(fn:document-uri($N)) is $N
will always be true
, unless fn:document-uri($N)
is an empty sequence. This
implies a requirement for the fn:doc
and fn:collection
functions to be
consistent in their effect. If the implementation uses catalogs or
user-supplied URI resolvers to dereference URIs supplied to the fn:doc
function, the implementation of the fn:collection
function must take these
mechanisms into account. For example, an implementation might achieve this
by mapping the collection URI to a set of document URIs, which are then
resolved using the same catalog or URI resolver that is used by the fn:doc
function.
[Definition:
Default collection.
This is the sequence of items that would result from calling the fn:collection
function
with no arguments.] The value of default collection may be initialized by the
implementation.
[Definition:
Available
URI collections. This is a mapping of
strings to sequences of URIs. The string
represents the absolute URI of a
resource which can be interpreted as an aggregation of a number of individual resources each of which
has its own URI. The sequence of URIs represents
the result of the fn:uri-collection
function when that URI is supplied as the
argument. ] There is no implication that the URIs in this sequence
can be successfully dereferenced, or that the resources they refer to have any particular media type.
Note:
An implementation may maintain some consistent relationship between the available
collections and the available URI collections, for example by ensuring that the result of
fn:uri-collection(X)!fn:doc(.)
is the same as the result of fn:collection(X)
.
However, this is not required. The fn:uri-collection
function is more
general than fn:collection
in that
fn:collection
allows access to
nodes that might lack individual URIs, for example nodes corresponding
to XML fragments stored in the rows of a relational database.
[Definition:
Default URI collection.
This is the sequence of URIs that would result from calling the fn:uri-collection
function
with no arguments.] The value of default URI collection may be initialized by the
implementation.
[Definition: Environment variables. This is a mapping from names to values. Both the names and the values are strings. The names are compared using an implementation-defined collation, and are unique under this collation. The set of environment variables is implementation-defined and may be empty.]
Note:
A possible implementation is to provide the set of POSIX environment variables (or their equivalent on other operating systems) appropriate to the process in which the expression is evaluated.
XPath 4.0 is defined in terms of the data model and the expression context.
Figure 1: Processing Model Overview
Figure 1 provides a schematic overview of the processing steps that are discussed in detail below. Some of these steps are completely outside the domain of XPath 4.0; in Figure 1, these are depicted outside the line that represents the boundaries of the language, an area labeled external processing. The external processing domain includes generation of XDM instances that represent the data to be queried (see 2.3.1 Data Model Generation), schema import processing (see 2.3.2 Schema Import Processing) and serialization. The area inside the boundaries of the language is known as the XPath processing domain , which includes the static analysis and dynamic evaluation phases (see 2.3.3 Expression Processing). Consistency constraints on the XPath processing domain are defined in 2.3.4 Consistency Constraints.
The input data for an expression must be represented as one or more XDM instances. This process occurs outside the domain of XPath 4.0, which is why Figure 1 represents it in the external processing domain. Here are some steps by which an XML document might be converted to an XDM instance:
A document may be parsed using an XML parser that generates an XML Information Set (see [XML Infoset]). The parsed document may then be validated against one or more schemas. This process, which is described in [XML Schema 1.0 Part 1] or [XML Schema 1.1 Part 1], results in an abstract information structure called the Post-Schema Validation Infoset (PSVI). If a document has no associated schema, its Information Set is preserved. (See DM1 in Fig. 1.)
The Information Set or PSVI may be transformed into an XDM instance by a process described in [XQuery and XPath Data Model (XDM) 4.0]. (See DM2 in Fig. 1.)
The above steps provide an example of how an XDM instance might be constructed. An XDM instance might also be synthesized directly from a relational database, or constructed in some other way (see DM3 in Fig. 1.) XPath 4.0 is defined in terms of the data model, but it does not place any constraints on how XDM instances are constructed.
[Definition: Each element node and attribute node in an XDM instance has a type annotation (described in Section 2.8 Schema InformationDM40).
The type annotation of a node is a reference to an XML Schema type.
] The type-name
of a node is the name of the type referenced by its type annotation.
If the XDM instance was derived from a validated XML document as described in Section 3.3 Construction from a PSVIDM40, the type annotations of the element and attribute nodes are derived from schema
validation. XPath 4.0 does
not provide a way to directly access the type annotation of an element
or attribute node.
The value of an attribute is represented directly within the
attribute node. An attribute node whose type is unknown (such as might
occur in a schemaless document) is given the type annotation
xs:untypedAtomic
.
The value of an element is represented by the children of the
element node, which may include text nodes and other element
nodes. The type annotation of an element node indicates how the values in
its child text nodes are to be interpreted. An element that has not been validated (such as might occur in a schemaless document) is annotated
with the schema type xs:untyped
. An element that has been validated and found to be partially valid is annotated with the schema type xs:anyType
. If an element node is annotated as xs:untyped
, all its descendant element nodes are also annotated as xs:untyped
. However, if an element node is annotated as xs:anyType
, some of its descendant element nodes may have a more specific type annotation.
The in-scope schema definitions in the static context are provided by the host language (see step SI1 in Figure 1) and must satisfy the consistency constraints defined in 2.3.4 Consistency Constraints.
XPath 4.0 defines two phases of processing called the static analysis phase and the dynamic evaluation phase (see Fig. 1). During the static analysis phase, static errors, dynamic errors, or type errors may be raised. During the dynamic evaluation phase, only dynamic errors or type errors may be raised. These kinds of errors are defined in 2.4.1 Kinds of Errors.
Within each phase, an implementation is free to use any strategy or algorithm whose result conforms to the specifications in this document.
[Definition: The static analysis phase depends on the expression itself and on the static context. The static analysis phase does not depend on input data (other than schemas).]
During the static analysis phase, the XPath expression is parsed into an internal representation called the operation tree (step SQ1 in Figure 1). A parse error is raised as a static error [err:XPST0003]. The static context is initialized by the implementation (step SQ2). The static context is used to resolve schema type names, function names, namespace prefixes, and variable names (step SQ4). If a name of one of these kinds in the operation tree is not found in the static context, a static error ([err:XPST0008] or [err:XPST0017]) is raised (however, see exceptions to this rule in 3.6.3.2 Element Test and 3.6.3.4 Attribute Test.)
The operation tree is then normalized by making explicit the implicit operations such as atomization and extraction of Effective Boolean Values (step SQ5).
During the static analysis phase, a processor may perform type analysis. The effect of type analysis is to assign a static type to each expression in the operation tree. [Definition: The static type of an expression is the best inference that the processor is able to make statically about the type of the result of the expression.] This specification does not define the rules for type analysis nor the static types that are assigned to particular expressions: the only constraint is that the inferred type must match all possible values that the expression is capable of returning.
Examples of inferred static types might be:
For the expression concat(a,b)
the inferred static type is xs:string
For the expression $a = $v
the inferred static type is xs:boolean
For the expression $s[exp]
the inferred static
type has the same item type as the static type of $s
,
but a cardinality that allows the empty sequence even if the
static type of $s
does not allow an empty
sequence.
The inferred static type of the expression data($x)
(whether written
explicitly or inserted into the operation tree in places where atomization
is implicit) depends on the inferred static type of $x
: for example, if $x
has type element(*, xs:integer)
then data($x)
has static type xs:integer
.
In XQuery 1.0 and XPath 2.0, rules for static type inferencing were published normatively in [XQuery 1.0 and XPath 2.0 Formal Semantics], but implementations were allowed to refine these rules to infer a more precise type where possible. In XQuery 3.1 and XPath 3.1, the rules for static type inferencing are entirely implementation-dependent.
Every kind of expression also imposes requirements on the type of its
operands. For example, with the expression substring($a, $b, $c)
, $a
must be
of type xs:string
(or something that can be converted to xs:string
by the
function calling rules), while $b
and $c
must be of type xs:double
.
If the
Static Typing Feature is in effect, a processor must raise a
type error during static analysis if the inferred static type of an
expression is not subsumed by the required type of the context where the
expression is used. For example, the call of substring above would cause a
type error if the inferred static type of $a
is xs:integer
; equally, a type
error would be reported during static analysis if the inferred static type
is xs:anyAtomicType
.
If the Static Typing Feature is not in effect, a processor may raise a type error during static analysis only when one of the following conditions is met:
When the inferred static type of an
expression has no overlap (intersection) with the required type, and cannot be converted
to the required type using the coercion rules. For example,
given the call fn:upper-case($s)
, the processor may raise an error if the
declared or inferred type of $s
is xs:integer
,
but not if it is xs:anyAtomicType
.
When the only possible value of an expression that is consistent with the required
type is the empty sequence. Consider for example the expression
fn:codepoints-to-string(fn:tokenize($in))
. Since fn:codepoints-to-string
requires xs:integer*
while fn:tokenize($in)
delivers xs:string*
,
this expression can succeed only in the special case where the value is empty,
so processors may report this as an error. An error
must not be raised under this rule unless both the inferred static type and the required type permit
values other than the empty sequence.
When an ForwardStep or ReverseStep is used, and it is known during static analysis that the step will select no nodes.
One example of this is an expression such as @price/text()
: attribute nodes
never have children, so this expression will never select anything.
Another example arises when schema information is available: if it is known
that the variable $emp
holds a value of type schema-element(employee)
,
and that no element of this type can have an attribute named @sallary
(sic), then
a type error may be reported if the expression $emp/@sallary
is encountered.
Note:
A static error must not be reported simply because a predicate
will always return false
: the expression a[name()='b']
will always return
an empty sequence, but it is not an error.
When the KeySpecifier in a
Lookup expression is such that the result of the lookup
will inevitably be empty. For example if the context value is known to be of type
record(longitude, latitude)
then a static type error may be raised
against the expression ?altitude
.
For backwards compatibility, processors should provide
an option to avoid reporting type errors in respect of constructs such as @a/@b
that were executed without error in previous versions. Note in particular that the construct
/..
was sometimes recommended in XPath 1.0 as the preferred way to denote an empty
node-set.
Alternatively, if the Static Typing Feature is not in effect, the processor may defer all type checking until the dynamic evaluation phase.
[Definition: The dynamic evaluation phase is the phase during which the value of an expression is computed.] It is dependent on successful completion of the static analysis phase.
The dynamic evaluation phase can occur only if no errors were detected during the static analysis phase. If the Static Typing Feature is in effect, all type errors are detected during static analysis and serve to inhibit the dynamic evaluation phase.
The dynamic evaluation phase depends on the operation tree of the expression being evaluated (step DQ1), on the input data (step DQ4), and on the dynamic context (step DQ5), which in turn draws information from the external environment (step DQ3) and the static context (step DQ2). The dynamic evaluation phase may create new data-model values (step DQ4) and it may extend the dynamic context (step DQ5)—for example, by binding values to variables.
[Definition: Every value matches one or more sequence types. A value is said to have a dynamic type T if it matches (or is an instance of) the sequence type T.]
In many cases (but not all), one of the dynamic types that a value matches will be a subtype of all the others, in which case it makes sense to speak of “the dynamic type” of the value as meaning this single most specific type. In other cases (examples are empty maps and empty arrays) none of the dynamic types is more specific than all the others.
Note:
An atomic value has a type annotation which will always be a subtype of all the other types that it matches; we can therefore refer to this as the dynamic type of the atomic value without ambiguity.
A value may match a dynamic type that is more specific than the static type
of the expression that computed it (for example, the static type of an expression might be xs:integer*
,
denoting a sequence of zero or more integers, but at evaluation time its value may be an instance of xs:integer
,
denoting exactly one integer).
If an operand of an expression does not have a dynamic type that is a subtype of the static type required for that operand, a type error is raised [err:XPTY0004].
Even though static typing can catch many type errors before an expression is executed, it is possible for an expression to raise an error during evaluation that was not detected by static analysis. For example, an expression may contain a cast of a string into an integer, which is statically valid. However, if the actual value of the string at run time cannot be cast into an integer, a dynamic error will result. Similarly, an expression may apply an arithmetic operator to a value whose static type is xs:untypedAtomic
. This is not a static error, but at run time, if the value cannot be successfully cast to a numeric type, a dynamic error will be raised.
When the Static Typing Feature is in effect, it is also possible for static analysis of an expression to raise a type error, even though execution of the expression on certain inputs would be successful. For example, an expression might contain a function that requires an element as its parameter, and the static analysis phase might infer the static type of the function parameter to be an optional element. This case is treated as a type error and inhibits evaluation, even though the function call would have been successful for input data in which the optional element is present.
In order for XPath 4.0 to be well defined, the input XDM instances, the static context, and the dynamic context must be mutually consistent. The consistency constraints listed below are prerequisites for correct functioning of an XPath 4.0 implementation. Enforcement of these consistency constraints is beyond the scope of this specification. This specification does not define the result of an expression under any condition in which one or more of these constraints is not satisfied.
For every node that has a type annotation, if that type annotation is found in the in-scope schema definitions (ISSD), then its definition in the ISSD must be compatibleDM40 with its definition in the schemaDM40 that was used to validate the node.
Every element name, attribute name, or schema type name referenced in in-scope variables or statically known function definitions must be in the in-scope schema definitions, unless it is an element name referenced as part of an ElementTest or an attribute name referenced as part of an AttributeTest.
Any reference to a global element, attribute, or type name in the in-scope schema definitions must have a corresponding element, attribute or type definition in the in-scope schema definitions.
For each mapping of a string to a document node in available documents, if there exists a mapping of the same string to a document type in statically known documents, the document node must match the document type, using the matching rules in 3.5 Sequence Type Matching.
For each mapping of a string to a sequence of items in available collections, if there exists a mapping of the same string to a type in statically known collections, the sequence of items must match the type, using the matching rules in 3.5 Sequence Type Matching.
The sequence of items in the default collection must match the statically known default collection type, using the matching rules in 3.5 Sequence Type Matching.
The context value must match the context value static type, using the matching rules in 3.5 Sequence Type Matching.
For each (variable, type) pair in in-scope variables and the corresponding (variable, value) pair in variable values such that the variable names are equal, the value must match the type, using the matching rules in 3.5 Sequence Type Matching.
In the statically known namespaces, the prefix xml
must not be bound to any namespace URI other than http://www.w3.org/XML/1998/namespace
, and no prefix other than xml
may be bound to this namespace URI.
The prefix xmlns
must not be bound to any namespace URI, and no prefix may be bound to the namespace URI http://www.w3.org/2000/xmlns/
.
As described in 2.3.3 Expression Processing, XPath 4.0 defines a static analysis phase, which does not depend on input data, and a dynamic evaluation phase, which does depend on input data. Errors may be raised during each phase.
[Definition: An error that can be detected during the static analysis phase, and is not a type error, is a static error.] A syntax error is an example of a static error.
[Definition: A dynamic error is an error that must be detected during the dynamic evaluation phase and may be detected during the static analysis phase.] Numeric overflow is an example of a dynamic error.
[Definition: A type error may be raised during the static analysis phase or the dynamic evaluation phase. During the static analysis phase, a type error occurs when the static type of an expression does not match the expected type of the context in which the expression occurs. During the dynamic evaluation phase, a type error occurs when the dynamic type of a value does not match the expected type of the context in which the value occurs.]
The outcome of the static analysis phase is either success or one or more type errors, static errors, or statically detected dynamic errors. The result of the dynamic evaluation phase is either a result value, a type error, or a dynamic error.
If more than one error is present, or if an error condition comes within the scope of more than one error defined in this specification, then any non-empty subset of these errors may be reported.
During the static
analysis phase, if the
Static Typing Feature is in effect and the static type assigned to an expression other than ()
or data(())
is empty-sequence()
, a static error is raised [err:XPST0005]. This catches cases in which a query refers to an element or attribute that is not present in the in-scope schema definitions, possibly because of a spelling error.
Independently of whether the Static Typing Feature is in effect, if an implementation can determine during the static analysis phase that an XPath expression, if evaluated, would necessarily raise a dynamic error or that an expression, if evaluated, would necessarily raise a type error, the implementation may (but is not required to) report that error during the static analysis phase.
An implementation can raise a dynamic error for an XPath expression statically only if the expression can never execute without raising that error, as in the following example:
error()
The following example contains a type error, which can be reported statically even if the implementation can not prove that the expression will actually be evaluated.
if (empty($arg)) then "cat" * 2 else 0
[Definition: In addition to static errors, dynamic errors, and type errors, an XPath 4.0 implementation may raise warnings, either during the static analysis phase or the dynamic evaluation phase. The circumstances in which warnings are raised, and the ways in which warnings are handled, are implementation-defined.]
In addition to the errors defined in this specification, an implementation may raise a dynamic error for a reason beyond the scope of this specification. For example, limitations may exist on the maximum numbers or sizes of various objects. An error must be raised if such a limitation is exceeded [err:XPDY0130].
The errors defined in this specification are identified by QNames that have the form err:XPYYnnnn
, where:
err
denotes the namespace for XPath and XQuery errors, http://www.w3.org/2005/xqt-errors
. This binding of the namespace prefix err
is used for convenience in this document, and is not normative.
XP
identifies the error as an XPath error (some errors, originally defined by XQuery and later added to XPath, use the code XQ
instead).
YY
denotes the error category, using the following encoding:
ST
denotes a static error.
DY
denotes a dynamic error.
TY
denotes a type error.
nnnn
is a unique numeric code.
Note:
The namespace URI for XPath and XQuery errors is not expected to change from one version of XPath to another. However, the contents of this namespace may be extended to include additional error definitions.
The method by which an XPath 4.0 processor reports error information to the external environment is implementation-defined.
An error can be represented by a URI reference that is derived from the error QName as follows: an error with namespace URI
NS
and local part
LP
can be represented as the URI reference
NS
#
LP
. For example, an error whose QName is err:XPST0017
could be represented as http://www.w3.org/2005/xqt-errors#XPST0017
.
Note:
Along with a code identifying an error, implementations may wish to return additional information, such as the location of the error or the processing phase in which it was detected. If an implementation chooses to do so, then the mechanism that it uses to return this information is implementation-defined.
Except as noted in this document, if any operand of an expression
raises a dynamic error, the expression also raises a dynamic error.
If an expression can validly return a value or raise a dynamic
error, the implementation may choose to return the value or raise
the dynamic error (see 2.4.4 Errors and
Optimization). For example, the logical expression
expr1 and expr2
may return the value false
if either operand returns false
,
or may raise a dynamic error if either operand raises a dynamic
error.
If more than one operand of an expression raises an error, the implementation may choose which error is raised by the expression. For example, in this expression:
($x div $y) + xs:decimal($z)
both the sub-expressions ($x div $y)
and xs:decimal($z)
may
raise an error. The
implementation may choose which error is raised by the +
expression. Once one operand raises an error, the implementation is
not required, but is permitted, to evaluate any other operands.
[Definition: In addition to its identifying QName, a dynamic error may also carry a descriptive string and one or more additional values called error values.] An implementation may provide a mechanism whereby an application-defined error handler can process error values and produce diagnostic messages. The host language may also provide error handling mechanisms.
A dynamic error may be raised by a system
function or operator. For example,
the div
operator raises an error if its operands are xs:decimal
values and its second operand
is equal to zero. Errors raised by system functions and operators are defined in [XQuery and XPath Functions and Operators 4.0] or the host language.
A dynamic error can also be raised explicitly by calling the
fn:error
function, which always raises a dynamic error and never
returns a value. This function is defined in Section 3.1.1 fn:errorFO40. For example, the following
function call raises a dynamic
error, providing a QName that identifies the error, a descriptive string, and a diagnostic value (assuming that the prefix app
is bound to a namespace containing application-defined error codes):
error(xs:QName("app:err057"), "Unexpected value", string($v))
Because different implementations may choose to evaluate or optimize an expression in different ways, certain aspects of raising dynamic errors are implementation-dependent, as described in this section.
An implementation is always free to evaluate the operands of an operator in any order.
In some cases, a processor can determine the result of an expression without accessing all the data that would be implied by the formal expression semantics. For example, the formal description of filter expressions suggests that $s[1]
should be evaluated by examining all the items in sequence $s
, and selecting all those that satisfy the predicate position()=1
. In practice, many implementations will recognize that they can evaluate this expression by taking the first item in the sequence and then exiting. If $s
is defined by an expression such as //book[author eq 'Berners-Lee']
, then this strategy may avoid a complete scan of a large document and may therefore greatly improve performance. However, a consequence of this strategy is that a dynamic error or type error that would be detected if the expression semantics were followed literally might not be detected at all if the evaluation exits early. In this example, such an error might occur if there is a book
element in the input data with more than one author
subelement.
The extent to which a processor may optimize its access to data, at the cost of not raising errors, is defined by the following rules.
Consider an expression Q that has an operand (sub-expression) E. In general the value of E is a sequence. At an intermediate stage during evaluation of the sequence, some of its items will be known and others will be unknown. If, at such an intermediate stage of evaluation, a processor is able to establish that there are only two possible outcomes of evaluating Q, namely the value V or an error, then the processor may deliver the result V without evaluating further items in the operand E. For this purpose, two values are considered to represent the same outcome if their items are pairwise the same, where nodes are the same if they have the same identity, and values are the same if they are equal and have exactly the same type.
There is an exception to this rule: If a processor evaluates an operand E (wholly or in part), then it is required to establish that the actual value of the operand E does not violate any constraints on its cardinality. For example, the expression $e eq 0
results in a type error if the value of $e
contains two or more items. A processor is not allowed to decide, after evaluating the first item in the value of $e
and finding it equal to zero, that the only possible outcomes are the value true
or a type error caused by the cardinality violation. It must establish that the value of $e
contains no more than one item.
These rules apply to all the operands of an expression considered in combination: thus if an expression has two operands E1 and E2, it may be evaluated using any samples of the respective sequences that satisfy the above rules.
The rules cascade: if A is an operand of B and B is an operand of C, then the processor needs to evaluate only a sufficient sample of B to determine the value of C, and needs to evaluate only a sufficient sample of A to determine this sample of B.
The effect of these rules is that the processor is free to stop examining further items in a sequence as soon as it can establish that further items would not affect the result except possibly by causing an error. For example, the processor may return true
as the result of the expression S1 = S2
as soon as it finds a pair of equal values from the two sequences.
Another consequence of these rules is that where none of the items in a sequence contributes to the result of an expression, the processor is not obliged to evaluate any part of the sequence. Again, however, the processor cannot dispense with a required cardinality check: if an empty sequence is not permitted in the relevant context, then the processor must ensure that the operand is not an empty sequence.
Examples:
If an implementation can find (for example, by using an index) that at
least one item returned by $expr1
in the following example has the value 47
, it is allowed to
return true
as the result of the some
expression, without searching for
another item returned by $expr1
that would raise an error if it were evaluated.
some $x in $expr1 satisfies $x = 47
In the following example, if an implementation can find (for example, by using an index) the
product
element-nodes that have an id
child with the value 47
, it is allowed to return these nodes as the
result of the path expression, without searching for another product
node that
would raise an error because it has an id
child whose value is not an integer.
//product[id = 47]
For a variety of reasons, including optimization, implementations may rewrite expressions into a different form. There are a number of rules that limit the extent of this freedom:
Other than the raising or not raising of errors, the result of evaluating a rewritten expression must conform to the semantics defined in this specification for the original expression.
Note:
This allows an implementation to return a result in cases where the original expression would have raised an error, or to raise an error in cases where the original expression would have returned a result. The main cases where this is likely to arise in practice are (a) where a rewrite changes the order of evaluation, such that a subexpression causing an error is evaluated when the expression is written one way and is not evaluated when the expression is written a different way, and (b) where intermediate results of the evaluation cause overflow or other out-of-range conditions.
Note:
This rule does not mean that the result of the expression will always be the same in non-error cases as if it had not been rewritten, because there are many cases where the result of an expression is to some degree implementation-dependent or implementation-defined.
The rules described in 2.4.5 Guarded Expressions ensure that for certain kinds of expression (for example conditional expressions), changing the order of evaluation of subexpressions does not result in dynamic errors that would not otherwise occur.
Expressions must not be rewritten in such a way as to create or remove static errors. The static errors in this specification are defined for the original expression, and must be preserved if the expression is rewritten.
As stated earlier, an expression
must not be rewritten to dispense with a
required cardinality check: for example, string-length(//title)
must raise an
error if the document contains more than one title element.
[Definition: An expression E is said to be guarded by some governing condition C if evaluation of E is not allowed to fail with a dynamic error except when C applies.]
For example, in a conditional expression if (P) then T else F
, the subexpression
T is guarded by P, and the subexpression F is guarded by
not(P)
. One way an implementation can satisfy this rule is by not evaluating T unless P
is true, and likewise not evaluating F unless P is false. Another
way of satisfying the rule is for the implementation to evaluate all the subexpressions, but to catch any errors that occur
in a guarded subexpression so they are not propagated.
The existence of this rule enables errors to be prevented by writing expressions such as
if ($y eq 0) then "N/A" else ($x div $y)
. This example will never fail with a divide-by-zero
error because the else
branch of the conditional is guarded.
Similarly, in the mapping expression E1!E2
, the subexpression E2
is guarded
by the existence of an item from E1
. This means, for example, that the expression (1 to $n)!doc('bad.xml')
must not raise a dynamic error if $n
is zero. The rule governing evaluation of guarded expressions
is phrased so as not to disallow “loop-lifting” or “constant-folding” optimizations
whose aim is to avoid repeated evaluation of a common subexpression;
but such optimizations must not result in errors that would not otherwise occur.
The complete list of expressions that have guarded subexpressions is as follows:
In a conditional expression (IfExpr) the then
branch
is guarded by the condition being true, and the else
branch
is guarded by the condition being false.
In an and
expression (AndExpr), the second operand
is guarded by the value of the first operand being true.
In an or
expression (OrExpr), the second operand
is guarded by the value of the first operand being false.
In an otherwise
expression (OtherwiseExpr), the second operand
is guarded by the value of the first operand being an empty sequence.
In a path expression of the form E1/E2
or E1//E2
, and in a mapping
expression of the form E1!E2
, the right-hand operand E2
is guarded by
the existence of at least one item in the result of evaluating E1
.
This rule applies even if E2
does not reference the context value.
For example, no dynamic error can be thrown by the expression
(1 to $n)!doc('bad.xml')
in the case where $n
is zero.
In a filter expression of the form E[P]
, the predicate P
is guarded by
the existence of at least one item in the result of evaluating E
.
This rule has the consequence that in a filter expression with multiple predicates, such as E[P1][P2]
,
evaluation of P2
must not raise a dynamic error unless P1
returns true
. This rule does
not prevent reordering of predicates (for example, to take advantage of indexes), but it does require that any
such reordering must not result in errors that would not otherwise occur.
In a for
expression (ForExpr) such
as for $x in S return E
, the expression E
is guarded by the existence of
an item bound to $x
.
This means that the expression for $x in 1 to $n return doc('bad.xml')
must not raise a dynamic error in the case where $n
is zero.
In a quantified
expression (QuantifiedExpr) such
as some $x in S satisfies P
, the expression P
is guarded by the existence of
an item bound to $x
.
The fact that an expression is guarded does not remove the obligation to report static errors in the expression; nor does it remove the option to report statically detectable type errors.
Note:
These rules do not constrain the order of evaluation of subexpressions. For example, given an expression
such as //person[@first="Winston"][@last="Churchill"]
, or equivalently
//person[@first="Winston" and @last="Churchill"]
, an implementation might use an index on the value of
@last
to select items that satisfy the second condition, and then filter these
items on the value of the first condition. Alternatively, it might evaluate both predicates in parallel.
Or it might interpose an additional redundant condition:
//person[string-length(@first)+string-length(@last)=16][@first="Winston"][@last="Churchill"]
.
But implementations must ensure that
such rewrites do not result in dynamic errors being reported that would not occur if the predicates
were evaluated in order as written.
Note:
Although the rules for guarded expressions prevent optimizations resulting in spurious errors,
they do not prevent optimizations whose effect is to mask errors. For example, the rules guarantee that
("A", 3)[. instance of xs:integer][. eq 3]
will not raise an error caused by the comparison
("A" eq 3)
, but they
do not guarantee the converse: the expression ("A", 3)[. eq 3][. instance of xs:integer]
may or may not raise a dynamic error.
Note:
The rules in this section do not disallow all expression rewrites that might result in dynamic
errors. For example, rewriting ($x - $y + $z)
as ($x + $z - $y)
is permitted
even though it might result in an arithmetic overflow.
Note:
Some implementations allow calls on external functions that have side-effects. The semantics of such function calls are entirely implementation defined. Processors may choose to reference the rules for guarded expressions when defining the behavior of such function calls, but this is outside the scope of the language specification.
[Definition: Certain expressions, while not erroneous, are classified as being implausible, because they achieve no useful effect.]
An example of an implausible expression is @code/text()
. This expression
will always evaluate to an empty sequence, because attribute nodes cannot have text
node children. The semantics of the expression are well defined, but it is likely
that the user writing this expression intended something different; if they wanted to an
expression that evaluated to an empty sequence, there would be easier ways to write it.
Where an expression is classified (by rules in this specification) as being implausible, a processor may (but is not required to) raise a static error.
For reasons of backwards compatibility and interoperability, and to facilitate automatic generation of XPath 4.0 code, a processor must provide a mode of operation in which implausible expressions are not treated as static errors, but are evaluated with the defined semantics for the expression.
Some other examples of implausible expressions include:
round(tokenize($input))
. The result of fn:tokenize
is a sequence of strings (xs:string*
), while the required type for the
first argument of fn:round
is optional numeric (xs:numeric?
).
The expression can succeed only in the exceptional case where the result of fn:tokenize
is an empty sequence, in which case the result of fn:round
will also be
an empty sequence; it is therefore highly likely that the expression was written in error.
parse-csv($input)?column-names
. The signature of the parse-csv
function declares its return type as record(columns, rows)
. There is no field in this
record named column-names
, and therefore the lookup expression will always
return an empty sequence. Again, there is no good reason that a user would write this,
so it is likely that it was written in error.
Note:
The specification is deliberately conservative in the choice of constructs that have been classified as implausible. Constructs have not been classified as implausible merely because there are better ways of writing the same thing, but only in cases where it is considered that no user in full understanding of the specification would intentionally write such a construct. All these cases correspond to situations that would be classed as errors in a language with stricter static typing rules.
Note:
In many cases the classification of constructs as implausible is designed to protect users from usability problems that have been found with earlier versions of the language. without introducing backwards incompatibilities.
This section explains some concepts that are important to the processing of XPath 4.0 expressions.
An ordering called document order is defined among all the nodes accessible during processing of a given expression, which may consist of one or more trees (documents or fragments). Document order is defined in Section 2.5 Document OrderDM40, and its definition is repeated here for convenience. Document order is a total ordering, although the relative order of some nodes is implementation-dependent. [Definition: Informally, document order is the order in which nodes appear in the XML serialization of a document.] [Definition: Document order is stable, which means that the relative order of two nodes will not change during the processing of a given expression, even if this order is implementation-dependent.] [Definition: The node ordering that is the reverse of document order is called reverse document order.]
Within a tree, document order satisfies the following constraints:
The root node is the first node.
Every node occurs before all of its children and descendants.
Namespace nodes immediately follow the element node with which they are associated. The relative order of namespace nodes is stable but implementation-dependent.
Attribute nodes immediately follow the namespace nodes of the element node with which they are associated. The relative order of attribute nodes is stable but implementation-dependent.
The relative order of siblings is the order in which they occur
in the children
property of their parent node.
Children and descendants occur before following siblings.
The relative order of nodes in distinct trees is stable but implementation-dependent, subject to the following constraint: If any node in a given tree T1 is before any node in a different tree T2, then all nodes in tree T1 are before all nodes in tree T2.
The semantics of some
XPath 4.0 operators depend on a process called atomization. Atomization is
applied to a value when the value is used in a context in which a
sequence of atomic values is required. The result of atomization is
either a sequence of atomic values or a type error
[err:FOTY0012]FO40. [Definition:
Atomization of a sequence
is defined as the result of invoking the fn:data
function, as defined in Section 2.4 fn:dataFO40.]
The semantics of
fn:data
are repeated here for convenience. The result of
fn:data
is the sequence of atomic values produced by
applying the following rules to each item in the input
sequence:
If the item is an atomic value, it is returned.
If the item is a node, its typed value is returned (a type error [err:FOTY0012]FO40 is raised if the node has no typed value.)
If the item is a function item (other than an array) or map a type error [err:FOTY0013]FO40 is raised.
If the item is an array $a
, atomization is defined as $a?* ! fn:data(.)
, which is equivalent to atomizing the members of the array.
Note:
This definition recursively atomizes members that are arrays. Hence, the result of atomizing the array [ [1, 2, 3], [4, 5, 6] ]
is the sequence (1, 2, 3, 4, 5, 6)
.
Atomization is used in processing the following types of expressions:
Arithmetic expressions
Comparison expressions
Function calls and returns
Cast expressions
Under certain circumstances (listed below), it is necessary to find
the effective boolean value of a
value. [Definition: The
effective boolean value of a value is defined as the result
of applying the fn:boolean
function to the value, as
defined in Section 7.3.1 fn:booleanFO40.]
The dynamic semantics of fn:boolean
are repeated here for convenience:
If its operand is an empty sequence, fn:boolean
returns false
.
If its operand is a sequence whose first item is a node, fn:boolean
returns true
.
If its operand is a singleton value of type xs:boolean
or derived from xs:boolean
, fn:boolean
returns the value of its operand unchanged.
If its operand is a singleton value of type xs:string
, xs:anyURI
, xs:untypedAtomic
, or a type derived from one of these, fn:boolean
returns false
if the operand value has zero length; otherwise it returns true
.
If its operand is a singleton value of any numeric type or derived from a numeric type, fn:boolean
returns false
if the operand value is NaN
or is numerically equal to zero; otherwise it returns true
.
In all other cases, fn:boolean
raises a type error [err:FORG0006]FO40.
Note:
For instance, fn:boolean
raises a type error if the operand is a function, a map, or an array.
The effective boolean value of a sequence is computed implicitly during processing of the following types of expressions:
Logical expressions (and
, or
)
The fn:not
function
Certain types of predicates, such as a[b]
Conditional expressions (if
)
Quantified expressions (some
, every
)
General comparisons, in XPath 1.0 compatibility mode.
Note:
The definition of effective boolean
value is not used when casting a value to the
type xs:boolean
, for example in a cast
expression or when passing a value to a function whose expected
parameter is of type xs:boolean
.
XPath 4.0 has a set of functions that provide access to XML documents (fn:doc
, fn:doc-available
), collections (fn:collection
, fn:uri-collection
), text files (fn:unparsed-text
, fn:unparsed-text-lines
, fn:unparsed-text-available
), and environment variables (fn:environment-variable
, fn:available-environment-variables
). These functions are defined in Section 14.6 Functions giving access to external informationFO40.
An expression can access input data either by calling one of these input functions or by referencing some part of the dynamic context that is initialized by the external environment, such as a variable or context value.
XPath 4.0 requires a statically known, valid URI in a BracedURILiteral. An implementation may raise a static error [err:XQST0046] if the value of a Braced URI Literal is of nonzero length and is neither an absolute URI nor a relative URI.
Note:
The xs:anyURI
type is designed to anticipate the introduction of
Internationalized Resource Identifiers (IRIs) as defined in
[RFC3987].
Whitespace is normalized using the whitespace normalization rules
of fn:normalize-space
. If the result of whitespace
normalization contains only whitespace, the corresponding URI
consists of the empty string.
A Braced URI Literal or URI Literal is not subjected to percent-encoding or decoding as defined in [RFC3986].
[Definition: To
resolve a relative URI
$rel
against a
base URI $base
is to expand it to an absolute URI,
as if by calling the function fn:resolve-uri($rel,
$base)
.] During static analysis, the base URI is
the Static Base URI. During dynamic evaluation, the base URI
used to resolve a relative URI reference depends on the semantics of the
expression.
Any process that attempts to resolve a URI against a base URI, or to dereference the URI, may apply percent-encoding or decoding as defined in the relevant RFCs.
The type system of XPath 4.0 is based on [XML Schema 1.0] or [XML Schema 1.1].
[Definition: A sequence type is a type that can be expressed using the SequenceType syntax. Sequence types are used whenever it is necessary to refer to a type in an XPath 4.0 expression. The term sequence type suggests that this syntax is used to describe the type of an XPath 4.0 value, which is always a sequence.]
[Definition: An item type is a type that can be expressed using the ItemType syntax, which forms part of the SequenceType
syntax. Item types match individual items.]
With the exception of the generic types item()
, which matches all items, and xs:error
,
which matches no items, the set of items matched by an item type consists either
exclusively of atomic values,
exclusively of nodes,
or exclusively of function itemsDM40.
Note:
These definitions require a caveat: types defined in a schema may be anonymous, in which case
they cannot be referenced directly using the ItemType or
SequenceType syntax. For example
an element that is validated against an anonymous complex type A conforms to an item type
which could be written element(*, A)
but for the fact that A is anonymous.
In many situations the terms item type and
sequence type are used interchangeably to refer either to the type itself,
or to the syntactic construct that designates the type: so in the expression
$x instance of xs:string*
, the construct xs:string*
uses the SequenceType syntax to designate a
sequence type whose instances are sequences of strings.
When more precision is required, the specification is careful to use the terms
item type and sequence type to
refer to the actual types, while using the production names ItemType
and SequenceType to refer to the syntactic
designators of these types.
[Definition: A sequence type designator is a syntactic construct conforming to the grammar rule SequenceType. A sequence type designator is said to designate a sequence type.]
[Definition: An item type designator is a syntactic construct conforming to the grammar rule ItemType. An item type designator is said to designate an item type.]
Note:
Two item type designators may designate the
same item type. For example, element()
and element(*)
are equivalent,
as are attribute(A)
and attribute(A, xs:anySimpleType)
.
[Definition: A schema type is a type that is (or could be) defined using the facilities of [XML Schema 1.0] or [XML Schema 1.1] (including the built-in types).] A schema type can be used as a type annotation on an
element or attribute node (unless it is a non-instantiable type such as xs:NOTATION
or xs:anyAtomicType
, in which case its derived
types can be so used). Every schema type is either a complex type or a
simple type; simple types are further subdivided into list types, union
types, and atomic types (see [XML Schema 1.0] or [XML Schema 1.1] for definitions and explanations of these terms.)
Note:
Local union types (see 3.6.2.1 Local Union Types) and enumeration types (see 3.6.2.2 Enumeration Types) are classified as schema types, even though they are not defined in any XSD schema.
[Definition: A generalized atomic type is a schema type that is either (a) an atomic type or (b) a pure union type ].
[Definition: A pure union type is a
simple type
that satisfies the following constraints:
(1) {variety}
is union
, (2) the {facets}
property is empty, (3) no type in the transitive membership of the union type has {variety}
list
, and (4) no type in the transitive membership of the union type is a type with {variety}
union
having a non-empty {facets}
property].
Note:
The definition of pure union type excludes union types derived by non-trivial restriction from other union types, as well as union types that include list types in their membership. Pure union types have the property that every instance of an atomic type defined as one of the member types of the union is also a valid instance of the union type.
Note:
The current (second) edition of XML Schema 1.0 contains an error in respect of the substitutability of a union type by one of its members: it fails to recognize that this is unsafe if the union is derived by restriction from another union.
This problem is fixed in XSD 1.1, but the effect of the resolution is that an atomic value labeled with an atomic type cannot be treated as being substitutable for a union type without explicit validation. This specification therefore allows union types to be used as item types only if they are defined directly as the union of a number of atomic types.
Note:
A local union type (see 3.6.2.1 Local Union Types) is always a pure union type
Generalized atomic types
represent the intersection between the categories of sequence type and schema type. A generalized atomic type, such as xs:integer
or my:hatsize
, is both a sequence type and a
schema type.
The in-scope schema types
in the static
context are initialized with a set of
predefined schema types that is determined by the host
language. This set may include some or all of the
schema types in the
namespace
http://www.w3.org/2001/XMLSchema
,
represented in this document by the namespace prefix
xs
. The schema types in this namespace are defined in [XML Schema 1.0] or [XML Schema 1.1]
and augmented by additional types defined in [XQuery and XPath Data Model (XDM) 4.0]. An implementation
that has based its type system on [XML Schema 1.0] is not required to support the xs:dateTimeStamp
or xs:error
types.
The schema types defined in Section 2.8.3 Predefined TypesDM40 are summarized below.
[Definition:
xs:untyped
is used as the type annotation of an element node that has not been validated, or has been validated in skip
mode.] No predefined schema types are derived from xs:untyped
.
[Definition:
xs:untypedAtomic
is an atomic type that is used to denote untyped atomic data, such as text that has not been assigned a more specific type.] An attribute that has been validated in skip
mode is represented in the data model by an attribute node with the type annotation
xs:untypedAtomic
. No predefined schema types are derived from xs:untypedAtomic
.
[Definition:
xs:dayTimeDuration
is derived by restriction from xs:duration
. The lexical representation of xs:dayTimeDuration
is restricted to contain only day, hour, minute, and second
components.]
[Definition:
xs:yearMonthDuration
is derived by restriction from xs:duration
. The lexical representation of xs:yearMonthDuration
is
restricted to contain only year and month
components.]
[Definition:
xs:anyAtomicType
is an atomic type that includes all atomic values (and no values that
are not atomic). Its base type is
xs:anySimpleType
from which all simple types, including atomic,
list, and union types, are derived. All primitive atomic types, such as
xs:decimal
and xs:string
, have xs:anyAtomicType
as their base type.]
Note:
xs:anyAtomicType
will not appear as the type of an actual value in an XDM instance.
[Definition:
xs:error
is a simple type with no value space. It is defined in Section
3.16.7.3 xs:error
XS11-1 and can be used in the 3.4 Sequence Types to raise errors.]
The relationships among the schema types in the xs
namespace are illustrated in Figure 2. A more complete description of the XPath 4.0 type hierarchy can be found in
Section 1.7 Type SystemFO40.
Figure 2: Hierarchy of Schema Types used in XPath 4.0.
[Definition: The namespace-sensitive
types are xs:QName
, xs:NOTATION
, types
derived by restriction from xs:QName
or
xs:NOTATION
, list types that have a namespace-sensitive
item type, and union types with a namespace-sensitive type in their
transitive membership.]
It is not possible to preserve the type of a namespace-sensitive value without also preserving the namespace binding that defines the meaning of each namespace prefix used in the value. Therefore, XPath 4.0 defines some error conditions that occur only with namespace-sensitive values. For instance, casting to a namespace-sensitive type raises a type error [err:FONS0004]FO40 if the namespace bindings for the result cannot be determined.
Every node has a typed value and a string value, except for nodes whose value is absentDM40. [Definition: The typed value of a node is a sequence of atomic values and can be extracted by applying the Section 2.4 fn:dataFO40 function to the node.] [Definition: The string value of a node is a string and can be extracted by applying the Section 2.3 fn:stringFO40 function to the node.]
An implementation may store both the typed value and the string value of a node, or it may store only one of these and derive the other as needed.
The string value of a node must be a valid lexical representation of the typed value of the node,
but the node is not required to preserve the string representation from the original source document.
For example, if the typed value of a node is the xs:integer
value 30
,
its string value might be "30"
or "0030"
.
The typed value, string value, and type annotation of a node are closely related. If the node was created by mapping from an Infoset or PSVI, the relationships among these properties are defined by rules in Section 2.8 Schema InformationDM40.
As a convenience to the reader, the relationship between typed value and string value for various kinds of nodes is summarized and illustrated by examples below.
For text and document nodes, the typed value of the node is the same as its
string value, as an instance of the type xs:untypedAtomic
. The
string value of a document node is formed by concatenating the string
values of all its descendant text nodes, in document
order.
The typed value of a comment, namespace, or processing instruction node is the same as its string value. It is an instance of the type xs:string
.
The typed value of an attribute node with
the type annotation
xs:anySimpleType
or xs:untypedAtomic
is the same as its
string value, as an instance of xs:untypedAtomic
. The
typed value of an attribute node with any other type annotation is
derived from its string value and type annotation using the lexical-to-value-space mapping defined in [XML Schema 1.0] or [XML Schema 1.1] Part 2 for
the relevant type.
Example: A1 is an attribute
having string value "3.14E-2"
and type annotation
xs:double
. The typed value of A1 is the
xs:double
value whose lexical representation is
3.14E-2
.
Example: A2 is an attribute with type
annotation xs:IDREFS
, which is a list datatype whose item type is the atomic datatype xs:IDREF
. Its string value is
"bar baz faz"
. The typed value of A2 is a sequence of
three atomic values ("bar"
, "baz"
",
"faz"
"), each of type xs:IDREF
. The typed
value of a node is never treated as an instance of a named list
type. Instead, if the type annotation of a node is a list type (such
as xs:IDREFS
), its typed value is treated as a sequence
of the generalized atomic type from which it is derived (such as
xs:IDREF
).
For an element node, the relationship between typed value and string value depends on the node’s type annotation, as follows:
If the type annotation is xs:untyped
or xs:anySimpleType
or
denotes a complex type with mixed content (including xs:anyType
), then the typed value of the
node is equal to its string value, as an instance of
xs:untypedAtomic
. However, if the nilled
property of the node is true
, then its typed value is the empty sequence.
Example: E1 is an element node
having type annotation xs:untyped
and string value
"1999-05-31"
. The typed value of E1 is
"1999-05-31"
, as an instance of
xs:untypedAtomic
.
Example: E2 is an element node
with the type annotation formula
, which is a complex type
with mixed content. The content of E2 consists of the character
H
, a child element named subscript
with
string value "2"
, and the character O
. The
typed value of E2 is "H2O"
as an instance of
xs:untypedAtomic
.
If the type
annotation denotes a simple type or a complex type with simple
content, then the typed value of the node is derived from its string
value and its type annotation in a way that is consistent with schema
validation. However, if the nilled
property of the node is true
, then its typed value is the empty sequence.
Example: E3 is an element node with the type
annotation cost
, which is a complex type that has several
attributes and a simple content type of xs:decimal
. The
string value of E3 is "74.95"
. The typed value of E3 is
74.95
, as an instance of
xs:decimal
.
Example: E4 is an element node with the
type annotation hatsizelist
, which is a simple type
derived from the atomic type hatsize
, which in turn is
derived from xs:integer
. The string value of E4 is
"7 8 9"
. The typed value of E4 is a sequence of three
values (7
, 8
, 9
), each of type
hatsize
.
Example: E5 is an element node with the type annotation my:integer-or-string
which is a union type with member types xs:integer
and xs:string
.
The string value of E5 is "47"
. The typed value of E5 is 47
as a
xs:integer
, since xs:integer
is the member type that validated the
content of E5. In general, when the type annotation of a node is a union type,
the typed value of the node will be an instance of one of the member types of the union.
Note:
If an implementation stores only the string value of a node, and the type annotation of the node is a union type, the implementation must be able to deliver the typed value of the node as an instance of the appropriate member type.
If the type annotation denotes a complex type with empty content, then the typed value of the node is the empty sequence and its string value is the zero-length string.
If the type annotation
denotes a complex type with element-only content, then the typed value
of the node is absentDM40. The fn:data
function raises a
type error
[err:FOTY0012]FO40 when applied to such a node. The string value of such a node is equal to the concatenated string values of all its text node descendants, in document order.
Example: E6 is an
element node with the type annotation weather
, which is a
complex type whose content type specifies
element-only
. E6 has two child elements named
temperature
and precipitation
. The typed
value of E6 is absentDM40, and the fn:data
function
applied to E6 raises an error.
Whenever it is necessary to refer to a type in an XPath 4.0 expression, the SequenceType syntax is used.
[112] | SequenceType |
::= | ("empty-sequence" "(" ")") |
|
[114] | ItemType |
::= |
AnyItemTest | TypeName | KindTest | FunctionTest | MapTest | ArrayTest | AtomicOrUnionType | RecordTest | LocalUnionType | EnumerationType | ParenthesizedItemType
|
|
[113] | OccurrenceIndicator |
::= | "?" | "*" | "+" |
/* xgc: occurrence-indicators */ |
With the exception of the special type
empty-sequence()
, a sequence type consists of an
item type that constrains the type of each item in the
sequence, and a cardinality that constrains the number of
items in the sequence. Apart from the item type item()
,
which permits any kind of item, item types divide into node
types (such as element()
), generalized atomic
types (such as xs:integer
) and function types
(such as function() as item()*
).
Lexical QNames appearing in a sequence type have their
prefixes expanded to namespace URIs by means of the
statically known namespaces and (where applicable) the
default namespace for elements and types.
Equality of QNames is defined by the eq
operator.
Item types representing element
and attribute nodes may specify the required type annotations of those nodes in
the form of a schema
type. Thus the item type element(*, us:address)
denotes any element node whose type annotation is (or is derived from)
the schema type named us:address
.
The occurrence indicators +
, *
, and ?
bind to the last ItemType in the SequenceType, as described in occurrence-indicators constraint.
Here are some examples of sequence types that might be used in XPath 4.0:
xs:date
refers to the built-in atomic schema type named xs:date
attribute()?
refers to an optional attribute node
element()
refers to any element node
element(po:shipto, po:address)
refers to an element node that has the name po:shipto
and has the type annotation po:address
(or a schema type derived from po:address
)
element(*, po:address)
refers to an element node of any name that has the type annotation po:address
(or a type derived from po:address
)
element(customer)
refers to an element node named customer
with any type annotation
schema-element(customer)
refers to an element node whose name is customer
(or is in the substitution group headed by customer
) and whose type annotation matches the schema type declared for a customer
element in the in-scope element declarations
node()*
refers to a sequence of zero or more nodes of any kind
item()+
refers to a sequence of one or more items
function(*)
refers to any function item, regardless of arity or type
function(node()) as xs:string*
refers to a function item that takes a single argument whose value is a single node,
and returns a sequence of zero or more xs:string
values
(function(node()) as xs:string)*
refers to a sequence of zero or more function items, each of which takes a single
argument whose value is a single node, and returns as its result a single xs:string
value
[Definition:
SequenceType matching compares a value with an expected sequence type. ] For example, an instance of
expression
returns true
if a given value matches a given sequence type, and false
if it does not.
An XPath 4.0 implementation must be able to determine relationships among the types in type annotations in an XDM instance and the types in the in-scope schema definitions (ISSD).
[Definition: The use of a value that has a dynamic type
that is a subtype of the
expected type is known as subtype substitution.]
Subtype substitution does not change the actual type of a value. For
example, if an xs:integer
value is used where an
xs:decimal
value is expected, the value retains its type
as xs:integer
.
The definition of SequenceType matching relies
on a pseudo-function named derives-from(
AT,
ET
)
, which takes an actual simple or complex
schema type AT and an expected simple or complex schema
type ET, and either returns a boolean value or raises a
type error
[err:XPTY0004]. This function is defined as follows:
derives-from(
AT, ET
)
raises a type error [err:XPTY0004] if ET is
not present in the in-scope schema definitions (ISSD).
derives-from(
AT,
ET
)
returns true
if any of the following conditions applies:
AT is ET
ET is the base type of AT
ET is a pure union type of which AT is a member type
There is a type MT such that derives-from(
AT, MT
)
and derives-from(
MT, ET
)
Otherwise, derives-from(
AT, ET
)
returns false
The rules for SequenceType matching are given below, with examples (the examples are for purposes of illustration, and do not cover all possible cases).
The sequence type
empty-sequence()
matches a value that is the empty sequence.
An ItemType with no OccurrenceIndicator matches any value that contains exactly one item if the ItemType matches that item (see 3.6 Item Types).
An ItemType with an OccurrenceIndicator matches a value if the number of items in the value matches the OccurrenceIndicator and the ItemType matches each of the items in the value.
An OccurrenceIndicator specifies the number of items in a sequence, as follows:
?
matches zero or one items
*
matches zero or more items
+
matches one or more items
As a consequence of these rules, any sequence type whose
OccurrenceIndicator is *
or ?
matches a
value that is an empty sequence.
[114] | ItemType |
::= |
AnyItemTest | TypeName | KindTest | FunctionTest | MapTest | ArrayTest | AtomicOrUnionType | RecordTest | LocalUnionType | EnumerationType | ParenthesizedItemType
|
|
[115] | AnyItemTest |
::= | "item" "(" ")" |
|
[133] | TypeName |
::= |
EQName
|
|
[117] | KindTest |
::= |
DocumentTest
|
|
[119] | DocumentTest |
::= | "document-node" "(" (ElementTest | SchemaElementTest)? ")" |
|
[127] | ElementTest |
::= | "element" "(" (NameTestUnion ("," TypeName "?"?)?)? ")" |
|
[128] | SchemaElementTest |
::= | "schema-element" "(" ElementDeclaration ")" |
|
[129] | ElementDeclaration |
::= |
ElementName
|
|
[124] | AttributeTest |
::= | "attribute" "(" (NameTestUnion ("," TypeName)?)? ")" |
|
[125] | SchemaAttributeTest |
::= | "schema-attribute" "(" AttributeDeclaration ")" |
|
[126] | AttributeDeclaration |
::= |
AttributeName
|
|
[131] | ElementName |
::= |
EQName
|
|
[130] | AttributeName |
::= |
EQName
|
|
[123] | PITest |
::= | "processing-instruction" "(" (NCName | StringLiteral)? ")" |
|
[121] | CommentTest |
::= | "comment" "(" ")" |
|
[122] | NamespaceNodeTest |
::= | "namespace-node" "(" ")" |
|
[120] | TextTest |
::= | "text" "(" ")" |
|
[118] | AnyKindTest |
::= | "node" "(" ")" |
|
[134] | FunctionTest |
::= |
AnyFunctionTest
|
|
[135] | AnyFunctionTest |
::= | "function" "(" "*" ")" |
|
[136] | TypedFunctionTest |
::= | "function" "(" (SequenceType ("," SequenceType)*)? ")" "as" SequenceType
|
|
[151] | ParenthesizedItemType |
::= | "(" ItemType ")" |
|
[137] | MapTest |
::= |
AnyMapTest | TypedMapTest
|
|
[140] | RecordTest |
::= |
AnyRecordTest | TypedRecordTest
|
|
[148] | ArrayTest |
::= |
AnyArrayTest | TypedArrayTest
|
|
[116] | AtomicOrUnionType |
::= |
EQName
|
|
[146] | LocalUnionType |
::= | "union" "(" ItemType ("," ItemType)* ")" |
|
[147] | EnumerationType |
::= | "enum" "(" StringLiteral ("," StringLiteral)* ")" |
This section defines the semantics of different ItemTypes
in terms of the values that they match.
An item type designator written simply
as an EQName
(that is, a NamedType
) is interpreted as follows:
If the name is written as a lexical QName, then it is expanded using the
in-scope namespaces in the static context. If the
name is an unprefixed NCName
, then it is expanded according to the
default namespace for elements and types.
If the name matches a named item type in the static context, then it is taken as a reference to the corresponding item type. The rules that apply are the rules for the expanded item type definition.
Otherwise, it must match the name of a type in the in-scope schema types in the static context: specifically, an atomic type or a plain union type. See 3.6.2 Atomic and Union Types for details.
Note:
A name in the xs
namespace will always fall into this category, since the namespace
is reserved.
If the name cannot be resolved to a type, a static error is raised [err:XPST0051].
item()
matches
any single item.
Example: item()
matches the atomic
value 1
, the element <a/>
, or the function fn:concat#3
.
A ParenthesizedItemType designates the same item type as the ItemType that is in parentheses.
Note:
Parenthesized item types are used primarily when defining nested item types in a function
signature. For example, a sequence of functions that each return a single boolean might be denoted
(function() as xs:boolean)*
. In this example the parentheses
are needed to indicate where the occurrence indicator belongs.
A generalized atomic type may be designated by an ItemType in any of the following ways:
Using the QName of a type in the in-scope schema definitions that is an atomic type or a pure union type.
Using a QName that identifies a named item type that resolves to a generalized atomic type.
Using a ParenthesizedItemType where the parentheses enclose a generalized atomic type.
Using a LocalUnionType as described below.
Using an EnumerationType as described below.
An atomic value A matches the
generalized atomic type
GAT
if the type annotation of A
(call it T) satisfies the condition derives-from(T, GAT)
.
Example: The ItemType
xs:decimal
matches any value of type
xs:decimal
. It also matches any value of type
shoesize
, if shoesize
is an atomic type
derived by restriction from xs:decimal
.
Example: Suppose ItemType
dress-size
is a union type that allows
either xs:decimal
values for numeric sizes (for example: 4, 6, 10, 12),
or one of an enumerated set of xs:strings
(for example: small
, medium
, large
). The ItemType
dress-size
matches any of these values.
Note:
The names of list
types such as xs:IDREFS
are not accepted in this context,
but can often be replaced by a generalized atomic type with an occurrence indicator, such as
xs:IDREF+
.
A LocalUnionType
defines an anonymous union type locally (for example,
within a function signature) which may be more convenient than defining the type in an
imported schema.
[146] | LocalUnionType |
::= | "union" "(" ItemType ("," ItemType)* ")" |
Although the grammar allows any ItemType
to appear, each ItemType
must identify a generalized atomic type
[err:XPST0147].
A LocalUnionType
is a
generalized atomic type. It is classified
as a schema type even though it is not defined in any
XSD schema.
An item matches a LocalUnionType
if it matches any of the
generalized atomic types
listed within the parentheses.
For example, the type union(xs:date, xs:dateTime, xs:time)
matches any value that is an instance
of xs:date
, xs:dateTime
, or xs:time
.
Similarly, the type union(xs:NCName, enum(""))
matches any value that is either
an instance of xs:NCName
, or a zero-length string. This might be a suitable type for
a variable that holds a namespace prefix.
Note:
Local union types are particularly useful in function signatures, allowing a function to take arguments of a variety of types. The semantics are identical to using a named union type, but a local union type is more convenient because it does not need to be defined in a schema, and does not require a schema-aware processor.
A local union type can also be used in a cast expression: cast @when as union(xs:date, xs:dateTime)
allows the attribute @when
to be either a date, or a dateTime.
An instance of
expression can be used to test whether a value belongs to one
of a number of specified types: $x instance of union(xs:string, xs:anyURI, xs:untypedAtomic)
returns true
if $x
is an instance of any of these three atomic types.
[Definition: An EnumerationType accepts a fixed set of string values.]
[147] | EnumerationType |
::= | "enum" "(" StringLiteral ("," StringLiteral)* ")" |
An EnumerationType
has a value space consisting of a set of xs:string
values. When matching strings against an enumeration type, strings are always compared
using the Unicode codepoint collation.
For example, if an argument of a function declares the required type
as enum("red", "green", "blue")
, then the string "green"
is accepted,
while "yellow"
is rejected with a type error.
Technically, enumeration types are defined as follows:
An enumeration type with a single enumerated value (such as
enum("red")
) is an atomic type
derived from xs:string
by restriction using an enumeration facet
that permits only the value "red"
. This is referred to
as a singleton enumeration type. It is equivalent to the XSD-defined type:
<xs:simpleType> <xs:restriction base="xs:string"> <xs:enumeration value="red"/> </xs:restriction> </xs:simpleType>
Two singleton enumeration types are the same type if and only if they have the same (single) enumerated value, as determined using the Unicode codepoint collation.
An enumeration type with multiple
enumerated values is a union of singleton enumeration types,
so enum("red", "green", "blue")
is equivalent to union(enum("red"), enum("green"), enum("blue"))
.
In consequence, an enumeration type T is a subtype of an enumeration type U if the enumerated values of T are a subset of the enumerated values of U: see 3.7.2 Subtypes of Item Types.
An enumeration type is thus a generalized atomic type.
It follows from these rules that an atomic value will only satisfy an instance of
test if it has the correct type annotation, and this can only be achieved using an explicit cast or
constructor function. So the expression "red" instance of enum("red", "green", "blue")
returns false
.
However, the coercion rules ensure that where a variable
or function declaration specifies an enumeration type as the required type, a string
(or indeed an xs:untypedAtomic
or xs:anyURI
value) equal
to one of the enumerated values will be accepted.
Some of the constructs described in this section include a TypeName. This appears as T in:
element(N, T)
attribute(N, T)
document-node(element(N, T))
In these constructs, the type name T is expanded using the in-scope namespaces
in the static context, using the default namespace for elements and types
if it is unprefixed. The resulting
QName must identify a type in the in-scope schema definitions. This can be any schema type: either a simple type,
or (except in the case of attributes) a complex type. If it is a simple type then it can be an atomic, union, or
list type. It can be a built-in type (such as xs:integer
) or a user-defined type. It must however
be the name of a type defined in a schema; it cannot be a named item type.
node()
matches any node.
text()
matches any
text node.
processing-instruction()
matches any processing-instruction
node.
processing-instruction(
N
)
matches any processing-instruction node whose PITarget is equal to fn:normalize-space(N)
. If the result of fn:normalize-space(N)
is not in the lexical space of NCName, a type error is raised [err:XPTY0004]
Example:
processing-instruction(xml-stylesheet)
matches any
processing instruction whose PITarget is
xml-stylesheet
.
For backward compatibility with
XPath 1.0, the PITarget of a
processing instruction may also be expressed as a
string literal, as in this example:
processing-instruction("xml-stylesheet")
.
If the specified PITarget is not a syntactically valid NCName, a type error is raised [err:XPTY0004].
comment()
matches any comment node.
namespace-node()
matches any
namespace node.
document-node()
matches any document
node.
document-node(
E
)
matches any document node that contains exactly one element node, optionally accompanied by one or more comment and processing instruction nodes, if
E is an ElementTest or SchemaElementTest that matches the element node (see
3.6.3.2 Element Test and 3.6.3.3 Schema Element Test).
Example:
document-node(element(book))
matches a document node
containing
exactly one element node that is matched by the ElementTest
element(book)
.
An ItemType that is an ElementTest, SchemaElementTest, AttributeTest, SchemaAttributeTest, or FunctionTest matches an item as described in the following sections.
[127] | ElementTest |
::= | "element" "(" (NameTestUnion ("," TypeName "?"?)?)? ")" |
|
[29] | NameTestUnion |
::= |
NameTest ("|" NameTest)* |
|
[68] | NameTest |
::= |
EQName | Wildcard
|
|
[69] | Wildcard |
::= | "*" |
/* ws: explicit */ |
[133] | TypeName |
::= |
EQName
|
An ElementTest is used to match an element node by its name and/or type annotation.
An unprefixed EQName
within the NameTestUnion
is interpreted according to the
default namespace for elements and types.
The name need not be present in the in-scope element declarations.
An unprefixed TypeName is interpreted according to the default namespace for elements and types. The TypeName must be present in the in-scope schema types [err:XPST0008]
Note:
Substitution groups do not affect the semantics of ElementTest.
An ElementTest ET matches an item E if the following conditions are satisfied:
E is an element node.
If ET includes a NameTestUnion, then the name of the element node E matches one or more of the NameTests in the NameTestUnion. A name N matches a NameTest NT if one of the following conditions is true:
NT is *
NT is *:local
and the local part
of N is local.
NT is
prefix:*
and the namespace URI
of N matches the namespace URI bound to prefix in the static
context.
NT is
BracedURILiteral*
and the namespace URI
of N matches the namespace URI found in the BracedURILiteral
.
NT is an EQName
equal to N.
If ET includes a TypeName, then the type annotation of the element node E is either the schema type identified by that type name, or a type derived from that type by restriction.
If E has the nilled
property, then ET
either includes no TypeName,
or includes a TypeName followed by the symbol ?
.
Here are some examples of ElementTests:
element()
and
element(*)
match any
single element node, regardless of its name or
type annotation.
element(person)
matches any element node whose name is person
,
in the default namespace for elements and types.
element(doctor|nurse)
matches any element node whose name is
doctor
or nurse
,
in the default namespace for elements and types.
element(xhtml:*)
matches any element node whose name is in the namespace
bound to the prefix xhtml
.
element(xhtml:*|svg:*|mathml|*)
matches any element node whose name is one of the
three namespaces identified, specifically the namespaces bound to the prefixes
xhtml
, svg
, and mathml
.
element(Q{"http://www.w3.org/2000/svg"}*)
matches any element node whose name is in the SVG namespace.
element(*:html)
matches any element node whose local name is "html"
,
in any namespace.
element(person, surgeon)
matches a
non-nilled element node whose name is person
and whose
type annotation is surgeon
(or is derived from surgeon
).
element(person, surgeon?)
matches a nilled or non-nilled element node whose name is person
and whose type
annotation is surgeon
(or is derived from surgeon
).
element(*, surgeon)
matches any non-nilled element node whose type annotation is
surgeon
(or is derived from surgeon
), regardless of its name.
element(*, surgeon?)
matches any nilled or non-nilled element node whose type annotation is
surgeon
(or is derived from surgeon
), regardless of its name.
Where a TypeName is included in an ElementTest T, and element node will only match the test if it has been validated against a schema that defines type T; furthermore, T must be present in the in-scope schema definitions of the static context of the ElementTest. Although it is guaranteed that type T will have compatibleDM40 definitions in the schema that was used for validation and in the in-scope schema definitions, it is not guaranteed that revalidation using the in-scope schema definitions would succeed. For example, if substitution group membership varies between the two schemas, the element node may contain children or descendants that the in-scope schema definitions would not allow.
[128] | SchemaElementTest |
::= | "schema-element" "(" ElementDeclaration ")" |
|
[129] | ElementDeclaration |
::= |
ElementName
|
|
[131] | ElementName |
::= |
EQName
|
A SchemaElementTest matches an element node against a corresponding element declaration found in the in-scope element declarations.
The ElementName of a SchemaElementTest has its prefixes expanded to a namespace URI by means of the statically known namespaces, or if unprefixed, the is interpreted according to the default namespace for elements and types. If the ElementName specified in the SchemaElementTest is not found in the in-scope element declarations, a static error is raised [err:XPST0008].
A SchemaElementTest matches a candidate element node if all of the following conditions are satisfied:
Either:
The name N of the candidate node matches the specified ElementName, or
The name N of the candidate node matches the name of an element declaration that is a member of the actual substitution group headed by the declaration of element ElementName.
Note:
The term “actual substitution group” is defined in [XML Schema 1.1]. The actual substitution group of an element declaration H includes those element declarations P that are declared to have H as their direct or indirect substitution group head, provided that P is not declared as abstract, and that P is validly substitutable for H, which means that there must be no blocking constraints that prevent substitution.
The schema element declaration named N is not abstract.
derives-from( AT, ET )
is true, where AT is the type annotation of the candidate node and ET is the schema type declared in the schema element declaration named N.
If the schema element declaration named N is not nillable, then the nilled property of the candidate node is false
.
Example: The SchemaElementTest
schema-element(customer)
matches a candidate element node
in the following two situations:
customer is a top-level element declaration in the in-scope element declarations; the name of the candidate node is customer; the element declaration of customer is not abstract; the type annotation of the candidate node is the same as or derived from the schema type declared in the customer element declaration; and either the candidate node is not nilled, or customer is declared to be nillable.
customer is a top-level element declaration in the in-scope element declarations; the name of the candidate node is client; client is an actual (non-abstract and non-blocked) member of the substitution group of customer; the type annotation of the candidate node is the same as or derived from the schema type declared for the client element; and either the candidate node is not nilled, or client is declared to be nillable.
In the case where the schema X used to validate an element node E (whose name is N) differs from the schema Y represented by the in-scope schema definitions in the static context of the SchemaElementTest, the following considerations apply:
In applying the test derives-from( AT, ET )
,
note that AT will necessarily be present in X,
but not necessarily in Y. However, ET will
necessarily be present in both; and because the two schemas
must be compatibleDM40,
ET will be the present in both schemas, will have the same
definition in both, and will be the declared type of N in both.
The test can therefore be applied from knowledge of type AT
as defined in schema X.
The test as to whether the element name N is a member of the actual substitution group is performed entirely by reference to schema Y. Although the two schemas are compatible, substitution group membership can vary.
[124] | AttributeTest |
::= | "attribute" "(" (NameTestUnion ("," TypeName)?)? ")" |
|
[29] | NameTestUnion |
::= |
NameTest ("|" NameTest)* |
|
[68] | NameTest |
::= |
EQName | Wildcard
|
|
[69] | Wildcard |
::= | "*" |
/* ws: explicit */ |
[133] | TypeName |
::= |
EQName
|
An AttributeTest is used to match an attribute node by its name and/or type annotation.
An unprefixed EQName
within the NameTestUnion
refers to a name in no namespace.
The name need not be present in the in-scope attribute declarations.
An unprefixed TypeName is interpreted according to the default namespace for elements and types. The TypeName must be present in the in-scope schema types [err:XPST0008]
An AttributeTest AT matches an item A if the following conditions are satisfied:
A is an attribute node.
If AT includes a NameTestUnion, then the name of the attribute node A matches one or more of the NameTests in the NameTestUnion. A name N matches a NameTest NT if one of the following conditions is true:
NT is *
NT is *:local
and the local part
of N matches local.
NT is
prefix:*
and the namespace URI
of N matches the namespace URI bound to prefix in the static
context.
NT is
BracedURILiteral*
and the namespace URI
of N matches the namespace URI found in the BracedURILiteral
.
NT is an EQName
equal to N.
If AT includes a TypeName, then the type annotation of the attribute node A is either the schema type identified by that type name, or a type derived from that type by restriction.
Here are some examples of AttributeTests:
attribute()
and attribute(*)
match any single attribute node,
regardless of its name or type annotation.
attribute(price)
matches any attribute node whose name is price
(in no namespace), regardless of its type annotation.
attribute(price|discount)
matches any attribute node whose name is price
or discount
(in no namespace).
attribute(xlink:*)
matches any attribute node whose name is in the namespace
bound to the prefix xlink
.
element(Q{"http://www.w3.org/2000/svg"}*)
matches any attribute node whose name is in the SVG namespace.
attribute(*:default-collation)
matches any attribute node
whose local name is default-collation
,
regardless of namespace, and regardless of type annotation.
attribute(*:price|*:discount)
matches any attribute node
whose local name is price
or discount
,
regardless of namespace, and regardless of type annotation.
attribute(price, currency)
matches an
attribute node whose name is price
(in no namespace) and whose type
annotation is
currency
(or is derived from currency
).
attribute(xlink:*, xs:string)
matches any attribute node whose name is in the namespace
bound to the prefix xlink
, and whose type annotation is xs:string
or a type derived from xs:string
.
attribute(*, currency)
matches any attribute node whose
type annotation is currency
(or is derived from currency
), regardless of its
name.
Unlike the situation with an ElementTest, few problems arise if the attribute was validated using a different schema. This is because simple types can never be derived by extension, and attributes do not have substitution groups.
[125] | SchemaAttributeTest |
::= | "schema-attribute" "(" AttributeDeclaration ")" |
|
[126] | AttributeDeclaration |
::= |
AttributeName
|
|
[130] | AttributeName |
::= |
EQName
|
A SchemaAttributeTest matches an attribute node against a corresponding attribute declaration found in the in-scope attribute declarations.
The AttributeName of a SchemaAttributeTest has its prefixes expanded to a namespace URI by means of the statically known namespaces. If unprefixed, an AttributeName is in no namespace. If the AttributeName specified in the SchemaAttributeTest is not found in the in-scope attribute declarations, a static error is raised [err:XPST0008].
A SchemaAttributeTest matches a candidate attribute node if both of the following conditions are satisfied:
The name of the candidate node matches the specified AttributeName.
derives-from(
AT, ET
)
is true
, where AT is the type annotation of the candidate node and ET is the schema type declared for attribute AttributeName in the in-scope attribute declarations.
Example: The SchemaAttributeTest
schema-attribute(color)
matches a candidate attribute node if color
is a top-level attribute declaration in the in-scope attribute declarations, the name of the candidate node is color
, and the type annotation of the candidate node is the same as or derived from the schema type declared for the color
attribute.
Unlike the situation with a SchemaElementTest, few problems arise if the attribute was validated using a different schema. This is because attributes do not have substitution groups.
The following sections describe the syntax for item types for functions, including arrays and maps.
The subtype relation among these types is described in the various subsections of 3.7.2 Subtypes of Item Types.
[134] | FunctionTest |
::= |
AnyFunctionTest
|
|
[135] | AnyFunctionTest |
::= | "function" "(" "*" ")" |
|
[136] | TypedFunctionTest |
::= | "function" "(" (SequenceType ("," SequenceType)*)? ")" "as" SequenceType
|
A FunctionTest matches selected function items, potentially checking their signatureDM40 (which includes the types of the arguments and results).
An AnyFunctionTest matches any item that is a function.
A TypedFunctionTest matches an item if it is a function item and the function’s type signature (as defined in Section 2.9.4 Function ItemsDM40) is a subtype of the TypedFunctionTest.
In addition, a TypedFunctionTest may match certain maps and arrays, as described in 3.6.4.2 Map Test and 3.6.4.5 Array Test
Here are some examples of FunctionTests:
function(*)
matches any function, including maps and arrays.
function(int, int) as int
matches any function item with the function signature function(int, int) as int
.
function(xs:anyAtomicType) as item()*
matches any map, or any function with the required signature.
function(xs:integer) as item()*
matches any array, or any function with the required signature.
[137] | MapTest |
::= |
AnyMapTest | TypedMapTest
|
|
[138] | AnyMapTest |
::= | "map" "(" "*" ")" |
|
[139] | TypedMapTest |
::= | "map" "(" ItemType "," SequenceType ")" |
The MapTest
map(*)
matches any map. The MapTest
map(K, V)
matches any map where every key
is an instance of K
and every value is an
instance of V
.
Although the grammar for TypedMapTest
allows the key to be described using the full ItemType
syntax, the item type used must be
a generalized atomic type. [TODO: error code].
Examples:
Given a map $M
whose keys are integers and whose
results are strings, such as map{0:"no", 1:"yes"}
,
consider the results of the following expressions:
$M instance of map(*)
returns true()
$M instance of map(xs:integer, xs:string)
returns true()
$M instance of map(xs:decimal, xs:anyAtomicType)
returns true()
not($M instance of map(xs:int, xs:string))
returns true()
not($M instance of map(xs:integer, xs:token))
returns true()
A map is also a function item, and therefore matches certain
function tests. Specifically, a map that matches map(K, V)
also matches a function
test of the form function(xs:anyAtomicType) as R
provided that both the following
conditions are satisfied:
Note:
To understand this rule, consider the use of a map $M
in a function
call $M($K)
, which is equivalent to the function call map:get($M, $K)
.
This function accepts any atomic value for the argument $K
, and hence satisfies
a function test that requires an argument type of xs:anyAtomicType
. If the
key $K
is present in the map, the result of the function will be a value of
type V; if not, it will be an empty sequence. The map is therefore substitutable
for the function test provided that the function test allows both a value of type V
and the empty sequence as possible results.
The key type K does not enter into this rule. That is because in the function call
$M($K)
, the sought key $K
does not have to be of the same
type as the keys actually present in the map.
The transitivity rules for item type matching mean that if an item M
matches a type T, and T is a subtype
of U, then M also matches type U. So the fact
that a map from integers to strings (map(xs:integer, xs:string)
)
matches function(xs:anyAtomicType) as xs:string?
means that it will also match other function tests such as
function(xs:integer) as xs:string?
and
function(xs:decimal) as xs:anyAtomicType?
Furthermore, the rules for
function coercion mean that any map can be supplied as a value in a
context where it does not actually match the required function type, but
can be coerced to a function that does. For example a map of type
map(xs:integer, xs:string)
can be coerced to a function of
type function(xs:integer) as xs:string
; in this situation a type
error will occur only if a call on the function actually returns an empty sequence.
Examples:
$M instance of function(*)
returns true()
$M instance of function(xs:anyAtomicType) as item()*
returns true()
$M instance of function(xs:integer) as item()*
returns true()
$M instance of function(xs:int) as item()*
returns true()
$M instance of function(xs:string) as item()*
returns true()
not($M instance of function(xs:integer) as xs:string)
returns true()
Note:
The last case might seem surprising;
however, function coercion ensures that $M
can be used successfully
anywhere that the required type is function(xs:integer) as xs:string
.
Rules defining whether one map type is a subtype of another are given in 3.7.2.7 Maps.
[140] | RecordTest |
::= |
AnyRecordTest | TypedRecordTest
|
|
[141] | AnyRecordTest |
::= | "record" "(" "*" ")" |
|
[142] | TypedRecordTest |
::= | "record" "(" FieldDeclaration ("," FieldDeclaration)* ExtensibleFlag? ")" |
|
[143] | FieldDeclaration |
::= |
FieldName "?"? ("as" SequenceType)? |
|
[144] | FieldName |
::= |
NCName | StringLiteral
|
|
[145] | ExtensibleFlag |
::= | "," "*" |
A RecordTest matches maps that meet specific criteria.
For example, the RecordTest
record(r as xs:double, i as xs:double)
matches a map if the map has exactly two entries: an entry with key "r"
whose value is a singleton xs:double
value, and an entry with key "i"
whose value is also a singleton xs:double
value.
If the list of fields ends with ",*"
then the record test is said to be
extensible. For example, the RecordTest
record(e as element(Employee), *)
matches a map if it has an entry with key "e"
whose value matches element(Employee)
,
regardless what other entries the map might contain.
For generality, the syntax record(*)
defines an extensible record type that has no explicit
field declarations. The item type denoted by record(*)
is equivalent to the item type
map(*)
: that is, it allows any map.
A record test can constrain only those entries whose keys are strings, but when the record test is marked as extensible, then other entries may be present in the map with non-string keys. Entries whose key is a string can be expressed using an (unquoted) NCName if the key conforms to NCName syntax, or using a (quoted) string literal otherwise.
Note:
Lookup expressions have been extended in 4.0 so that non-NCName keys can be used without
parentheses: employee?"middle name"
If the type declaration for a field is omitted, then item()*
is assumed: that is,
the map entry may have any type.
If the field name is followed by a question mark,
then the value must have the specified type if it is present, but it may also be absent. For example,
the RecordTest
record(first as xs:string, middle? as xs:string, last as xs:string, *)
requires the map to have string-valued entries with keys "first"
and "last"
;
it also declares that if the map has an entry with key "middle"
, the value of that
entry must be a single xs:string
. Declaring the type as
record(first as xs:string, middle? as xs:string?, last as xs:string, *)
also allows
the entry with key "middle"
to be present but empty.
Note:
Within an extensible record test, a FieldDeclaration
that is marked optional
and has no declared type does not constrain the
map in any way, so it serves no practical purpose, but it is permitted because it may have
documentary value.
Record tests describe a subset of the value space of maps. They do not define any new kinds of values, or any additional operations. They are useful in many cases to describe more accurately the type of a variable, function parameter, or function result, giving benefits both in the readability of the code, and in the ability of the processor to detect and diagnose type errors and to optimize execution.
If a variable $rec
is known to conform to a particular
record type, then when a lookup expression $rec?field
is used, (a) the processor
can report a type error if $rec
cannot contain an entry with name field
(see 4.15.3.4 Implausible Lookup Expressions),
and (b) the processor can make static type inferences about the type of value returned by
$rec?field
.
Note:
(TODO: change function signatures as suggested here!) A number of functions in the standard function library use maps as function arguments;
this is a useful technique where the information to be supplied across the interface is highly
variable. However, the type signature for such functions typically declares the argument type
as map(*)
, which gives very little information (and places very few constraints)
on the values that are actually passed across. Using record tests offers the possibility of
improving this: for example, the options argument of fn:parse-json
, previously
given as map(*)
, can now be expressed as record(liberal? as xs:boolean,
duplicates? as xs:string, escape? as xs:boolean, fallback as function(xs:string) as xs:string, *)
.
In principle the xs:string
type used to describe the duplicates
option could also be replaced by a schema-defined subtype
of xs:string
that enumerates the permitted values ("reject"
,
"use-first"
, "use-last"
).
The use of a record test in the signature of such a function causes the
coercion rules
to be invoked. So, for example, if the function expects an entry in the map to be an xs:double
value, it becomes possible to supply a map in which the corresponding entry has type xs:integer
.
Greater precision in defining the types of such arguments also enables better type checking, better diagnostics, better optimization, better documentation, and better syntax-directed editing tools.
Note:
One of the motivations for introducing record tests is to enable better pattern matching
in XSLT when processing JSON input. With XML input, patterns are often based
around XML element names. JSON has no direct equivalent of XML’s element names; matching a JSON object
such as {longitude: 130.2, latitude: 53.4}
relies instead on recognizing the property
names appearing in the object. XSLT 4.0, by integrating record tests into pattern matching syntax,
allows such an object to be matched with a pattern of the form
match="record(longitude, latitude)"
Rules defining whether one record test is a subtype of another are given in 3.7.2.9 Record Tests.
A named item type N is said to be recursive if its definition includes a direct or indirect reference to N.
For example, the following XQuery declaration defines a linked list:
declare item type my:list as record(value as item()*, next? as my:list);
The equivalent in XSLT is:
<xsl:item-type name="my:list" as="record(value as item()*, next? as my:list)"/>
A recursive named item type N is permitted only if it satisfies all the following conditions:
The item type must be a record test.
Within the record test, every item type reference R that refers directly or indirectly to N must satisfy one or more of the following conditions, where F is the field declaration of N in which R appears:
F is an optional field declaration: for example
next? as N
.
The SequenceType of F has an occurrence indicator
of ?
or *
: for example next as N?
or next as N*
.
The item type of F is a function test, map test, or array test:
for example next as (function() as N)
or next as array(N)
.
Note:
These conditions are designed to ensure that finite instances of N can be constructed.
Instances of recursive record types can be constructed and interrogated in the normal way. For example a list of length 3 can be constructed as:
map{"value":1, "next":map{"value":2, "next":map{"value":3}}}
and the third value in the map can be retrieved as $list?next?next?value
.
In practice, recursive data structures are usually manipulated using recursive functions.
Note:
For an example of a practical use of recursive record types, see the
specification of the function fn:random-number-generator
.
Recursive type definitions need to be handled specially by the subtyping rules; a naïve approach of simply replacing each reference to a named item type with its definition would make the assessment of the subtype relationship non-terminating. For details see 3.7.2 Subtypes of Item Types.
A record used to represent a node in a binary tree might be represented (using XQuery syntax) as:
declare item-type t:binary-tree as record(left? as t:binary-tree, value, right? as t:binary-tree)
A function to walk this tree and enumerate all the values in depth-first order might be written (again using XQuery syntax) as:
declare function t:values($tree as t:binary-tree?) as item()* { $tree ! (t:values(?left), ?value, t:values(?right)) }
A record used to represent a node in a tree where each node has an arbitrary number of children might be represented (using XQuery syntax) as:
declare item-type t:tree as record(value, children as t:tree*);
A function to walk this tree and enumerate all the values in order might be written as:
declare function t:flatten($tree as t:tree) as item()* { $tree?value, $tree?children ! t:flatten(.)) }
The usual textbook example of mutually-recursive types is that of a forest consisting of a list of trees, where each tree is a record comprising a value and a forest. As the previous example shows, this structure can be defined straightforwardly in XPath 4.0 without recourse to mutual recursion.
A more realistic example where mutual recursion is needed is for the schema component model used in [XML Schema 1.0] or [XML Schema 1.1]. Simplifying greatly, the data representing an element declaration in XSD may contain references to a complex type, which in turn will typically contain references to further element declarations. The structure therefore involves mutual recursion.
[148] | ArrayTest |
::= |
AnyArrayTest | TypedArrayTest
|
|
[149] | AnyArrayTest |
::= | "array" "(" "*" ")" |
|
[150] | TypedArrayTest |
::= | "array" "(" SequenceType ")" |
The AnyArrayTest
array(*)
matches any
array. The TypedArrayTest
array(X)
matches any array
in which every array member matches the SequenceType
X
.
Examples:
[ 1, 2 ] instance array(*)
returns true()
[] instance of array(xs:string)
returns true()
[ "foo" ] instance of array(xs:string)
returns true()
[ "foo" ] instance of array(xs:integer)
returns false()
[(1,2),(3,4)] instance of array(xs:integer)
returns false()
[(1,2),(3,4)] instance of array(xs:integer+)
returns true()
An array also matches certain other item types, including:
item()
function(*)
function(xs:integer) as item()*
An array that matches array(T)
also matches the function test function(xs:integer) as T
.
Note:
To understand this rule, consider the use of an array $A
in a function
call $A($I)
, which is equivalent to the function call array:get($A, $I)
.
This function accepts any integer for the argument $I
, and the result
will either be an instance of T, or an error.
The transitivity rules for item type matching mean that if an item A
matches a type T, and T is a subtype
of U, then A also matches type U. So the fact
that an array of strings (array(xs:string)
)
matches function(xs:integer) as xs:string
means that it will also match other function tests such as
function(xs:long) as item()*
.
Furthermore, the rules for
function coercion mean that any array can be supplied as a value in a
context where it does not actually match the required function type, but
can be coerced to a function that does. For example an array of type
array(node())
can be coerced to a function of
type function(xs:integer) as element()
; in this situation a type
error will occur only if a call on the function actually returns a node
that is not an element node.
Rules defining whether one array type is a subtype of another are given in 3.7.2.8 Arrays.
The type xs:error
has an empty value space; it never appears as a dynamic type or as the content type of a dynamic element or attribute type.
It was defined in XML Schema in the interests of making the type system complete and closed, and it is also available in XPath 4.0
for similar reasons.
Note:
Even though it cannot occur in an instance, xs:error
is a valid type name in a sequence type. The
practical uses of xs:error
as a sequence type are limited, but they do exist. For instance, an error-handling function that always raises a dynamic error
never returns a value, so xs:error
is a good choice for the return type of the function.
The semantics of xs:error
are well defined as a consequence of the fact that xs:error
is defined as a union type with
no member types. For example:
$x instance of xs:error
always returns false
, regardless of the value of $x
.
$x cast as xs:error
fails dynamically with error [err:FORG0001]FO40, regardless of the value of $x
.
$x cast as xs:error?
raises a dynamic error
[err:FORG0001]FO40 if exists($x)
returns true
, and evaluates to the empty sequence if empty($x)
returns true
.
xs:error($x)
has the same semantics as $x cast as xs:error?
(see the previous bullet point)
$x castable as xs:error
evaluates to false
, regardless of the value of $x
.
$x treat as xs:error
raises a dynamic error
[err:XPDY0050] if evaluated, regardless of the value of $x
. It never fails statically.
All of the above examples assume that $x
is actually evaluated. The rules specified in 2.4.4 Errors and
Optimization permit an implementation to avoid evaluating $x
if the result of the query does not depend upon the value of $x
and thus to avoid raising an error.
[Definition: Given two sequence types or item types, the rules in this section determine if one is a subtype of the other. If a type A is a subtype of type B, it follows that every value matched by A is also matched by B.]
Note:
The relationship subtype(A, A)
is always true:
every type is a subtype of itself.
Note:
The converse is not necessarily true: we cannot infer that
if every value matched by A is also matched by B, then
A is a subtype of type B. For example, A might be
defined as the set of strings matching the regular expression [A-Z]*
, while B
is the set of strings matching the regular expression [A-Za-z]*
; no subtype
relationship holds between these types.
The rules for deciding whether one sequence type is a subtype of another are given in 3.7.1 Subtypes of Sequence Types. The rules for deciding whether one item type is a subtype of another are given in 3.7.2 Subtypes of Item Types.
Note:
The subtype relationship is not acyclic. There are cases where subtype(A, B)
and
subtype(B, A)
are both true. This implies that A and B
have the same value space, but they can still be different types. For example this applies when A
is a union type with member types xs:string
and xs:integer
, while
B is a union type with member types xs:integer
and xs:string
.
These are different types ("23" cast as A
produces a string, while "23" cast as B
produces an integer, because casting is attempted to each member type in order) but both types have the same value space.
We use the notation A ⊑ B
, or subtype(A, B)
to indicate that
a sequence type
A
is a subtype of a sequence type B
.
This section defines the rules for deciding whether any two sequence types have this relationship.
To define the rules, we divide sequence types into six categories:
The category empty
includes the sequence types empty-sequence()
,
xs:error*
and xs:error?
. All these sequence types
match the empty sequence as their only instance.
The category void
includes the sequence types xs:error
and xs:error+
,
which have no instances (not even the empty sequence).
The categories X?
, X*
, X
and X+
includes all sequence types
having an item type X
other than xs:error
, together with an occurrence indicator of
?
(zero or more), *
(one or more), absent (exactly one), or +
(one or more)
respectively. We use the notation Xi to indicate the item type of such a sequence type.
The judgement A ⊑ B
is then determined by the categories of the two sequence types, as defined
in the table below. In many cases this depends on the relationship between the item types of A
and B
. This is denoted using the notation
Ai ⊆ Bi
,
as defined in 3.7.2 Subtypes of Item Types.
Sequence type B | |||||||
---|---|---|---|---|---|---|---|
empty
|
Bi?
|
Bi*
|
Bi
|
Bi+
|
void
|
||
Sequence type A |
empty
|
true | true | true | false | false | false |
Ai?
|
false |
Ai ⊆ Bi
|
Ai ⊆ Bi
|
false | false | false | |
Ai*
|
false | false |
Ai ⊆ Bi
|
false | false | false | |
Ai
|
false |
Ai ⊆ Bi
|
Ai ⊆ Bi
|
Ai ⊆ Bi
|
Ai ⊆ Bi
|
false | |
Ai+
|
false | false |
Ai ⊆ Bi
|
false |
Ai ⊆ Bi
|
false | |
void
|
true | true | true | true | true | true |
We use the notation A ⊆ B
, or itemtype-subtype(A, B)
to indicate that
an item type
A
is a subtype of an item type B
.
This section defines the rules for deciding whether any two item types have this relationship.
The rules in this section apply to
item types, not to
item type designators.
For example, if the name STR
has been defined in the
static context as a named item type referring to the type xs:string
,
then anything said here about the type xs:string
applies equally
whether it is designated as xs:string
or as STR
,
or indeed as the parenthesized forms (xs:string)
or
(STR)
.
References to named item types are handled as described in 3.7.2.10 Named Item Types.
The relationship A ⊆ B
is true
if and only if at least one of the conditions listed in the following subsections applies:
Given item types
A and B,
A ⊆ B
is true if any of the following apply:
A is xs:error
.
B is item()
.
A and B are the same item type.
There is an item type
X such that
A ⊆ X
and
X ⊆ B
. (This is referred to below as the transitivity rule).
Note:
The first rule is technically redundant: it is implied by the second rule in 3.7.2.2 Atomic and Union Types.
The type xs:error
is defined as a union type with no member types; therefore it is automatically true that
every member type T satisfies
T ⊆ B
.
Given item types A and B,
A ⊆ B
is true if any of the following apply:
A and B are generalized atomic types,
and derives-from(A, B)
returns true
.
The derives-from
relationship is defined in 3.5 Sequence Type Matching.
xs:integer ⊆ xs:decimal
because xs:integer
is derived
by restriction from xs:decimal
.
xs:decimal ⊆ xs:numeric
because xs:numeric
is a pure union
type that includes xs:decimal
as a member type.
enum("red") ⊆ xs:string
because the singleton
enumeration type enum("red")
is defined to be an atomic
type derived from xs:string
.
enum("red") ⊆ enum("red", "green")
because the
enumeration type enum("red", "green")
is defined to be a union type
that has the atomic type enum("red")
as a member type.
A is a pure union type,
and every type T in the transitive membership of A
satisfies
T ⊆ B
.
union(xs:short, xs:long) ⊆ xs:integer
because xs:short ⊆ xs:integer
and xs:long ⊆ xs:integer
.
union(P, Q) ⊆ union(P, Q, R)
because P ⊆ union(P, Q, R)
and Q ⊆ union(P, Q, R)
.
enum("red", "green") ⊆ xs:string
because the
enumeration type enum("red") ⊆ xs:string
and enum("green") ⊆ xs:string
.
enum("red", "green") ⊆ enum("red", "green", "blue")
because
enum("red") ⊆ enum("red", "green", "blue")
and
enum("green") ⊆ enum("red", "green", "blue")
.
enum("red", "green", "blue") ⊆ union(enum("red", "green"), enum("blue"))
because
each of the types enum("red")
, enum("green")
, and enum("blue")
is a subtype of one of the two members of the union type.
Note:
This rule applies both when A
is a schema-defined union type
and when it is a LocalUnionType; in addition it
applies when A
is an enumeration type with multiple enumerated values,
which is defined to be equivalent to a union type.
Given item types A and B,
A ⊆ B
is true if any of the following apply:
A is a KindTest and B is node()
.
A is processing-instruction(N)
for any name N,
and B is processing-instruction()
.
A is document-node(E)
for any ElementTest
E,
and B is document-node()
.
All the following are true:
A is document-node(Ae)
B is document-node(Be)
Ae ⊆ Be
[Definition: In these rules, if MU and NU are NameTestUnions, then MU wildcard-matches NU is true if every name that matches MU also matches NU.]
More specifically, this is the case if for every NameTest M in MU there is a NameTest N in NU where at least one of the following applies:
M and N are the same NameTest
.
M is an EQName
and N is a
Wildcard that matches M.
N is the Wildcard
*
.
Given item types A and B,
A ⊆ B
is true if any of the following apply.
A is an ElementTest and
B is either element()
or element(*)
All the following are true:
A is either element(An)
or element(An, T)
or element(An, T?)
for any type T
B is either element(Bn)
or element(Bn, xs:anyType?)
An wildcard-matches Bn
element(title) ⊆ element(*)
element(title, xs:string) ⊆ element(*)
element(title|heading, xs:string) ⊆ element(*)
element(title, xs:string) ⊆ element(title|heading)
element(title, xs:string?) ⊆ element(*)
element(title|heading, xs:string) ⊆ element(*)
element(title) ⊆ element(title, xs:anyType?)
element(title, xs:integer) ⊆ element(title|heading, xs:anyType?)
element(title, xs:string?) ⊆ element(title, xs:anyType?)
element(my:title|your:title) ⊆ element(*:title)
element(my:title|my:heading) ⊆ element(my:*)
All the following are true:
A is element(An, At)
B is element(Bn, Bt)
An wildcard-matches Bn
derives-from(At, Bt)
.
All the following are true:
A is either element(An, At)
or
element(An, At?)
B is element(Bn, Bt?)
An wildcard-matches Bn
derives-from(At, Bt)
.
All the following are true:
A is schema-element(An)
B is schema-element(Bn)
Every element declaration that is an actual member of the substitution group of An is also an actual member of the substitution group of Bn.
Note:
The fact that P is a member of the substitution group of Q does not mean that every element declaration in the substitution group of P is also in the substitution group of Q. For example, Q might block substitution of elements whose type is derived by extension, while P does not.
Given item types A and B,
A ⊆ B
is true if any of the following apply:
A is an AttributeTest and
B is either attribute()
or attribute(*)
All the following are true:
A is either attribute(An)
or
attribute(An, T)
for any type T.
B is either attribute(Bn)
or
attribute(Bn, xs:anyAtomicType)
An wildcard-matches Bn
All the following are true:
A is attribute(An, At)
B is attribute(Bn, Bt)
An wildcard-matches Bn
derives-from(At, Bt)
.
attribute(*, xs:ID) ⊆ attribute(*, xs:string)
attribute(my:*, xs:ID) ⊆ attribute(*, xs:string)
attribute(code, xs:ID) ⊆ attribute(code|status, xs:string)
attribute(code, xs:ID) ⊆ attribute(*, xs:string)
attribute(code, xs:ID) ⊆ attribute(*:code, xs:ID)
attribute(my:code|my:status, xs:ID) ⊆ attribute(my:*, xs:string)
All the following are true:
A is schema-attribute(An)
B is schema-attribute(Bn)
the expanded QName of An equals the expanded QName of Bn
Given item types A and B,
A ⊆ B
is true if any of the following apply:
All the following are true:
A is a FunctionTest
B is
function(*)
All the following are true:
A is
function(a1, a2, ... aM) as RA
B is
function(b1, b2, ... bN) as RB
N (the arity of B) equals M (the arity of A)
RA ⊑ RB
For all values of p between 1 and N,
bp ⊑ ap
function(xs:integer) as xs:string ⊆ function(xs:long) as xs:string
function(xs:integer) as xs:ID ⊆ function(xs:integer) as xs:string
function(xs:integer) as xs:ID ⊆ function(xs:long) as xs:string
Note:
Function return types are covariant because this rule requires
RA ⊑ RB
for return types.
Function parameter types are contravariant because this rule requires
bp ⊑ ap
for parameter types.
Given item types A and B,
A ⊆ B
is true if any of the following apply:
Both of the following are true:
A is map(K, V)
,
for any K and V
B is map(*)
All the following are true:
A is map(Ka, Va)
B is map(Kb, Vb)
Ka ⊆ Kb
Va ⊑ Vb
Both the following are true:
A is map(*)
(or, because of the transitivity rules, any other map type)
B is function(*)
Both the following are true:
A is map(*)
(or, because of the transitivity rules, any other map type)
B is
function(xs:anyAtomicType) as item()*
All the following are true:
A is map(K, V)
B is function(xs:anyAtomicType) as R
V ⊆ R
empty-sequence()
⊆ R
map(xs:int, node()) ⊆ function(xs:anyAtomicType) as node()?
map(xs:int, node()+) ⊆ function(xs:anyAtomicType) as node()*
The function accepts type xs:anyAtomicType
rather than xs:int
,
because $M("xyz")
is a valid call on a map (treated as a function) even
when all the keys in the map are integers.
The return type of the function is extended from node()
or node()+
to allow an empty sequence
because $M("xyz")
can return an empty sequence even if none of the entries
in the map contains an empty sequence.
Given item types A and B,
A ⊆ B
is true if any of the following apply:
Both the following are true:
A is array(X)
B is array(*)
All the following are true:
A is array(X)
B is array(Y)
X ⊑ Y
Both the following are true:
A is array(*)
(or, because of the transitivity rules, any other array type)
B is function(*)
Both the following are true:
A is array(*)
(or, because of the transitivity rules, any other array type)
B is function(xs:integer) as item()*
Both the following are true:
A is array(X)
B is function(xs:integer) as X
Given item types A and B, A
⊆
B is true if any of the following apply:
A is map(*)
and B is record(*)
.
All of the following are true:
A is a record test.
B is map(*)
or record(*)
.
All of the following are true:
A is a non-extensible record test
B is map(K, V)
K is either xs:string
or xs:anyAtomicType
For every field F in A,
where T is the declared type of F (or its default, item()*
),
T ⊑ V
.
All of the following are true:
A is a non-extensible record test.
B is a non-extensible record test.
Every field in A is also declared in B.
Every mandatory field in B is also declared in A.
For every field that is declared in both A and B,
where the declared type in A is T
and the declared type in B is U,
T ⊑ U
.
All of the following are true:
A is an extensible record test
B is an extensible record test
Every mandatory field in B is also declared in A.
For every field that is declared in both A and B,
where the declared type in A is T
and the declared type in B is U,
T ⊑ U
.
For every field that is declared in B
but not in A, the declared type in B is item()*
.
All of the following are true:
A is a non-extensible record test.
B is an extensible record test.
Every mandatory field in B is also declared in A.
Every field that is declared in B with a type other than item()*
is also declared in A.
For every field that is declared in both A and B,
where the declared type in A is T
and the declared type in B is U,
T ⊑ U
.
This section describes how references to named item types are handled when evaluating the subtype relationship.
Named item types can be classified as recursive or non-recursive. A recursive type is one that references itself, directly or indirectly. Only record tests are allowed to be recursive.
Where an item type contains a reference to a named item type that is non-recursive, the reference
is expanded, recursively, as the first step in evaluating the subtype relationship. For example
this means that if U is a named item type with the expansion
union(xs:integer, xs:double)
,
then xs:integer ⊆ U
is true, because
xs:integer ⊆ union(xs:integer, xs:double)
is true.
Recursive types are considered to be, in the terminology of the computer science
literature, iso-recursive (rather than equi-recursive).
This means that a recursive type name is not
treated as being equivalent to its expansion (at any depth).
For example, if the named item type T
has the expansion record(A as item()*, B as T?)
, then the type
array(T)
is not considered to be equivalent to
array(record(A as item()*, B as T?))
, despite the fact
that the two types have exactly the same instances.
The rules are therefore defined as follows:
If B is a reference to a recursive named item type, then A ⊆ B is true if and only if A and B are references to the same named item type.
If A is a reference to a recursive named item type, then A ⊆ B is true if either:
A and B are references to the same named item type.
record(*) ⊆ B
.
Note:
This is because only record tests are allowed to be recursive.
Note:
The decision to make recursive types iso-recursive rather than equi-recursive was made largely because it saves a great deal of implementation complexity without any serious adverse effects for users. In practice, problems can be avoided by using named item type references consistently (for example, avoiding having two named item types with different names but identical definitions).
[Definition: The coercion rules are rules used to convert a supplied value to a required type, for example when converting an argument of a function call to the declared type of the function parameter. ] The required type is expressed as a sequence type. The effect of the coercion rules may be to accept the value as supplied, to convert it to a value that matches the required type, or to reject it with a type error.
This section defines how the coercion rules operate; the situations in which the rules apply are defined elsewhere, by reference to this section.
Note:
In previous versions of this specification, the coercion rules were referred to as the function conversion rules. The terminology has changed because the rules are not exclusively associated with functions or function calling.
If the required type is empty-sequence()
,
no coercion takes place (the supplied
value must be an empty sequence, or a type error occurs). In all other cases, the required
sequence type
T comprises a required item type
R
and an optional occurrence indicator.
The coercion rules are applied to a supplied value V and the required type T as follows:
If XPath
1.0 compatibility mode is true
and V
is not an instance of the required type T, then the following conversions are applied
sequentially to V:
If the occurrence indicator of T is either absent or ?
(examples: xs:string
, xs:string?
,
xs:untypedAtomic
, xs:untypedAtomic?
,
node()
, node()?
, item()
,
item()?
), then V is effectively replaced by V[1]
.
If T is xs:string
or xs:string?
,
then V is effectively replaced by
fn:string(V)
.
Note:
This rule does not apply where T is derived from xs:string
or xs:string?
, because derived types did not arise in XPath 1.0.
If T is xs:double
or xs:double?
, then
V
is effectively replaced by fn:number(V)
.
Note:
This rule does not apply where T is derived from xs:double
or xs:double?
, because derived types did not arise in XPath 1.0.
Note:
The special rules for XPath 1.0 compatibility
mode are used for converting the arguments of a static function call, and
in certain XSLT constructs. They are not invoked in other contexts such as dynamic function calls,
for converting the result of an inline function to its required type,
for partial function application, or for implicit function calls such as
occur when evaluating functions such as fn:for-each
and fn:filter
.
If R is a generalized atomic type, then the following conversions are applied, in order:
Note:
Enumeration types are generalized atomic types, so these rules apply.
Atomization is applied to V, resulting in a sequence of atomic values V1.
Each item A in V1 is processed using the first rule in the following list that is applicable.
If A is an instance of R then it is used unchanged.
If A is an instance of type xs:untypedAtomic
then:
If R is an
EnumerationType then
A is cast to xs:string
.
If R is namespace-sensitive then:
If the result of casting A to xs:string
is an instance of R, then A is cast to
xs:string
.
Note:
This rule is new in 4.0. It allows
an xs:untypedAtomic
value to be supplied where the
required type is union(xs:string, xs:QName)
, which
was previously an error.
Otherwise, a type error [err:XPTY0117] is raised.
Otherwise, A is cast to type R.
If there is an entry (from, to) in the following table such that A is an instance of from, and to is R, then A is cast to type R.
from | to |
---|---|
xs:decimal
|
xs:double
|
xs:double
|
xs:decimal
|
xs:decimal
|
xs:float
|
xs:float
|
xs:decimal
|
xs:float
|
xs:double
|
xs:double
|
xs:float
|
xs:string
|
xs:anyURI
|
xs:anyURI
|
xs:string
|
xs:hexBinary
|
xs:base64Binary
|
xs:base64Binary
|
xs:hexBinary
|
Note:
The item type in the to column must match R exactly; however, A may belong to a subtype of the type in the from column.
For example, an xs:NCName
will be cast to type xs:anyURI
,
but an xs:anyURI
will not be cast to type xs:NCName
.
Similarly, an xs:integer
will be cast to type xs:double
,
but an xs:double
will not be cast to type xs:integer
.
If R is derived from some primitive atomic type P, then A is relabeled as an instance of R if it satisfies all the following conditions:
A is an instance of P.
A is not an instance of R.
The datumDM40 of A is within the value space of R.
Relabeling an atomic value changes the type annotation but not the
datumDM40. For example, the
xs:integer
value 3 can be relabeled as an instance of xs:unsignedByte
, because
the datum is within the value space of xs:unsignedByte
.
Note:
Relabeling is not the same as casting. For example, the xs:decimal
value 10.1
can be cast to xs:integer
, but it cannot be relabeled as xs:integer
,
because its datum not within the value space of xs:integer
.
Note:
The effect of this rule is that if, for example, a function parameter is declared
with an expected type of xs:positiveInteger
, then a call that supplies the literal
value 3 will succeed, whereas a call that supplies -3 will fail.
Note:
This differs from previous versions of this specification, where both these calls would fail.
This change allows the arguments of existing functions to be defined with a
more precise type. For example, the $position
argument of array:get
can be defined as xs:positiveInteger
rather than xs:integer
. To enable
this to be done without breaking backwards compatibility in respect of error behavior,
system functions in many cases define custom error codes to be raised where
relabeling of argument values fails.
Note:
Numeric promotion and xs:anyURI
promotion occur only when T
is a primitive type (xs:double
, xs:float
, or xs:string
).
Relabeling occurs only when T is a derived type. Promotion and relabeling are therefore
never combined.
If R is a pure union type, then A is relabeled as an instance of some member type M in the transitive membership of R if M satisfies all the conditions for relabeling defined in the previous rule, and if it is the first member type in the transitive membership of R to satisfy those conditions.
Note:
For example, if T
is the type union(xs:negativeInteger, xs:positiveInteger)*
and the supplied value is the
sequence (20, -20)
, then the first item 20
is relabeled as type
xs:positiveInteger
and the second item -20
is relabeled as type
xs:negativeInteger
.
Note:
This rule also ensures that if the required type is enum("red", "green", "blue")
and the supplied value is "green"
, then the supplied value will be accepted, and
will be relabeled as an instance of xs:string
.
If R is a RecordTest, then any item in V that is a map is converted to a new map as follows:
The keys in the supplied map are unchanged.
In any map entry whose key is an xs:string
equal to the
name of one of the field declarations in R, then the corresponding
value is converted to the required type defined by that field declaration,
by applying the coercion rules recursively
(but with XPath 1.0 compatibility mode treated as false).
Note:
For example, if the required type is
record(longitude as xs:double, latitude as xs:double)
and the supplied value is map{"longitude": 0, "latitude":53.2}
,
then the map is converted to map{"longitude": 0.0e0, "latitude": 53.2e0}
.
If R is a TypedFunctionTest, function coercion is applied to each function in V.
Note:
Maps and arrays are functions, so function coercion applies to them as well.
If, after the above conversions, the resulting value does not match the expected type T according to the rules for SequenceType Matching, a type error is raised [err:XPTY0004].
Note:
Under the general rules for type errors
(see 2.4.1 Kinds of Errors), a processor
may report a type error during static
analysis if it will necessarily occur when the expression is evaluated.
For example, the function call fn:abs("beer")
will necessarily fail when evaluated, because the function requires
a numeric value as its argument; this may be detected and reported
as a static error.
An expression is deemed to be implausible [err:XPTY0006] if the static type of the expression, after applying all necessary coercions, is substantively disjoint with the required type T. Two sequence types are considered to be substantively disjoint if (a) neither is a subtype of the other (see 3.7.1 Subtypes of Sequence Types) and (b) the only values that are instances of both types are one or more of the following:
The empty sequence, ()
.
The empty map, map{}
.
The empty array, []
.
Note:
Examples of pairs of sequence types that are substantively disjoint include:
xs:integer*
and xs:string*
map(xs:integer, node())
and map(xs:string, node())
array(xs:integer)
and array(xs:string)
For example, supplying a value whose static type is xs:integer*
when the required type is xs:string*
is implausible,
because it can succeed only in the special case where the actual value supplied
is an empty sequence.
Note:
The case where the supplied type and the required type are completely
disjoint (for example map(*)
and array(*)
) is covered
by the general rules for type errors: that case can always be reported as a static
error.
Examples of implausible coercions include the following:
round(timezone-from-time($now))
. The result of fn:timezone-from-time
is of type xs:dayTimeDuration?
, which is substantively disjoint with the required type
of fn:round
, namely xs:numeric?
.
function($x as xs:integer) as array(xs:string) { array{1 to $x} }
. The type
of the function body is array(xs:integer)
, which is substantively disjoint with the
required type array(xs:string)
: the function can succeed only in the exceptional case
where the function body delivers an empty array.
Function coercion is a transformation applied to function items during application of the coercion rules. [Definition: Function coercion wraps a function item in a new function whose signature is the same as the expected type. This effectively delays the checking of the argument and return types until the function is called.]
Given a function F, and an expected function type, function coercion proceeds as follows:
If F has higher arity than the expected type, a type error is raised [err:XPTY0004]
If F has lower arity than the expected type, then F is wrapped in a new function that declares and ignores the additional argument; the following steps are then applied to this new function.
For example, if the expected type is function(node(), xs:boolean) as xs:string
,
and the supplied function is fn:name#1
, then the supplied function is effectively
replaced by function($n as node(), $b as xs:boolean) as xs:string {fn:name($n)}
Note:
This mechanism makes it easier to design versatile and extensible higher-order functions.
For example, in previous versions of this specification, the second argument of
the fn:filter
function expected an argument of type
function(item()) as xs:boolean
. This has now been extended to
function(item(), xs:integer) as xs:boolean
, but existing code continues
to work, because callback functions that are not interested in the value of the second
argument simply ignore it.
Function coercion then returns a new function item with the following properties (as defined in Section 2.9.4 Function ItemsDM40):
name: The name of F (if not absent).
identity: A new function identity distinct from the identity of any other function item.
Note:
See also 4.6.2.7 Function Identity.
parameter names: The parameter names of F.
signature:
Annotations
is set to the annotations of F. TypedFunctionTest
is set to the expected type.
implementation:
In effect,
a FunctionBody
that calls F,
passing it the parameters of this new function,
in order.
nonlocal variable bindings: An empty mapping.
If the result of invoking the new function would necessarily result in a type error, that error may be raised during function coercion. It is implementation dependent whether this happens or not.
These rules have the following consequences:
SequenceType matching of the function’s arguments and result are delayed until that function is called.
The coercion rules rules applied to the function’s arguments and result are defined by the SequenceType it has most recently been coerced to. Additional coercion rules could apply when the wrapped function is called.
If an implementation has static type information about a function, that can be used to type check the function’s argument and return types during static analysis.
Note:
Although the semantics of function coercion are specified in terms of wrapping the functions, static typing will often be able to reduce the number of places where this is actually necessary.
Since maps and arrays are also functions in XPath 4.0, function coercion applies to them as well. For instance, consider the following expression:
let $m := map { "Monday" : true(), "Wednesday" : true(), "Friday" : true(), "Saturday" : false(), "Sunday" : false() }, $days := ("Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday") return filter($days,$m)
The map $m
has a function signature of function(xs:anyAtomicType) as item()*
. When the fn:filter()
function is called, the following occurs to the map:
The map $m
is treated as function($f)
, equivalent to map:get($m,?)
.
The coercion rules result in applying
function coercion
to $f
, wrapping $f
in a new function ($p
) with the
signature function(item()) as xs:boolean
.
$p
is matched against the SequenceType function(item()) as xs:boolean
, and succeeds.
When $p
is called by fn:filter()
, coercion
and SequenceType matching rules are applied to the argument, resulting in an item()
value
($a
) or a type error.
$f
is called with $a
, which returns an xs:boolean
or the empty sequence.
$p
applies coercion rule and SequenceType matching to the result sequence from $f
. When the result is an xs:boolean
the SequenceType matching succeeds. When it is an empty sequence (such as when $m
does not contain a key for "Tuesday"
), SequenceType matching results in a type error [err:XPTY0004], since the expected type is xs:boolean
and the actual type is an empty sequence.
Consider the following expression:
let $m := map { "Monday" : true(), "Tuesday" : false(), "Wednesday" : true(), "Thursday" : false(), "Friday" : true(), "Saturday" : false(), "Sunday" : false() } let $days := ("Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday") return filter($days,$m)
The result of the expression is the sequence ("Monday", "Wednesday", "Friday")
Note:
Function coercion applies even if the supplied function matches the required type.
For example, consider this case:
declare function local:filter($s as item()*, $p as function(xs:string) as xs:boolean) as item()* { $s[$p(.)] }; let $f := function($a) { $a mod 2 = 0 } return local:filter(1 to 10, $f)
Here the supplied function $f
is an instance of the required type,
because its signature defaults the argument type to item()*
, which
is a supertype of xs:string
. The expression $s[$p(.)]
could in principle succeed. However, function coercion ensures that the supplied function
is wrapped in a function that requires the argument to be of type xs:string
,
so the call fails with a type error when the wrapping function is invoked supplying an
xs:integer
as the argument.
This section discusses each of the basic kinds of expression. Each kind of expression has a name such as PathExpr
, which is introduced on the left side of the grammar production that defines the expression. Since XPath 4.0 is a composable language, each kind of expression is defined in terms of other expressions whose operators have a higher precedence. In this way, the precedence of operators is represented explicitly in the grammar.
The order in which expressions are discussed in this document does not reflect the order of operator precedence. In general, this document introduces the simplest kinds of expressions first, followed by more complex expressions. For the complete grammar, see Appendix [A XPath 4.0 Grammar].
The highest-level symbol in the XPath grammar is XPath.
[1] | XPath |
::= |
Expr
|
|
[10] | Expr |
::= |
ExprSingle ("," ExprSingle)* |
|
[11] | ExprSingle |
::= |
WithExpr
|
The XPath 4.0 operator that has lowest precedence is the comma operator, which is used to combine two operands to form a sequence. As shown in the grammar, a general expression (Expr) can consist of multiple ExprSingle operands, separated by commas. The name ExprSingle denotes an expression that does not contain a top-level comma operator (despite its name, an ExprSingle may evaluate to a sequence containing more than one item.)
The symbol ExprSingle is used in various places in the grammar where an expression is not allowed to contain a top-level comma. For example, each of the arguments of a function call must be an ExprSingle, because commas are used to separate the arguments of a function call.
After the comma, the expressions that have next lowest precedence are ForExpr, LetExpr, QuantifiedExpr, IfExpr, and OrExpr. Each of these expressions is described in a separate section of this document.
[12] | WithExpr |
::= | "with" NamespaceDeclaration ("," NamespaceDeclaration)* EnclosedExpr
|
|
[13] | NamespaceDeclaration |
::= |
QName "=" StringLiteral
|
|
[9] | EnclosedExpr |
::= | "{" Expr? "}" |
The namespace context for an expression can be set using a construct of the form:
with xmlns="http://example.com/, xmlns:a="http://example.com/a" { /doc/a:element/b }
The static context for the enclosed expression will be the same as the static context for the WithExpr itself, except for modifications defined below.
The QName
used in a NamespaceDeclaration
must be either xmlns
or xmlns:prefix
where prefix
is some
NCName
.
If more than one NamespaceDeclaration specifies
the same QName
, all but the last of the duplicates are ignored.
If the QName is "xmlns"
then:
If the StringLiteral
is a zero-length string:
The default namespace for elements and types is set to absent, meaning that unprefixed element names are treated as being in no namespace.
TODO: reconcile this with the "fixed" setting in the XQuery prolog.
Any binding for the zero-length prefix in the statically known namespaces is removed.
If the StringLiteral
is not zero-length:
The default namespace for elements and types is set to the supplied namespace URI, meaning that unprefixed element names are treated as being in that namespace.
TODO: reconcile this with the "fixed" setting in the XQuery prolog.
A binding that maps the zero-length prefix to the specified namespace URI is added to the statically known namespaces.
If the QName is in the form xmlns:prefix
then the StringLiteral
must not be zero-length; the effect is that a binding that maps the given prefix
to
the specified namespace URI is added to the statically known namespaces.
For example, the expression:
with xmlns="http://www.acme.com/" {a/b[c=3]}
is equivalent to the expression:
Q{http://www.acme.com/}a/Q{http://www.acme.com/}b[Q{http://www.acme.com/}c=3]
[166] | Comment |
::= | "(:" (CommentContents | Comment)* ":)" |
/* ws: explicit */ |
/* gn: comments */ | ||||
[177] | CommentContents |
::= | (Char+ - (Char* ('(:' | ':)') Char*)) |
Comments may be used to provide information relevant to programmers who read an expression. Comments are lexical constructs only, and do not affect expression processing.
Comments are strings, delimited by the symbols (:
and :)
. Comments may be nested.
A comment may be used anywhere ignorable whitespace is allowed (see A.3.5.1 Default Whitespace Handling).
The following is an example of a comment:
(: Houston, we have a problem :)
[Definition: Primary expressions are the basic primitives of the language. They include literals, variable references, context value references, and function calls. A primary expression may also be created by enclosing any expression in parentheses, which is sometimes helpful in controlling the precedence of operators.] Map and Array Constructors are described in 4.15 Maps and Arrays.
[86] | PrimaryExpr |
::= |
Literal
|
|
[96] | FunctionItemExpr |
::= |
NamedFunctionRef | InlineFunctionExpr
|
[87] | Literal |
::= |
NumericLiteral | StringLiteral
|
[Definition: A literal is a direct syntactic representation of an atomic value.] XPath 4.0 supports two kinds of literals: numeric literals and string literals.
[88] | NumericLiteral |
::= |
IntegerLiteral | HexIntegerLiteral | BinaryIntegerLiteral | DecimalLiteral | DoubleLiteral
|
|
[153] | IntegerLiteral |
::= |
Digits
|
/* ws: explicit */ |
[154] | HexIntegerLiteral |
::= | "0x" HexDigits
|
/* ws: explicit */ |
[155] | BinaryIntegerLiteral |
::= | "0b" BinaryDigits
|
/* ws: explicit */ |
[156] | DecimalLiteral |
::= | ("." Digits) | (Digits "." [0-9]*) |
/* ws: explicit */ |
[157] | DoubleLiteral |
::= | (("." Digits) | (Digits ("." [0-9]*)?)) [eE] [+-]? Digits
|
/* ws: explicit */ |
[171] | Digits |
::= |
DecDigit ((DecDigit | "_")* DecDigit)? |
|
[172] | DecDigit |
::= | [0-9] |
|
[173] | HexDigits |
::= |
HexDigit ((HexDigit | "_")* HexDigit)? |
|
[174] | HexDigit |
::= | [0-9a-fA-F] |
|
[175] | BinaryDigits |
::= |
BinaryDigit ((BinaryDigit | "_")* BinaryDigit)? |
|
[176] | BinaryDigit |
::= | [01] |
The value of a numeric literal is determined as follows (taking the rules in order):
Underscore characters are stripped out. Underscores may be included in a numeric
literal to aid readability, but have no effect on the value. For example, 1_000_000
is equivalent to 1000000
.
Note:
Underscores must not appear at the beginning or end of a sequence of digits, only in intermediate positions. Multiple adjacent underscores are allowed.
A HexIntegerLiteral
represents a non-negative integer
expressed in hexadecimal: for example 0xffff
represents the integer 65535, and
0xFFFF_FFFF
represents the integer 4294967295.
A BinaryIntegerLiteral
represents a non-negative integer
expressed in binary: for example 0b101
represents the integer 5, and
0b1111_1111
represents the integer 255.
The value of a numeric literal containing no .
and
no e
or E
character is an atomic value of type xs:integer
;
the value is obtained by casting from xs:string
to xs:integer
as specified in
Section 21.2 Casting from xs:string and xs:untypedAtomicFO40.
The value of a numeric literal containing .
but no e
or E
character is an atomic value of type xs:decimal
;
the value is obtained by casting from xs:string
to xs:decimal
as specified in
Section 21.2 Casting from xs:string and xs:untypedAtomicFO40.
The value of a numeric literal
containing an e
or E
character is an atomic value of type
xs:double
;
the value is obtained by casting from xs:string
to xs:double
as specified in
Section 21.2 Casting from xs:string and xs:untypedAtomicFO40.
Note:
The value of a numeric literal is always non-negative. An expression may
appear to include a negative number such as -1
, but this is technically
an arithmetic expression comprising a unary minus operator followed by a numeric literal.
Note:
The effect of the above rules is that in the case of an integer or decimal literal, a dynamic error [err:FOAR0002]FO40 will generally be raised if the literal is outside the range of values supported by the implementation (other options are available: see Section 4.2 Arithmetic operators on numeric valuesFO40 for details.)
The XML Schema specification allows implementations to impose a limit (which
must not be less than 18 digits) on the size of integer and decimal
values. The full range of values of built-in subtypes of xs:integer
,
such as xs:long
and xs:unsignedLong
, can be supported only if the
limit is 20 digits or higher. Negative numbers such as the minimum
value of xs:long
(-9223372036854775808
) are technically unary
expressions rather than literals, but implementations may prefer to
ensure that they are expressible.
Here are some examples of numeric literals:
"12.5"
denotes the string containing the characters 1
, 2
, .
, and
5
.
12
denotes the xs:integer
value twelve.
1_000_000
denotes the xs:integer
value one million.
12.5
denotes the xs:decimal
value twelve and one half.
125E2
denotes the xs:double
value twelve thousand, five hundred.
0xffff
denotes the xs:integer
value 65535.
0b1000_0001
denotes the xs:integer
value 129.
[158] | StringLiteral |
::= |
AposStringLiteral | QuotStringLiteral
|
/* ws: explicit */ |
[159] | AposStringLiteral |
::= | "'" (EscapeApos | [^'])* "'" |
/* ws: explicit */ |
[160] | QuotStringLiteral |
::= | '"' (EscapeQuot | [^"])* '"' |
/* ws: explicit */ |
[164] | EscapeQuot |
::= | '""' |
/* ws: explicit */ |
[165] | EscapeApos |
::= | "''" |
/* ws: explicit */ |
The value of a string literal is an atomic value whose type is
xs:string
and whose value is the string denoted by the characters between the
delimiting apostrophes or quotation marks. If the literal is delimited by apostrophes, two adjacent
apostrophes within the literal are interpreted as a single apostrophe. Similarly, if the literal
is delimited by quotation marks, two adjacent quotation marks within the literal are interpreted
as one quotation mark.
Here are some examples of string literals:
"He said, ""I don't like it."""
denotes a string containing two quotation marks and one apostrophe.
Note:
When XPath or XQuery expressions are embedded in contexts where quotation marks have special significance, such as inside XML attributes, or in string literals in a host language such as Java or C#, then additional escaping may be needed.
An alternative solution is to use a
StringTemplate, for example
`He said, "I don't like it."`
. Technically this is not a literal, but
a string template with no embedded expressions can be used in the same way as a string
literal in nearly all contexts.
In XQuery, the string literal "<"
denotes a string of length 1 containing the single character
"<"
. In XPath, the string literal "<"
denotes a string of length 4 containing the four
characters "&"
, "l"
, "t"
, ";"
. (However, when the XPath
expression is embedded in an XML document, the sequence "<"
will typically have already been converted
to "<"
by the XML parser.)
The xs:boolean
values true
and false
can be constructed by calls to the
system functions
fn:true()
and fn:false()
, respectively.
Values of other simple types can be constructed by calling the constructor function for the given type. The constructor functions for XML Schema built-in types are defined in Section 20.1 Constructor functions for XML Schema built-in atomic typesFO40. In general, the name of a constructor function for a given type is the same as the name of the type (including its namespace). For example:
xs:integer("12")
returns the integer value twelve.
xs:date("2001-08-25")
returns an item whose type is xs:date
and whose value represents the date 25th August 2001.
xs:dayTimeDuration("PT5H")
returns an item whose type is xs:dayTimeDuration
and whose value represents a duration of five hours.
Constructor functions can also be used to create special values that have no literal representation, as in the following examples:
xs:float("NaN")
returns the special floating-point value, "Not a Number."
xs:double("INF")
returns the special double-precision value, "positive infinity."
Constructor functions are available for all simple types,
including union types. For example, if my:dt
is a user-defined union
type whose member types are xs:date
, xs:time
, and xs:dateTime
, then
the expression my:dt("2011-01-10")
creates an atomic value of type
xs:date
. The rules follow XML Schema validation rules for union types:
the effect is to choose the first member type that accepts the given
string in its lexical space.
It is also possible to construct values of various types by using a cast
expression. For example:
9 cast as
hatsize
returns the atomic value 9
whose type is hatsize
.
[89] | VarRef |
::= | "$" VarName
|
|
[90] | VarName |
::= |
EQName
|
[Definition: A variable reference is an EQName preceded by a $-sign.]
An unprefixed variable reference is in no namespace. Two variable references are equivalent if their expanded QNames are equal (as defined by the eq
operator). The scope of a variable binding is defined separately for each kind of
expression that can bind variables.
Every variable reference must match a name in the in-scope variables.
Every variable binding has a static scope. The scope defines where references to the variable can validly occur. It is a static error [err:XPST0008] to reference a variable that is not in scope. If a variable is bound in the static context for an expression, that variable is in scope for the entire expression except where it is occluded by another binding that uses the same name within that scope.
At evaluation time, the value of a variable reference is the value to which the relevant variable is bound.
[92] | ContextValueRef |
::= | "." |
A context value reference evaluates to the context value.
In many syntactic contexts, the context value will be a single item.
For example this applies on the right-hand side of the /
or !
operators, or within a Predicate.
If the context value is absentDM40, a context value reference raises a type error [err:XPDY0002].
Note:
Being absent is not the same thing as being empty.
[91] | ParenthesizedExpr |
::= | "(" Expr? ")" |
Parentheses may be used to override the precedence rules.
For example, the expression (2 + 4)
* 5
evaluates to thirty, since the parenthesized expression (2 + 4)
is evaluated first and its result is multiplied by five. Without
parentheses, the expression 2 + 4 * 5
evaluates to twenty-two, because the multiplication operator has higher
precedence than the addition operator.
Empty parentheses are used to denote an empty sequence, as described in 4.8.1 Sequence Concatenation.
[9] | EnclosedExpr |
::= | "{" Expr? "}" |
[Definition: An enclosed expression is an instance of the EnclosedExpr production, which allows an optional expression within curly braces.]
[Definition: In an enclosed expression, the optional expression enclosed in curly braces is called the content expression.] If the content expression is not provided explicitly, the content expression is ()
.
Note:
Despite the name, an enclosed expression is not actually an expression in its own right; rather it is a construct that is used in the grammar of many other expressions.
[71] | PostfixExpr |
::= |
PrimaryExpr | FilterExpr | DynamicFunctionCall | LookupExpr
|
|
[70] | FilterExpr |
::= |
PostfixExpr
Predicate
|
|
[72] | DynamicFunctionCall |
::= |
PostfixExpr
PositionalArgumentList
|
|
[80] | LookupExpr |
::= |
PostfixExpr
Lookup
|
A postfix expression takes one of the following forms:
[Definition:
A filter expression is an expression in the form E1[E2]
:
its effect is
to return those items from the value of E1
that
satisfy the predicate in E2.]
Filter expressions are described in 4.5 Filter Expressions.
An example of a filter expression is (1 to 100)[. mod 2 = 0]
which returns all even numbers in the range 1 to 100.
The base expression E1
can itself be a postfix expression,
so multiple predicates are allowed, in the form E1[E2][E3][E4]
.
An expression (other than a raw EQName) followed by an argument
list in parentheses (that is, E1(E2, E3, ...)
) is
referred to as a dynamic function call. Its
effect is to evaluate E1
to obtain a function,
and then call that function, with
E2
, E3
, ...
as
arguments. Dynamic function calls are described in 4.6.2.1 Dynamic Function Calls.
An example of a dynamic function call is $f("a", 2)
where
the value of variable $f
must be a function item.
A lookup-expression
takes the form
E1?K
, where E1
is an expression returning a sequence
of maps or arrays, and K
is a key specifier, which indicates which
entries in a map, or members in an array, should be selected.
Lookup expressions are described in 4.15.3.1 Postfix Lookup Expressions.
An example of a lookup expression is $emp?name
, where
the value of variable $emp
is a map, and the string "name"
is the key of one of the entries in the map.
Postfix expressions are evaluated from left-to-right. For example, the
expression $E1[E2]?(E3)(E4)
is evaluated by first evaluating
the filter expression $E1[E2]
to produce a sequence of maps and arrays
(say $S
), then evaluating the lookup expression $S?(E3)
to produce a function item (say $F
), then evaluating the dynamic
function call $F(E4)
to produce the final result.
Note:
The grammar for postfix expressions is defined here in a way designed to link clearly to the semantics of the different kinds of expression. For parsing purposes, the equivalent production rule:
PostfixExpr := PrimaryExpr (Predicate | PositionalArgumentList | Lookup)*
(as used in XPath 3.1) is probably more convenient.
[70] | FilterExpr |
::= |
PostfixExpr
Predicate
|
|
[79] | Predicate |
::= | "[" Expr "]" |
A filter expression consists of a base expression followed by a predicate, which is an expression written in square brackets. The result of the filter expression consists of the items returned by the base expression, filtered by applying the predicate to each item in turn. The ordering of the items returned by a filter expression is the same as their order in the result of the primary expression.
Note:
Where the expression before the square brackets is a ReverseStep or ForwardStep, the expression is technically not a filter expression but an AxisStep. There are minor differences in the semantics: see 4.7.5 Predicates within Steps
Here are some examples of filter expressions:
Given a sequence of products in a variable, return only those products whose price is greater than 100.
$products[price gt 100]
List all the integers from 1 to 100 that are divisible by 5. (See 4.8.1 Sequence Concatenation for an explanation of the to
operator.)
(1 to 100)[. mod 5 eq 0]
The result of the following expression is the integer 25:
(21 to 29)[5]
The following example returns the fifth through ninth items in the sequence bound to variable $orders
.
$orders[5 to 9]
The following example illustrates the use of a filter expression as a step in a path expression. It returns the last chapter or appendix within the book bound to variable $book
:
$book/(chapter | appendix)[last()]
For each item in the input sequence, the predicate expression is evaluated using an inner focus, defined as follows: The context value is the item currently being tested against the predicate. The context size is the number of items in the input sequence. The context position is the position of the context value within the input sequence.
For each item in the input sequence, the result of the
predicate expression is coerced to an xs:boolean
value, called the predicate truth value, as
described below. Those items for which the predicate truth value
is true
are retained, and those for which the
predicate truth value is false
are discarded.
The predicate truth value is derived by applying the following rules, in order:
If the value V of the predicate expression
is a sequence whose first item is an instance of the type xs:numeric
,
then:
V must be an instance of the type
xs:numeric+
(that is, every item in V
must be numeric). A type error [err:XPTY0004] is
raised if this is not the case.
The predicate truth value is true
if
V is equal (by the
=
operator) to the context
position, and is false
otherwise.
In effect this means that an item in the input sequence is selected
if its position in the sequence is equal to one or more of the numeric
values in the predicate. For example, the predicate [3 to 5]
is true for the third, fourth, and fifth items in the input sequence.
Note:
It is possible, though not generally useful, for the value of a numeric
predicate to depend on the focus, and thus to differ for different items
in the input sequence. For example, the predicate [xs:integer(@seq)]
selects those items in the input sequence whose @seq
attribute
is numerically equal to their position in the input sequence.
It is also possible, and again not generally useful, for the value of the predicate
to be numeric for some items in the input sequence, and boolean for others.
For example, the predicate [@special otherwise xs:integer(@seq)]
is true for an item that either has an @special
attribute,
or if not, has an @seq
attribute
that is numerically equal to the position of the item in the input sequence.
Note:
The truth value of a numeric predicate does not depend on the order
of the numbers in V. The predicates [1, 2, 3]
and [3, 2, 1]
have exactly the same effect. The items in
the result of a filter expression always retain the ordering of the input
sequence.
Note:
The truth value of a numeric predicate whose value is non-integral or non-positive is always false.
Editorial note | |
Add entry to changes appendix. |
Otherwise, the predicate truth value is the effective boolean value of the predicate expression.
Functions in XPath 4.0 arise in two ways:
A function definition contains information about a family of functions with the same name and a defined arity range. These functions are in most cases known statically (they appear in the statically known function definitions), but there may be further function definitions that are known only dynamically (appearing in the dynamically known function definitions).
Function items are XDM items that can be called using a dynamic function call. They are values that can be bound to variables, passed as arguments, returned as function results, and generally manipulated in the same way as other XDM values.
The functions defined by a statically known function definition can be invoked using a
static function call. Function items corresponding
to these definitions can also be obtained, as dynamic values, by evaluating a named function reference.
Function items can also be obtained using the fn:function-lookup
function: in this case the function name and arity do not need to be known statically, and the function definition
need not be present in the static context, so long as it is in the dynamic context.
Static and dynamic function calls are described in the following sections.
The static context for an expression includes a set of statically known function definitions. Every function definition in the static context has a name (which is an expanded QName) and an arity range, which is a range of permitted arities for calls on that function. Two function definitions having the same name must not have overlapping arity ranges. This means that for a given static function call, it is possible to identify the target function definition in the static context unambiguously from knowledge of the function name and the number of supplied arguments.
A static function call is bound to a function definition in the static context by matching the name and arity. If the function call has P positional arguments followed by K keyword arguments, then the required arity is P+K, and the static context must include a function definition whose name matches the expanded QName in the function call, and whose arity range includes this required arity. This is the function chosen to be called. The result of the function is obtained by evaluating the expression that forms its implementation, with a dynamic context that provides values for all the declared parameters, initialized as described in 4.6.1.2 Evaluating Static Function Calls below.
Similarly, a function reference of the form f#N
binds to a
function definition in the
static context whose name matches f where MinP ≤ N and MaxP ≥ N
.
The result of evaluating a function reference is a function item which can be called
using a dynamic function call. Function items are never variadic and their arguments
are always supplied positionally. For example, the function reference fn:concat#3
returns a function item with arity 3, which is always called by supplying three positional
arguments, and whose effect is the same as a static call on fn:concat
with
three positional arguments.
The detailed rules for evaluating static function calls and function references are defined in subsequent sections.
[93] | FunctionCall |
::= |
EQName
ArgumentList
|
/* xgc: reserved-function-names */ |
/* gn: parens */ | ||||
[73] | ArgumentList |
::= | "(" ((PositionalArguments ("," KeywordArguments)?) | KeywordArguments)? ")" |
|
[75] | PositionalArguments |
::= |
Argument ("," Argument)* |
|
[94] | Argument |
::= |
ExprSingle | ArgumentPlaceholder
|
|
[95] | ArgumentPlaceholder |
::= | "?" |
|
[76] | KeywordArguments |
::= |
KeywordArgument ("," KeywordArgument)* |
|
[77] | KeywordArgument |
::= |
EQName ":=" Argument
|
[Definition: A static function call consists of an EQName followed by a parenthesized list of zero or more arguments.]
The argument list consists of zero or more positional arguments, followed by zero or more keyword arguments.
[Definition: An argument to a function call is either an
argument expression or an ArgumentPlaceholder
(?
); in both cases it may
either be supplied positionally, or identified by a name (called a keyword).]
This section is concerned with static function calls in which none of the arguments are ArgumentPlaceholders. Calls using one or more ArgumentPlaceholders are covered in the section 4.6.2.3 Partial Function Application.
If the function name supplied in a static function call is an unprefixed lexical QName, it is expanded using the default function namespace in the static context.
The expanded QName used as the function name and the number of arguments used in the static function call (the required arity) must match the name and arity range of a function definition in the static context using the rules defined in the previous section; if there is no match, a static error is raised [err:XPST0017].
Evaluation of static function calls is described in 4.6.1.2 Evaluating Static Function Calls .
Since the arguments of a function call are separated by commas, any argument expression that contains a top-level comma operator must be enclosed in parentheses. Here are some illustrative examples of static function calls:
my:three-argument-function(1, 2, 3)
denotes a static function call with three
positional arguments. The
corresponding function declaration must define at least three parameters, and may define
more, provided they are optional.
my:two-argument-function((1,2), 3)
denotes a static function call with two arguments, the first of which is a
sequence of two values. The
corresponding function declaration must define at least two parameters, and may define
more, provided they are optional.
my:two-argument-function(1, ())
denotes a static function call with two arguments,
the second of which is an empty sequence.
my:one-argument-function((1, 2,
3))
denotes a static function call with one argument that is a sequence of three
values.
my:one-argument-function(( ))
denotes a static function call with one argument that is an empty sequence.
my:zero-argument-function( )
denotes a static function call with zero arguments.
lang(node := $n, language := 'de')
is a static function
call with two keyword arguments. The corresponding function declaration defines two parameters,
a required parameter language
and an optional parameter node
.
This call supplies values for both parameters. It is equivalent to the call
fn:lang('de', $n)
. Note that the keyword arguments are in a different
order from the parameter declarations.
sort(//employee, key := fn($e) { xs:decimal($e/salary) })
is a static function
call with one positional argument and one keyword argument.
The corresponding function declaration defines three parameters,
a required parameter $input
, an optional parameter $collation
,
and an optional parameter $key
This call supplies values for the first and third parameters, leaving the second parameter ($collation
)
to take its default value. The default value of the $collation
parameter
is given as fn:default-collation()
, so the value supplied to the function is the
default collation from the dynamic context of the caller. It is equivalent to the call
fn:sort(//employee, fn:default-collation(), fn($e) { xs:decimal($e/salary) })
.
An EQName
in a KeywordArgument
is expanded to a QName value; if there
is no prefix, then the name is in no namespace (otherwise the prefix is resolved in the usual way).
The keywords used in a function call (after expansion to QNames) must be distinct
[err:XPST0017]; [err:XPST0017].
This section applies to static function calls where none of the
arguments is an ArgumentPlaceholder
. For function calls involving
placeholders, see 4.6.2.3 Partial Function Application.
When a static function call FC is evaluated with respect to a static context SC and a dynamic context DC, the result is obtained as follows:
The function definition FD to be used is found in the statically known function definitions of SC.
The required arity is the total number of arguments in the function call, including both positional and keyword arguments.
There can be at most one function definition
FD in the
statically known function definitions component of SC whose function name
matches the expanded QName in FC and whose arity range
includes the arity of FC’s ArgumentList
.
If there is no such function definition, a static error [err:XPST0017] is raised.
Each parameter in the function definition FD is matched to an argument expression as follows:
If there are N positional arguments in the function call, the corresponding argument expressions are matched pairwise to the first N parameters in the declaration. For this purpose the required parameters and optional parameters in FD are concatenated into a single list, in order.
Any keyword arguments in FC are then matched to parameters (whether required or optional) in FD by comparing the keyword used in FC with the paramater name declared in FD. Each keyword must match the name of a declared parameter [err:XPST0017], and this must be one that has not already been matched to a positional argument. [err:XPST0017].
If any required parameter has not been matched to any argument in FC by applying the above rules, a static error is reported [err:XPST0017]
If any optional parameter has not been matched to any argument in FC by applying the above rules, then the parameter is matched to the default value expression for that parameter in FD.
Each argument expression established by the above rules is evaluated with respect to DC. The order of argument evaluation is implementation dependent and it is not required that an argument be evaluated if the function body can be evaluated without evaluating that argument.
Note:
All argument expressions, including default value expressions, are evaluated in the dynamic
context of the function call. It is therefore possible to use a default value expression such as
.
, or /
, or fn:current-dateTime()
, whose value depends on the
dynamic context of the function call.
If the expression used for the default value of a parameter has no dependencies on the dynamic context, then an implementation may choose to reuse the same value on repeated function calls rather than re-evaluating it on each function call.
Note:
This is relevant, for example, if the expression constructs new nodes.
The result of evaluating the argument expression is converted to the required type (the
declared type associated with the corresponding parameter in the function declaration, defaulting
to item()*
) by applying the coercion rules.
This applies both to explicitly supplied arguments, and
to values obtained by evaluating default value expressions. In both cases a type
error will be raised if the value (after coercion) does not match the required type.
The result of the function call is obtained as follows:
FD’s body is invoked in an implementation-dependent way. The processor makes the following information available to that invocation:
The converted argument values;
If the function is context dependent, the static context SC and dynamic context DC of the function call.
The result is converted to the required type (the
declared return type in the function declaration, defaulting
to item()*
) by applying the coercion rules.
The result of applying the coercion rules is either an instance of FD’s return type or a dynamic error. This result is then the result of evaluating FC.
Note:
A host language may define alternative rules for processing the result, especially in the case of external functions implemented using a non-XDM type system.
Errors raised by system functions are defined in [XQuery and XPath Functions and Operators 4.0].
Errors raised by host-language-dependent functions are implementation-defined.
The following function call uses the function
Section 2.5 fn:base-uriFO40. Use of SC
and DC
and errors raised by this function are all defined in
[XQuery and XPath Functions and Operators 4.0].
base-uri()
A function item is an XDM value that can be bound to a variable, or manipulated in various ways by XPath 4.0 expressions. The most significant such expression is a dynamic function call, which supplies values of arguments and evaluates the function to produce a result.
The syntax of dynamic function calls is defined in 4.6.2.1 Dynamic Function Calls.
A number of constructs can be used to produce a function item, notably:
A named function reference (see 4.6.2.4 Named Function References)
constructs a function item by reference to function definitions
in the static context. For example, fn:node-name#1
returns a function item whose effect is to call the static fn:node-name
function
with one argument.
An inline function (see 4.6.2.5 Inline Function Expressions
)
constructs a function item whose body is defined locally. For example, the
construct fn($x) { $x + 1 }
returns a function item whose effect is to increment
the value of the supplied argument.
A partial function application (see
4.6.2.3 Partial Function Application) derives one function item from another by supplying
the values of some of its arguments. For example, fn:ends-with(?, ".txt")
returns
a function item with one argument that tests whether the supplied string ends with the substring
".txt"
.
Maps and arrays are also function items. See 4.15.1.1 Map Constructors and 4.15.2.1 Array Constructors.
The fn:function-lookup
function can be called to discover functions
that are present in the dynamic context.
The fn:load-xquery-module
function can be called to load functions
dynamically from an external XQuery library module.
Some system functions such as fn:random-number-generator
and fn:op
return a function item as their result.
These constructs are described in detail in the following sections, or in [XQuery and XPath Functions and Operators 4.0].
[72] | DynamicFunctionCall |
::= |
PostfixExpr
PositionalArgumentList
|
|
[74] | PositionalArgumentList |
::= | "(" PositionalArguments? ")" |
|
[75] | PositionalArguments |
::= |
Argument ("," Argument)* |
|
[94] | Argument |
::= |
ExprSingle | ArgumentPlaceholder
|
|
[95] | ArgumentPlaceholder |
::= | "?" |
[Definition: A dynamic function call consists of a base expression that returns the function and a parenthesized list of zero or more arguments (argument expressions or ArgumentPlaceholders).]
A dynamic function call is evaluated as described in 4.6.2.2 Evaluating Dynamic Function Calls.
The following are examples of some dynamic function calls:
This example calls the function contained in $f
, passing the arguments 2 and 3:
$f(2, 3)
This example fetches the second item from sequence $f
, treats it as a function and calls it, passing an xs:string
argument:
$f[2]("Hi there")
This example calls the function $f
passing no arguments, and filters the result with a positional predicate:
$f()[2]
Note:
Arguments in a dynamic function call are always supplied positionally.
This section applies to dynamic function calls whose arguments do not include
an ArgumentPlaceholder
. For function calls that include a placeholder,
see 4.6.2.3 Partial Function Application.
[Definition: A dynamic function call is an expression that is evaluated by calling a function item, which is typically obtained dynamically.]
When a dynamic function call FC is evaluated, the result is obtained as follows:
The function item
FI to be called
is obtained by evaluating the base expression of the function call.
If this yields a sequence consisting of a single function item
whose arity matches the number of arguments in the ArgumentList
,
let FI denote that function item.
Otherwise, a type error is raised
[err:XPTY0004].
Note:
Keyword arguments are not allowed in a dynamic function call.
Argument expressions are evaluated, producing argument values. The order of argument evaluation is implementation-dependent and an argument need not be evaluated if the function body can be evaluated without evaluating that argument.
Each argument value is converted to the corresponding parameter type in FI’s signature by applying the coercion rules, resulting in a converted argument value
If FI is a map, it is evaluated as described in 4.15.1.2 Map Lookup using Function Call Syntax.
If FI is an array, it is evaluated as described in 4.15.2.2 Array Lookup using Function Call Syntax.
If FI’s body is an XPath 4.0 expression (for example, if FI is an anonymous function, or a partial application of such a function):
FI’s body
is evaluated.
The static context for this evaluation
is the static context of the XPath 4.0 expression.
The dynamic context for this evaluation is obtained
by taking the dynamic context of the
InlineFunctionExpr
that contains the FunctionBody
, and
making the following changes:
The focus (context value, context position, and context size) is absentDM40.
In the variable values component of the dynamic context, each converted argument value is bound to the corresponding parameter name.
When this is done,
the converted argument values retain
their dynamic types,
even where these are subtypes
of the declared parameter types.
For example, a function with
a parameter $p
of type xs:decimal
can be called with an argument of type xs:integer
,
which is derived from xs:decimal
.
During the processing of this function
call, the value of $p
inside the body of the function
retains its dynamic type of xs:integer
.
FI’s nonlocal variable bindings are also added to the variable values. (Note that the names of the nonlocal variables are by definition disjoint from the parameter names, so there can be no conflict.)
The value returned by evaluating the function body is then converted to the declared return type of FI by applying the coercion rules. The result is then the result of evaluating FC.
As with argument values,
the value returned by a function
retains its dynamic type,
which may be a subtype of the declared return type of FI.
For example, a function that has
a declared return type of xs:decimal
may in fact return a value of dynamic type xs:integer
.
$incr
is a nonlocal variable that is available within the function because its variable binding has been added to the variable values of the function.. Even though the parameter and return type of this function are both xs:decimal
,
the more specific type xs:integer
is preserved in both cases.
let $incr := 1 let $f := function($i as xs:decimal) as xs:decimal { $i + $incr } return $f(5)
The following example will raise a type error [err:XPDY0002]:
let $vat := function() { @vat + @price } return doc('wares.xml')/shop/article/$vat()
Instead, the context value can be used as an argument to the anonymous function:
let $vat := function($art) { $art/@vat + $art/@price } return doc('wares.xml')/shop/article/$vat(.)
Alternatively, the value can be referenced as a nonlocal variable binding:
let $ctx := doc('wares.xml')/shop/article let $vat := function() { for $a in $ctx return $a/@vat + $a/@price } return $vat()
Finally, a focus function can be used. This binds the value of the argument to the context value within the function body:
let $vat := function { @vat + @price } return $vat(doc('wares.xml')/shop/article)
If the implementation of FI is not an XPath 4.0 expression (for example, FI is a system function the body of the function is evaluated, and the result is converted to the declared return type, in the same way as for a static function call (see 4.6.1.1 Static Function Call Syntax).
Errors may be raised in the same way.
[Definition: A static or dynamic function call is a partial function application if one or more arguments is an ArgumentPlaceholder.]
The rules for partial function application in static function calls and dynamic function calls have a great deal in common, but they are stated separately below for clarity.
In each case, the result of a partial function application is a function item, whose arity is equal to the number of placeholders in the call.
More specifically, the result of the partial function application is a partially applied function. [Definition: A partially applied function is a function created by partial function application.]
For static function calls, the result is obtained as follows:
The function definition
FD to be partially applied
is determined in the same way as for a static function call without placeholders,
as described in 4.6.1.1 Static Function Call Syntax.
For this purpose an ArgumentPlaceholder
contributes to the count of
arguments.
The parameters of FD are classified into three categories:
Parameters that map to a placeholder, referred to as placeholder parameters.
Parameters for which an explicit value is given in the function call (either positionally or by keyword), referred to as explicitly supplied parameters.
Parameters (which are necessarily optional parameters) for which no corresponding argument is supplied, either as a placeholder or with an explicit value. These are referred to as defaulted parameters.
Note:
A partial function application need not have any explicitly supplied parameters.
For example, the partial function application fn:string(?)
is allowed; it has exactly the same effect as the named function reference
fn:string#1
.
Explicitly supplied parameters and defaulted parameters are evaluated and converted to the required type using the rules for a static function call. This may result in an error being raised.
A type error is raised if any of the explicitly supplied or defaulted parameters, after applying the coercion rules, does not match the required type of the corresponding parameter.
In addition, a dynamic error may be raised if any of the explicitly supplied or defaulted parameters does not match other constraints on the value of that parameter (for example, if the value supplied for a parameter expecting a regular expression is not a valid regular expression); or if the processor is able to establish that evaluation of the resulting function will fail for any other reason (for example, if an error is raised while evaluating a subexpression in the function body that depends only on explicitly supplied and defaulted parameters).
In all cases the error code is the same as for a static function call supplying the same invalid value(s).
In the particular case where all the supplied arguments
are placeholders, the error behavior should be the same as
for an equivalent named function reference: for example, fn:id#1
fails if there is no context node, and fn:id(?)
should
fail likewise.
The result is a partially applied function having the following properties (which are defined in Section 2.9.4 Function ItemsDM40):
name: The name of FD if all parameters map to placeholders, that is, if the partial function application is equivalent to the corresponding named function reference. Otherwise, the name is absent.
identity: A new function identity distinct from the identity of any other function item.
Note:
See also 4.6.2.7 Function Identity.
arity: The number of placeholders in the function call.
parameter names: The names of the parameters of FD that have been identified as placeholder parameters, retaining the order in which the placeholders appear in the function call.
Note:
A partial function application can be used to change the order
of parameters, for example fn:contains(substring:=?, value:=?)
returns a function item that is equivalent to fn:contains#2
,
but with the order of arguments reversed.
signature: The parameters in the returned function are the parameters of FD that have been identified as placeholder parameters, retaining the order in which the placeholders appear in the function call. The result type of the returned function is the same as the result type of FD.
An implementation which can determine a more specific signature (for example, through use of type analysis) is permitted to do so.
body: The body of FD.
captured context: The static and dynamic context of the function call, augmented, for each explicitly supplied parameter and each defaulted parameter, with a binding of the converted argument value to the corresponding parameter name.
The following partial function application creates a function item that computes the sum of squares of a sequence.
let $sum-of-squares := fold-right(?, 0, function($a, $b) { $a*$a + $b }) return $sum-of-squares(1 to 3)
$sum-of-squares
is an anonymous function. It has one parameter, named $seq
, which is taken from the corresponding parameter in fn:fold-right
(the other two parameters are fixed). The implementation is the implementation of fn:fold-right
, which is a context-independent system function. The nonlocal bindings contain the fixed bindings for the second and third parameters of fn:fold-right
.
For dynamic function calls, the result is obtained as follows:
The function item
FI to be partially applied is
determined in the same way as for a
dynamic function call without placeholders, as described in 4.6.2.1 Dynamic Function Calls.
For this purpose an ArgumentPlaceholder
contributes to the count of
arguments.
The parameters of FI are classified into two categories:
Parameters that map to a placeholder, referred to as placeholder parameters.
Parameters for which an explicit value is given in the function call, referred to as supplied parameters.
Note:
A partial function application need not have any explicitly supplied parameters.
For example, if $f
is a function with arity 2, then
the partial function application $f(?, ?)
returns
a function that has exactly the same effect as $f
.
Arguments corresponding to supplied parameters are evaluated and converted to the required type of the parameter, using the rules for dynamic function calls.
A type error is raised if any of the supplied parameters, after applying the coercion rules, does not match the required type.
In addition, a dynamic error may be raised if any of the supplied parameters does not match other constraints on the value of that parameter (for example, if the value supplied for a parameter expecting a regular expression is not a valid regular expression); or if the processor is able to establish that evaluation of the resulting function will fail for any other reason (for example, if an error is raised while evaluating a subexpression in the function body that depends only on explicitly supplied parameters).
In both cases the error code is the same as for a dynamic function call supplying the same invalid value.
The result of the partial function application is a partially applied function with the following properties (which are defined in Section 2.9.4 Function ItemsDM40):
name: Absent.
arity: The number of placeholders in the function call.
parameter names: The names of parameters in FI that have been identified as placeholder parameters, in order.
Note:
In a dynamic partial function application, argument keywords are not available, so it is not possible to change the order of parameters.
signature: The signature of FI, removing the types of supplied parameters. An implementation which can determine a more specific signature (for example, through use of type analysis) is permitted to do so.
body: The body of FI.
captured context: the captured context of FI, augmented, for each supplied parameter, with a binding of the converted argument value to the corresponding parameter name.
In the following example, $f
is an anonymous function, and $paf
is a partially applied function created from $f
.
let $f := function($seq, $delim) { fold-left($seq, "", concat(?, $delim, ?)) } let $paf := $f(?, ".") return $paf(1 to 5)
$paf
is also an anonymous function. It has one parameter, named $delim
, which is taken from the corresponding parameter in $f
(the other parameter is fixed). The implementation of $paf
is the implementation of $f
, which is fn:fold-left($seq, "", fn:concat(?, $delim, ?))
. This implementation is associated with the SC
and DC
of the original expression in $f
. The nonlocal bindings associate the value "."
with the parameter $delim
.
Partial function application never returns a map or an array. If $f
is a map or an array, then $f(?)
is
a partial function application that returns a function, but the function it returns is neither a map nor an array.
[97] | NamedFunctionRef |
::= |
EQName "#" IntegerLiteral
|
/* xgc: reserved-function-names */ |
[152] | EQName |
::= |
QName | URIQualifiedName
|
[Definition:
A named function reference is an expression (written name#arity
)
which evaluates to a function item, the details
of the function item being based on the properties of a function definition
in the static context
.]
The name and arity of the required function are known statically.
If the EQName is a lexical QName, it is expanded using the default function namespace in the static context.
The expanded QName and arity must correspond to a function definition
present in the static context.
More specifically, for a named function reference F#N
,
there must be a function definition in the statically known function definitions
whose name matches F, and whose arity range includes N
.
Call this function definition
FD.
If FD is context dependent for the given arity, then the returned function item has a captured context comprising the static and dynamic context of the named function reference.
Note:
In practice, it is necessary to retain only those parts of the static and dynamic context that can affect the outcome. These means it is unnecessary to retain parts of the context that no system function depends on (for example, local variables), or parts that are invariant within an execution scope (for example, the implicit timezone).
Consider:
let $f := <foo/>/fn:name#0 return <bar/>/$f()
The function fn:name()
, with no arguments, returns the name of the context node. The function
item delivered by evaluating the expression fn:name#0
returns the name of the element that was the
context node at the point where the function reference was evaluated (that is, the <foo>
element).
This expression therefore returns "foo"
, not "bar"
.
An error is raised if the identified function depends on components of the static or dynamic
context that are not present, or that have unsuitable values. For example [err:XPDY0002] is raised for the expression fn:name#0
if the context item is absent, and [err:FODC0001]FO is raised for the call fn:id#1
if the context item is not a node
in a tree that is rooted at a document node. The error that is raised is the same as the error that would
be raised by the corresponding function if called with the same static and dynamic context.
If the expanded QName and arity in a named function reference do not match the name and arity range of a function definition in the static context, a static error is raised [err:XPST0017].
The value of a NamedFunctionRef
is a function item
FI
obtained from FD
as follows:
name: The name of FD.
identity:
If FD is context dependent for the given arity, then a new function identity distinct from the identity of any other function item.
Note:
In the general case, a function reference to a context-dependent function will produce different results every time it is evaluated, because the resulting function item has a captured context (see Section 2.9.4 Function ItemsDM40) that includes the dynamic context of the particular evaluation. Optimizers, however, are allowed to detect cases where the captured context happens to be the same, or where any variations are immaterial, and where it is therefore safe to return the same function item each time. This might be the case, for example, where the only context dependency of a function is on the default collation, and the default collation for both evaluations is known to be the same.
Otherwise, a function identity that is the same as that produced by the evaluation of any other named function reference with the same function name and arity.
This rule applies even across different
execution scopesFO40:
for example if a parameter to a call to fn:transform
is set to the
result of the expression fn:abs#1
, then the function item passed as the parameter
value will be identical to that obtained by evaluating the expression fn:abs#1
within the target XSLT stylesheet.
This rule also applies when the target function definition is
nondeterministicFO40.
For example all evaluations of the named function reference map:keys#2
return identical function items, even though two evaluations of map:keys
with the same arguments may produce different results.
Note:
See also 4.6.2.7 Function Identity.
arity: As specified in the named function reference.
parameter names: The first A parameter names of FD, where A is the required arity.
signature: Formed from the required types of the first A parameters of FD, and the function result type of FD.
body: The body of FD.
captured context: Comprises the static and dynamic context of the named function reference, augmented with bindings of the names of parameters of FD beyond the A’th parameter, to their respective default values.
Note:
In practice, it is necessary to retain only the parts of the context that the function actually depends on (if any).
Note:
Consider the system function fn:format-date
,
which has an arity range of 2 to 5. The named function reference fn:format-date#3
returns a function item whose three parameters correspond to the first three parameters
of fn:format-date
; the remaining two arguments will take their default values.
To obtain an arity-3 function that binds to arguments 1, 2, and 5 of fn:format-date
,
use the partial function application format-date(?, ?, place:=?)
.
The following are examples of named function references:
fn:abs#1
references the fn:abs
function which takes a single argument.
fn:concat#5
references the fn:concat
function which takes 5 arguments.
local:myfunc#2
references a function named local:myfunc
which takes 2 arguments.
Note:
Function items, as values in the data model, have a fixed arity, and
a dynamic function call always supplies the arguments positionally. Although the base function
referred to may be variadic, the result of evaluating the function reference is a function that
has fixed arity. In effect, the result of evaluating my:func#3
is the
same as the result of evaluating the inline function expression function($x, $y, $z){my:func($x, $y, $z)}
,
except that the returned function has a name (it retains the name my:func
).
[98] | InlineFunctionExpr |
::= | ("function" | "fn") FunctionSignature? FunctionBody
|
|
[3] | FunctionSignature |
::= | "(" ParamList? ")" TypeDeclaration? |
|
[6] | ParamList |
::= |
Param ("," Param)* |
|
[7] | Param |
::= | "$" EQName
TypeDeclaration? |
|
[111] | TypeDeclaration |
::= | "as" SequenceType
|
|
[8] | FunctionBody |
::= |
EnclosedExpr
|
[Definition: An inline function expression , when evaluated, creates an anonymous function defined directly in the inline function expression.] An inline function expression specifies the names and SequenceTypes of the parameters to the function, the SequenceType of the result, and the body of the function.
An inline function expression whose FunctionSignature is omitted is known as a focus function. Focus functions are described in 4.6.2.6 Focus Functions.
[Definition: An anonymous function is a function item with no name. Anonymous functions may be created, for example, by evaluating an inline function expression or by partial function application.]
The keywords function
and fn
are synonymous.
The syntax allows the names and types of the function argument to be declared, along with the type of the result:
function($x as xs:integer, $y as xs:integer) as xs:integer {$x + $y}
The types can be omitted, and the keyword can be abbreviated:
fn($x, $y) {$x + $y}
A zero-arity function can be written as, for example, fn(){current-date()}
.
If a function parameter is declared using a name but no type, its default type is item()*
.
If the result type is omitted, its default result type is item()*
.
The parameters of an inline function expression are considered to be variables whose scope is the function body. It is a static error [err:XQST0039] for an inline function expression to have more than one parameter with the same name.
The static context for the function body is inherited from the location of the inline function expression, with the exception of the static type of the context value which is initially absentDM40.
The variables in scope for the function body include all variables representing the function parameters, as well as all variables that are in scope for the inline function expression.
Note:
Function parameter names can mask variables that would otherwise be in scope for the function body.
The result of an inline function expression is a single function item with the following properties (as defined in Section 2.9.4 Function ItemsDM40):
name: Absent.
identity: A new function identity distinct from the identity of any other function item.
Note:
See also 4.6.2.7 Function Identity.
parameter names:
The parameter names in
the InlineFunctionExpr
’s
ParamList
.
signature:
A FunctionTest
constructed from the
SequenceType
s in the InlineFunctionExpr
.
An implementation which can determine a more specific signature (for example, through use of type analysis of the function’s body) is permitted to do so.
body:
The InlineFunctionExpr
’s FunctionBody
.
captured context: the static context
is the static context of the inline function expression,
with the exception of the static context value type which is
absentDM40. The dynamic context has an absent
focus, and a set of variable bindings
comprising the variable values component
of the dynamic context of the InlineFunctionExpr
.
The following are examples of some inline function expressions:
This example creates a function that takes no arguments and returns a sequence of the first 6 primes:
function() as xs:integer+ { 2, 3, 5, 7, 11, 13 }
This example creates a function that takes two xs:double
arguments and returns their product:
fn($a as xs:double, $b as xs:double) as xs:double { $a * $b }
This example creates and invokes a function that captures the value of a local variable in its scope:
let $incrementors := for $x in 1 to 10 return function($y) as xs:integer { $x + $y } return $incrementors[2](4)
The result of this expression is 6
[Definition: A focus function
is an inline function expression in which the function signature is implicit: the function takes
a single argument of type item()*
(that is, any value), and binds this to the
context value when evaluating
the function body, which returns a result of type item()*
.]
Here are some examples of focus functions:
fn{@age}
- a function that expects a node as its argument, and returns
the @age
attribute of that node.
fn{.+1}
- a function that expects a number as its argument, and returns
that number plus one.
function{`${.}`}
- a function that expects a string as its argument, and prepends
a "$"
character.
function{head(.)+foot(.)}
- a function that expects a sequence of numbers
as its argument, and returns the sum of the first and last items in the sequence.
Focus functions are often useful as arguments to simple higher-order functions such as fn:sort
.
For example, to sort employees by salary, write sort(//employee, (), fn { +@salary })
.
(The unary plus has the effect of converting the attribute’s value to a number, for numeric sorting).
Focus functions can also be useful on the right-hand side of the sequence arrow operator
and mapping arrow operator.
For example, $s => tokenize() =!> fn { `"{.}"` }()
first tokenizes the string $s
,
then wraps each token in double quotation marks.
The expression function{EXPR}
(or fn{EXPR}
) is a syntactic shorthand for the expression
function($Z as item()*) as item()* {$Z!(EXPR)}
, where $Z
is a variable name that is
otherwise unused. Note that the function body (EXPR
) is evaluated with a fixed focus:
the context position and context size will always be 1 (one).
Editorial note | 2023-09-14 |
TODO: The above no longer works. We don't currently have any construct (other than this one) that sets the context value to something other than a singleton. |
It is sometimes useful to be able to establish whether two variables refer to the same function or to different functions. For this purpose, every function item has an identity. Functions with the same identity are indistinguishable in every way; in particular, any function call with identical arguments will produce an identical result.
In general, evaluation of an expression that returns a function item other than one that was present in its operands delivers a function item whose identity is unique, and thus distinct from any other function item. There are two exceptions to this rule:
Evaluating a function reference such as count#1
returns the same function
every time. Specifically, if the function name identifies a function definition
that is not context dependent (which is the most usual case), then all
function references using this function name and arity return the same function.
For more details see 4.6.2.4 Named Function References.
An optimizer is permitted to rewrite expressions in such a way that repeated
evaluation is avoided if it can be established that the result will be the same each time,
and this may be done without consideration of function identity. For example, if the
expression contains(?, "e")
appears within the body of a for
expression, or if the same expression is written repeatedly in a query, then an optimizer
may decide to evaluate it once only, and thus return the same function item each time.
Similarly, optimizers are allowed to replace any expression with an equivalent
expression; for example, count(?)
may be rewritten as count#1
.
[55] | PathExpr |
::= | ("/" RelativePathExpr?) |
/* xgc: leading-lone-slash */ |
[56] | RelativePathExpr |
::= |
StepExpr (("/" | "//") StepExpr)* |
[Definition: A path expression consists of a series of one or more
steps, separated by /
or
//
, and optionally beginning with
/
or //
.
A path expression is typically used to locate nodes
within trees. ]
Absolute path expressions (those starting with an initial /
or //
), start their selection from the root node of a tree;
relative path expressions (those without a leading /
or
//
) start from the context value.
A path expression consisting of a single step is evaluated as described in 4.7.4 Steps.
A path expression consisting of /
on its own
is treated as an abbreviation for /.
.
An expression of the form /PP
(that is, a path expression
with a leading /
) is treated as an abbreviation for
the expression self::node()/(fn:root(.) treat as document-node())/PP
.
The effect of this expansion is that for every item J
in the context value V:
A type error occurs if J is not a node [err:XPTY0020].
The root node R of the tree containing J is selected.
A dynamic error occurs if R is not a document node [err:XPDY0050].
The expression that follows the leading /
is evaluated with
R as the context value.
The results of these multiple evaluations are then combined into a single sequence; if the result is a set of nodes, the nodes are delivered in document order with duplicates eliminated.
An expression of the form //PP
(that is, a path expression
with a leading //
) is treated as an abbreviation for
the expression self::node()/(fn:root(.) treat as document-node())/descendant-or-self:node()/PP
.
The effect of this expansion is that for every item J
in the context value V:
A type error occurs if J is not a node [err:XPTY0020].
The root node R of the tree containing J is selected.
A dynamic error occurs if R is not a document node [err:XPDY0050].
The descendants of R are selected, along with R itself.
For every node D in this set of nodes, the expression that
follows the leading //
is evaluated with D as the context value.
The results of these multiple evaluations are then combined into a single sequence; if the result is a set of nodes, the nodes are delivered in document order with duplicates eliminated.
If the context value is not a node, a type error is raised [err:XPTY0020]. At evaluation time, if the root node of the context node is not a document node, a dynamic error is raised [err:XPDY0050].
Note:
The descendants of a node do not include attribute nodes or namespace nodes.
Note:
A //
on its own is not allowed by the grammar.
[56] | RelativePathExpr |
::= |
StepExpr (("/" | "//") StepExpr)* |
A relative path expression is a path expression that selects nodes within a tree by following a series of steps starting at the nodes in the context value (which may be any kind of node, not necessarily the root of the tree).
Each non-initial occurrence of //
in a path expression is
expanded as described in 4.7.7 Abbreviated Syntax, leaving a
sequence of steps separated by /
. This sequence of steps
is then evaluated from left to right. So a path such as
E1/E2/E3/E4
is evaluated
as ((E1/E2)/E3)/E4
. The semantics of a path
expression are thus defined by the semantics of the
binary /
operator, which is defined in
4.7.3 Path operator (/).
Note:
Although the semantics describe the evaluation of a path with
more than two steps as proceeding from left to right, the /
operator is in most cases associative, so evaluation from
right to left usually delivers the same result. The cases
where /
is not associative arise when the functions
fn:position()
and fn:last()
are
used: A/B/position()
delivers a sequence of
integers from 1 to the size of (A/B)
, whereas
A/(B/position())
restarts the counting at each B
element.
The following example illustrates the use of relative path expressions. In each case it is assumed that the context value is a single node, referred to as the context node.
child::div1/child::para
Selects the
para
element children of the div1
element children of the context node; that is, the
para
element grandchildren of the context node
that have div1
parents.
Note:
Since each step in a path provides context nodes for the following step, in effect, only the last step in a path is allowed to return a sequence of non-nodes.
Note:
The /
character
can be used either as a complete path expression or as the
beginning of a longer path expression such as
/*
. Also, *
is both the multiply operator and a wildcard in path
expressions. This can cause parsing difficulties when
/
appears on the left-hand side of
*
. This is resolved using the leading-lone-slash
constraint. For example, /*
and /
*
are valid path expressions containing wildcards,
but /*5
and / * 5
raise syntax
errors. Parentheses must be used when /
is
used on the left-hand side of an operator, as in (/) * 5
. Similarly, 4 + / *
5
raises a syntax error, but 4 + (/) * 5
is a valid expression.
The expression 4 + /
is also
valid, because /
does not occur on the left-hand
side of the operator.
Similarly, in the expression /
union /*
, union
is interpreted as an element name
rather than an operator. For it to be parsed as an operator,
the expression should be written (/)
union /*
.
/
)The path operator /
is primarily used for
locating nodes within trees. Its left-hand operand must return
a sequence of nodes. The result of the operator is either a sequence of nodes
(in document order, with no duplicates), or a sequence of non-nodes.
The operation E1/E2
is evaluated as follows: Expression E1
is evaluated, and if the result is not a (possibly empty) sequence S
of nodes,
a type error is raised [err:XPTY0019]. Each node in S
then serves in turn to provide an inner focus
(the node as the context value, its position in S
as the context
position, the length of S
as the context size) for an evaluation
of E2
, as described in 2.2.2 Dynamic Context. The sequences resulting from all the evaluations of E2
are combined as follows:
If every evaluation of E2
returns a (possibly empty) sequence of nodes, these sequences are combined, and duplicate nodes are eliminated based on node identity.
The resulting node sequence is returned in document order.
If every evaluation of E2
returns a (possibly empty) sequence of non-nodes, these sequences are concatenated, in order, and returned.
The returned sequence preserves the orderings within and among the subsequences generated by the evaluations of E2
.
If the multiple evaluations of E2
return at least one node and at least one non-node, a type error is raised [err:XPTY0018].
Note:
The semantics of the path operator can also be defined using the simple map operator (!
) as follows (forming the union with an empty sequence ($R | ())
has the effect of eliminating duplicates and sorting nodes into document order):
E1/E2 ::= let $R := E1!E2 return if (every $r in $R satisfies $r instance of node()) then ($R|()) else if (every $r in $R satisfies not($r instance of node())) then $R else error()
For a table comparing the step operator to the map operator, see 4.20 Simple map operator (!).
[57] | StepExpr |
::= |
PostfixExpr | AxisStep
|
|
[58] | AxisStep |
::= | (ReverseStep | ForwardStep) PredicateList
|
|
[59] | ForwardStep |
::= | (ForwardAxis
NodeTest) | AbbrevForwardStep
|
|
[62] | ReverseStep |
::= | (ReverseAxis
NodeTest) | AbbrevReverseStep
|
|
[78] | PredicateList |
::= |
Predicate* |
[Definition: A step is a part of a path expression that generates a sequence of items and then filters the sequence by zero or more predicates. The value of the step consists of those items that satisfy the predicates, working from left to right. A step may be either an axis step or a postfix expression.] Postfix expressions are described in 4.4 Postfix Expressions.
[Definition: An axis step returns a sequence of nodes that are reachable from a starting node via a specified axis. Such a step has two parts: an axis, which defines the "direction of movement" for the step, and a node test, which selects nodes based on their kind, name, and/or type annotation .]
If the context value is a sequence of zero or more nodes, an axis step returns a sequence of zero or more nodes; otherwise, a type error is raised [err:XPTY0020].
The step expression S
is equivalent to ./S
.
Thus, if the context value is a sequence containing multiple nodes,
the semantics of a step expression are equivalent to a path expression
in which the step is always applied to a single node. The following
description therefore explains the semantics for the case where
the context value is a single node, called the context node.
Note:
The equivalence of a step S
to the
path expression ./S
means that
the resulting node sequence is returned in document
order.
An axis step may be either a forward step or a reverse step, followed by zero or more predicates.
In the abbreviated syntax for a step, the axis can be omitted and other shorthand notations can be used as described in 4.7.7 Abbreviated Syntax.
The unabbreviated syntax for an axis step consists of the axis name
and node test separated by a double colon. The result of the step consists of the nodes
reachable from the starting node via the specified axis that have the node kind, name,
and/or type annotation specified by the node test. For example, the
step child::para
selects the para
element children of the context node: child
is the name of the axis, and para
is the name of the element nodes to be selected on this axis. The available axes are described in 4.7.4.1 Axes. The
available node tests are described in 4.7.4.2 Node Tests. Examples of
steps are provided in 4.7.6 Unabbreviated Syntax and 4.7.7 Abbreviated Syntax.
[60] | ForwardAxis |
::= | ("child" "::") |
|
[63] | ReverseAxis |
::= | ("parent" "::") |
XPath defines a full set of axes for traversing documents, but a host language may define a subset of these axes. The following axes are defined:
The child
axis
contains the children of the context
node, which are the nodes returned by the
Section 4.3 children AccessorDM40.
Note:
Only document nodes and element nodes have children. If the context node is any other kind of node, or if the context node is an empty document or element node, then the child axis is an empty sequence. The children of a document node or element node may be element, processing instruction, comment, or text nodes. Attribute, namespace, and document nodes can never appear as children.
The descendant
axis is defined as the transitive closure of
the child axis; it contains the descendants
of the context node (the children, the children of the children, and so on).
The parent
axis contains the sequence
returned by the
Section 4.11 parent AccessorDM40,
which returns
the parent of the context
node, or an empty sequence
if the context node has no
parent.
Note:
An attribute node may have an element node as its parent, even though the attribute node is not a child of the element node.
The
ancestor
axis is
defined as the transitive
closure of the parent axis; it
contains the ancestors of the
context node (the parent, the
parent of the parent, and so
on).
Note:
The ancestor axis includes the root node of the tree in which the context node is found, unless the context node is the root node.
the following-sibling
axis contains the context node’s following
siblings, those children of the context
node’s parent that occur after the context
node in document order; if the context node
is an attribute or namespace node, the
following-sibling
axis is
empty.
the preceding-sibling
axis contains the context node’s preceding
siblings, those children of the context
node’s parent that occur before the context
node in document order; if the context node
is an attribute or namespace node, the
preceding-sibling
axis is
empty.
The following
axis
contains all nodes that are
descendants of the root of the tree in
which the context node is found, are
not descendants of the context node,
and occur after the context node in
document order.
The preceding
axis
contains all nodes that are
descendants of the root of the tree in
which the context node is found, are
not ancestors of the context node, and
occur before the context node in
document order.
The attribute
axis
contains the attributes of the context node,
which are the nodes returned by the
Section 4.1 attributes AccessorDM40
; the axis will be
empty unless the context node is an
element.
The self
axis contains just the context node itself.
The descendant-or-self
axis contains the context node and the descendants of the context
node.
The ancestor-or-self
axis contains the context node and the ancestors of the context node;
thus, the ancestor-or-self axis will always include the root node.
The namespace
axis
contains the namespace nodes of the
context node, which are the nodes
returned by the
Section 4.7 namespace-nodes AccessorDM40; this axis
is empty unless the context node is an
element node. The
namespace
axis is
deprecated as of XPath 2.0. If XPath 1.0
compatibility mode is true
, the namespace
axis must be supported. If XPath 1.0
compatibility mode is false
, then support for the
namespace
axis is
implementation-defined. An implementation
that does not support the
namespace
axis when XPath 1.0
compatibility mode is false
must raise
a static
error
[err:XPST0010] if it is
used. Applications needing information
about the in-scope namespaces of an element
should use the functions
Section 10.2.7 fn:in-scope-prefixesFO40,
and
Section 10.2.8 fn:namespace-uri-for-prefixFO40.
Axes can be categorized as forward axes and reverse axes. An axis that only ever contains the context node or nodes that are after the context node in document order is a forward axis. An axis that only ever contains the context node or nodes that are before the context node in document order is a reverse axis.
The parent
, ancestor
, ancestor-or-self
, preceding
, and preceding-sibling
axes are reverse axes; all other axes are forward axes. The ancestor
, descendant
, following
, preceding
and self
axes partition a document (ignoring attribute and namespace nodes):
they do not overlap and together they contain all the nodes in the
document.
[Definition: Every axis has a principal node kind. If an axis can contain elements, then the principal node kind is element; otherwise, it is the kind of nodes that the axis can contain.] Thus:
For the attribute axis, the principal node kind is attribute.
For the namespace axis, the principal node kind is namespace.
For all other axes, the principal node kind is element.
[Definition: A node test is a condition on the name, kind (element, attribute, text, document, comment, or processing instruction), and/or type annotation of a node. A node test determines which nodes contained by an axis are selected by a step.]
[65] | NodeTest |
::= |
UnionNodeTest | SimpleNodeTest
|
|
[66] | UnionNodeTest |
::= | "(" SimpleNodeTest ("|" SimpleNodeTest)* ")" |
|
[67] | SimpleNodeTest |
::= |
KindTest | NameTest
|
|
[68] | NameTest |
::= |
EQName | Wildcard
|
|
[69] | Wildcard |
::= | "*" |
/* ws: explicit */ |
[152] | EQName |
::= |
QName | URIQualifiedName
|
|
[117] | KindTest |
::= |
DocumentTest
|
A UnionNodeTest matches a node N if at least one of the constituent SimpleNodeTests matches N.
For example, (div1|div2|div3)
matches a node named div1
, div2
, or div3
[Definition: A node test that consists only of an EQName or a
Wildcard is called a name test.] A name
test that consists of an EQName matches a node N if and only if the kind of
node N is the principal node kind for the step axis and the
expanded QName of the node is equal (as defined by the eq
operator) to the
expanded QName specified by the name test. For
example, child::para
selects the para
element children of
the context node; if the context node has no
para
children, it selects an empty set
of nodes. attribute::abc:href
selects
the attribute of the context node with the QName
abc:href
; if the context node has no
such attribute, it selects an empty set of
nodes.
If the EQName is a lexical QName, it is resolved into an expanded QName using the
statically known namespaces in the expression
context. It is a static error
[err:XPST0081] if the QName has a prefix that does not
correspond to any statically known namespace.
An unprefixed QName, when used as a
name test on an axis whose principal node kind is element
, is interpreted as follows:
If the default namespace for elements and types is a namespace URI, then the name is interpreted as having that namespace URI.
If the default namespace for elements and types is absent, then the name is interpreted as being in no namespace.
A name test is not satisfied by an element node whose name does not match the expanded QName of the name test, even if it is in a substitution group whose head is the named element.
A node test *
is true for any node of the
principal node
kind of the step axis. For example, child::*
will select all element
children of the context node, and attribute::*
will select all
attributes of the context node.
A node test can have the form
NCName:*
. In this case, the prefix is
expanded in the same way as with a lexical QName, using the
statically known
namespaces in the static context. If
the prefix is not found in the statically known namespaces,
a static
error is raised [err:XPST0081].
The node test is true for any node of the principal
node kind of the step axis whose expanded QName has the namespace URI
to which the prefix is bound, regardless of the
local part of the name.
A node test can contain a BracedURILiteral, for example
Q{http://example.com/msg}*
. Such a node test is true for any node of the principal
node kind of the step axis whose expanded QName has the namespace URI specified in
the BracedURILiteral, regardless of the local part of the name.
A node test can also
have the form *:NCName
. In this case,
the node test is true for any node of the principal
node kind of the step axis whose local name matches the given NCName,
regardless of its namespace or lack of a namespace.
[Definition: An alternative form of a node test called a kind test can select nodes based on their kind, name, and type annotation.] The syntax and semantics of a kind test are described in 3.4 Sequence Types and 3.5 Sequence Type Matching. When a kind test is used in a node test, only those nodes on the designated axis that match the kind test are selected. Shown below are several examples of kind tests that might be used in path expressions:
node()
matches any
node.
text()
matches
any text
node.
comment()
matches any comment
node.
namespace-node()
matches any
namespace node.
element()
matches any element
node.
schema-element(person)
matches any element node whose name is
person
(or is in the substitution group
headed by person
), and whose type
annotation is the same as (or is derived from) the declared type of the person
element in the in-scope element declarations.
element(person)
matches any element node whose name is
person
, regardless of its type annotation.
element(doctor|nurse)
matches any element node whose name is
doctor
or nurse
, regardless of its type annotation.
element(person, surgeon)
matches any non-nilled element node whose name
is person
, and whose type
annotation is
surgeon
or is derived from surgeon
.
element(doctor|nurse, medical-staff)
matches any non-nilled element node whose name
is doctor
or nurse
, and whose type
annotation is
medical-staff
or is derived from medical-staff
.
element(*,
surgeon)
matches any non-nilled element node whose type
annotation is surgeon
(or is derived from surgeon
), regardless of
its
name.
attribute()
matches any
attribute node.
attribute(price)
matches
any attribute whose name is price
,
regardless of its type annotation.
attribute(*,
xs:decimal)
matches any attribute whose type
annotation is xs:decimal
(or is derived from xs:decimal
), regardless of
its
name.
document-node()
matches any document
node.
document-node(element(book))
matches any document node whose content consists of
a single element node that satisfies the kind test
element(book)
, interleaved with zero or more
comments and processing
instructions.
Certain axis steps, given an inferred type for the context value, are classified as implausible. During the static analysis phase, a processor may (subject to the rules in 2.4.6 Implausible Expressions) report a static error when such axis steps are encountered: [err:XPTY0144].
More specifically, an axis step is classified as implausible if any of the following conditions applies:
The inferred item type of the context value is a node kind for which the
specified axis is always empty: for example, the inferred item type
of the context value is attribute()
and the axis is child
.
The node test exclusively selects node kinds that cannot appear
on the specified axis: for example, the axis is child
and the node test is document-node()
.
In a schema-aware environment, when using the child
,
descendant
, descendant-or-self
, or attribute
axes, the inferred item type of the
context value has a content type that does not allow any node matching
the node test to be present on the relevant axis. For example, if the inferred
item type of the context value
is schema-element(list)
and the relevant element declaration
(taking into account substitution group membership and wildcards)
only allows item
children,
the axis step child::li
will never select anything and is therefore
classified as implausible.
Examples of implausible axis steps include the following:
@code/text()
: attributes cannot have text node children.
/@code
: document nodes cannot have attributes.
ancestor::text()
: the ancestor axis never returns text nodes.
Note:
Processors may choose not to classify the expression /..
as implausible, since XSLT 1.0 users were sometimes advised to use this construct
as an explicit way of denoting the empty sequence.
[58] | AxisStep |
::= | (ReverseStep | ForwardStep) PredicateList
|
|
[78] | PredicateList |
::= |
Predicate* |
|
[79] | Predicate |
::= | "[" Expr "]" |
A predicate within a Step has similar syntax and semantics to a predicate within a filter expression. The only difference is in the way the context position is set for evaluation of the predicate.
For the purpose of evaluating the context position within a predicate, the input sequence is considered to be sorted as follows: into document order if the predicate is in a forward-axis step, into reverse document order if the predicate is in a reverse-axis step, or in its original order if the predicate is not in a step.
Here are some examples of axis steps that contain predicates:
This example selects the second chapter
element that is a child
of the context node:
child::chapter[2]
This example selects all the descendants of the
context node that are elements named
"toy"
and whose color
attribute has the value "red"
:
descendant::toy[attribute::color = "red"]
This example selects all the employee
children of the context node
that have both a secretary
child element and an assistant
child element:
child::employee[secretary][assistant]
Note:
When using predicates with a sequence of nodes selected using a
reverse axis, the
context positions for such a sequence are assigned in reverse
document order. For example, preceding::foo[1]
returns the first qualifying foo
element in reverse document order, because the predicate is part of an axis step using a reverse axis. By
contrast, (preceding::foo)[1]
returns the first qualifying foo
element in document order, because the parentheses cause (preceding::foo)
to be parsed as a primary expression in which context positions are assigned in document order. Similarly, ancestor::*[1]
returns the nearest ancestor element, because the ancestor
axis is a
reverse axis, whereas (ancestor::*)[1]
returns the root element (first ancestor in document order).
The fact that a reverse-axis step assigns context positions in reverse document order for the purpose of evaluating predicates does not alter the fact that the final result of the step is always in document order.
The expression ancestor::(div1|div2)[1]
does not have the same meaning as (ancestor::div1|ancestor::div2)[1]
.
In the first expression,
the predicate [1]
is within a step that uses a reverse axis, so nodes are counted
in reverse document order. In the second expression, the predicate applies to the result of
a union expression, so nodes are counted in document order.
When the context value for evaluation of a step includes multiple nodes, the step is evaluated
separately for each of those nodes, and the results are combined. This means, for example, that
if the context value contains three list
nodes, and each of those nodes has multiple
item
children, then the step item[1]
will deliver a sequence of three item
elements, namely the first item
from each list
.
This section provides a number of examples of path expressions in which the axis is explicitly specified in each step. The syntax used in these examples is called the unabbreviated syntax. In many common cases, it is possible to write path expressions more concisely using an abbreviated syntax, as explained in 4.7.7 Abbreviated Syntax.
These examples assume that the context value is a single node, referred to as the context node.
child::para
selects
the para
element children of the context node.
child::(para|bullet)
selects
the para
and bullet
element children of the context node.
child::*
selects all element children of the context node.
child::text()
selects all text node children of the context node.
child::(text()|comment())
selects all text node and comment node children of the context node.
child::node()
selects all the children of the context node. Note that no attribute nodes are returned, because attributes are not children.
attribute::name
selects the name
attribute of the context node.
attribute::*
selects all the attributes of the context node.
parent::node()
selects the parent of the context node. If the context node is an attribute node, this expression returns the element node (if any) to which the attribute node is attached.
descendant::para
selects the para
element descendants of the context node.
ancestor::div
selects all div
ancestors of the context node.
ancestor-or-self::div
selects the div
ancestors of the context node and, if the context node is a div
element, the context node as well.
descendant-or-self::para
selects the para
element descendants of the context node and, if the context node is a para
element, the context node as well.
self::para
selects the context node if it is a para
element, and otherwise returns an empty sequence.
self::(chapter|appendix)
selects the context node if it is a
chapter
or appendix
element, and otherwise returns an empty sequence.
child::chapter/descendant::para
selects the para
element
descendants of the chapter
element children of the context node.
child::*/child::para
selects all para
grandchildren of the context node.
/
selects the root of the tree that contains the context node, but raises a dynamic error if this root is not a document node.
/descendant::para
selects all the para
elements in the same document as the context node.
/descendant::list/child::member
selects all
the member
elements that have a list
parent and that are in the same document as the context node.
child::para[position() = 1]
selects the first para
child of the context node.
child::para[position() = last()]
selects the last para
child of the context node.
child::para[position() = last()-1]
selects the last but one para
child of the context node.
child::para[position() > 1]
selects all the para
children of the context node other than the first para
child of the context node.
following-sibling::chapter[position() = 1]
selects the next chapter
sibling of the context node.
following-sibling::(chapter|appendix)[position() = 1]
selects the next sibling of the context node
that is either a chapter
or an appendix
.
preceding-sibling::chapter[position() = 1]
selects the previous chapter
sibling of the context node.
/descendant::figure[position() = 42]
selects the forty-second figure
element in the document containing the context node.
/child::book/child::chapter[position() = 5]/child::section[position() = 2]
selects the
second section
of the fifth chapter
of the book
whose parent is the document node that contains the context node.
child::para[attribute::type eq "warning"]
selects
all para
children of the context node that have a type
attribute with value warning
.
child::para[attribute::type eq 'warning'][position() = 5]
selects the fifth para
child of the context node that has a type
attribute with value warning
.
child::para[position() = 5][attribute::type eq "warning"]
selects the fifth para
child of the context node if that child has a type
attribute with value warning
.
child::chapter[child::title = 'Introduction']
selects
the chapter
children of the context node that have one or
more title
children whose typed value is equal to the
string Introduction
.
child::chapter[child::title]
selects the chapter
children of the context node that have one or more title
children.
child::*[self::chapter or self::appendix]
selects the chapter
and appendix
children of the context node.
child::*[self::(chapter|appendix)][position() = last()]
selects the
last chapter
or appendix
child of the context node.
[61] | AbbrevForwardStep |
::= | ("@" NodeTest) | SimpleNodeTest
|
|
[64] | AbbrevReverseStep |
::= | ".." |
The abbreviated syntax permits the following abbreviations:
The attribute axis attribute::
can be
abbreviated by @
. For example, a path expression para[@type="warning"]
is short
for child::para[attribute::type="warning"]
and
so selects para
children with a type
attribute with value
equal to warning
.
If the axis name is omitted from an axis step, the default axis is
child
, with two exceptions:
(1) if the NodeTest in an axis step contains an AttributeTest or SchemaAttributeTest then the
default axis is attribute
;
(2) if the NodeTest in an axis step is a NamespaceNodeTest
then the default axis is namespace
, but in an implementation that does not support
the namespace axis, an error is raised [err:XQST0134].
Note:
The namespace axis is deprecated as of XPath 2.0, but is required in some languages that use XPath, including XSLT.
For example, the path expression section/para
is an abbreviation for child::section/child::para
, and the path
expression section/@id
is an
abbreviation for child::section/attribute::id
. Similarly,
section/attribute(id)
is an
abbreviation for child::section/attribute::attribute(id)
. Note
that the latter expression contains both an axis specification and
a node test.
Note:
An abbreviated axis step that omits the axis name must use a
SimpleNodeTest rather than a UnionNodeTest.
This means that a construct such as (ul|ol)
is treated as an abbreviation for (child::ul|child::ol)
rather than child::(ul|ol)
.
Since the two constructs have exactly the same semantics, this is not actually a restriction.
Each non-initial occurrence of //
is effectively replaced by /descendant-or-self::node()/
during processing of a path expression. For example, div1//para
is
short for child::div1/descendant-or-self::node()/child::para
and so will select all para
descendants of div1
children.
Note:
The path expression //para[1]
does not mean the same as the path
expression /descendant::para[1]
. The latter selects the first descendant para
element; the former
selects all descendant para
elements that are the first para
children of their respective parents.
A step consisting
of ..
is short
for parent::node()
. For example, ../title
is short for parent::node()/child::title
and so will select the title
children of the parent of the context node.
Note:
The expression .
, known as a context value
reference, is a primary expression,
and is described in 4.3.3 Context Value References.
Here are some examples of path expressions that use the abbreviated syntax. These examples assume that the context value is a single node, referred to as the context node:
para
selects the para
element children of the context node.
*
selects all element children of the context node.
text()
selects all text node children of the context node.
@name
selects
the name
attribute of the context node.
@(id|name)
selects
the id
and name
attributes of the context node.
@*
selects all the attributes of the context node.
para[1]
selects the first para
child of the context node.
para[last()]
selects the last para
child of the context node.
*/para
selects
all para
grandchildren of the context node.
/book/chapter[5]/section[2]
selects the
second section
of the fifth chapter
of the book
whose parent is the document node that contains the context node.
chapter//para
selects the para
element descendants of the chapter
element children of the context node.
//para
selects all
the para
descendants of the root document node and thus selects all para
elements in the same document as the context node.
//@version
selects all the version
attribute nodes that are in the same document as the context node.
//list/member
selects all the member
elements in the same document as the context node that have a list
parent.
.//para
selects
the para
element descendants of the context node.
..
selects the parent of the context node.
../@lang
selects
the lang
attribute of the parent of the context node.
para[@type="warning"]
selects all para
children of the context node that have a type
attribute with value warning
.
para[@type="warning"][5]
selects the fifth para
child of the context node that has a type
attribute with value warning
.
para[5][@type="warning"]
selects the fifth para
child of the context node if that child has a type
attribute with value warning
.
chapter[title="Introduction"]
selects the chapter
children of the context node that have one
or more title
children whose typed value is equal to the string Introduction
.
chapter[title]
selects the chapter
children of the context node that have one or more title
children.
employee[@secretary and @assistant]
selects all
the employee
children of the context node that have both a secretary
attribute and
an assistant
attribute.
book/(chapter|appendix)/section
selects
every section
element that has a parent that is either a chapter
or an appendix
element, that in turn is a child of a book
element that is a child of the context node.
If E
is any expression that returns a sequence of nodes, then the expression E/.
returns the same nodes in document order, with duplicates eliminated based on node identity.
XPath 4.0 supports operators to construct, filter, and combine
sequences of items.
Sequences are never nested—for
example, combining the values 1
, (2, 3)
, and ( )
into a single sequence results
in the sequence (1, 2, 3)
.
[10] | Expr |
::= |
ExprSingle ("," ExprSingle)* |
[Definition: One way to construct a sequence is by using the comma operator, which evaluates each of its operands and concatenates the resulting sequences, in order, into a single result sequence.] Empty parentheses can be used to denote an empty sequence.
A sequence may contain duplicate items, but a sequence is never an item in another sequence. When a new sequence is created by concatenating two or more input sequences, the new sequence contains all the items of the input sequences and its length is the sum of the lengths of the input sequences.
Note:
In places where the grammar calls for ExprSingle, such as the arguments of a function call, any expression that contains a top-level comma operator must be enclosed in parentheses.
Here are some examples of expressions that construct sequences:
The result of this expression is a sequence of five integers:
(10, 1, 2, 3, 4)
This expression combines four sequences of length one, two, zero, and two, respectively, into a single sequence of length five. The result of this expression is the sequence 10, 1, 2, 3, 4
.
(10, (1, 2), (), (3, 4))
The result of this expression is a sequence containing
all salary
children of the context node followed by all bonus
children.
(salary, bonus)
Assuming that $price
is bound to
the value 10.50
, the result of this expression is the sequence 10.50, 10.50
.
($price, $price)
[35] | RangeExpr |
::= |
AdditiveExpr ( "to" AdditiveExpr )? |
A RangeExpression can be used to construct a sequence of
integers. Each of the operands is
converted as though it was an argument of a function with the expected
parameter type xs:integer?
.
If either operand is an empty sequence, or if the integer derived from the first operand is greater than the integer derived from the second operand, the result of the range expression is an empty sequence. If the two operands convert to the same integer, the result of the range expression is that integer. Otherwise, the result is a sequence containing the two integer operands and
every integer between the two operands, in increasing order.
The following examples illustrate the semantics:
1 to 4
returns the sequence 1, 2, 3, 4
10 to 10
returns the singleton sequence 10
10 to 1
returns the empty sequence
-13 to -10
returns the sequence -13, -12, -11, -10
More formally, a RangeExpression is evaluated as follows:
Each of the operands of the to
operator is converted as though it was an argument of a function
with the expected parameter type xs:integer?
.
If either operand is an empty sequence, or if the integer derived from the first operand is greater than the integer derived from the second operand, the result of the range expression is an empty sequence.
If the two operands convert to the same integer, the result of the range expression is that integer.
Otherwise, the result is a sequence containing the two integer operands and every integer between the two operands, in increasing order.
The following examples illustrate the use of RangeExpressions
.
This example uses a range expression as one operand in constructing a sequence.
It evaluates to the sequence 10, 1, 2, 3, 4
.
(10, 1 to 4)
This example selects the first four items from an input sequence:
$input[position() = 1 to 4]
This example returns the sequence (0, 0.1, 0.2, 0.3, 0.5)
:
$x = (1 to 5)!.*0.1
This example constructs a sequence of length one containing the single integer 10.
10 to 10
The result of this example is a sequence of length zero.
15 to 10
This example uses the fn:reverse
function to construct a sequence of six integers in decreasing order.
It evaluates to the sequence 15, 14, 13, 12, 11, 10.
reverse(10 to 15)
Note:
To construct a sequence of integers based on steps other than 1, use the fn:slice
function, as defined in Section
14.1 General functions and operators on sequences
FO31.
[38] | UnionExpr |
::= |
IntersectExceptExpr ( ("union" | "|") IntersectExceptExpr )* |
|
[39] | IntersectExceptExpr |
::= |
InstanceofExpr ( ("intersect" | "except") InstanceofExpr )* |
XPath 4.0 provides the following operators for combining sequences of nodes:
The union
and |
operators are equivalent. They take two node sequences as operands and
return a sequence containing all the nodes that occur in either of the
operands.
The intersect
operator takes two node sequences as operands and returns a sequence
containing all the nodes that occur in both operands.
The except
operator takes two node sequences as operands and returns a sequence
containing all the nodes that occur in the first operand but not in the second
operand.
All these operators eliminate duplicate nodes from their result sequences based on node identity. The resulting sequence is returned in document order.
If an operand
of union
, intersect
, or except
contains an item that is not a node, a type error is raised [err:XPTY0004].
If an IntersectExceptExpr contains more than two InstanceofExprs, they are grouped from left to right. With a UnionExpr, it makes no difference how operands are grouped, the results are the same.
Here are some examples of expressions that combine sequences. Assume the existence of three element nodes that we will refer to by symbolic names A, B, and C. Assume that the variables $seq1
, $seq2
and $seq3
are bound to the following sequences of these nodes:
$seq1
is bound to (A, B)
$seq2
is bound to (A, B)
$seq3
is bound to (B, C)
Then:
$seq1 union $seq2
evaluates to the sequence (A, B).
$seq2 union $seq3
evaluates to the sequence (A, B, C).
$seq1 intersect $seq2
evaluates to the sequence (A, B).
$seq2 intersect $seq3
evaluates to the sequence containing B only.
$seq1 except $seq2
evaluates to the empty sequence.
$seq2 except $seq3
evaluates to the sequence containing A only.
In addition to the sequence operators described here, see Section 14 Functions and operators on sequencesFO40 for functions defined on sequences.
XPath 4.0 provides arithmetic operators for addition, subtraction, multiplication, division, and modulus, in their usual binary and unary forms.
[36] | AdditiveExpr |
::= |
MultiplicativeExpr ( ("+" | "-") MultiplicativeExpr )* |
|
[37] | MultiplicativeExpr |
::= |
UnionExpr ( ("*" | "×" | "div" | "÷" | "idiv" | "mod") UnionExpr )* |
|
[45] | UnaryExpr |
::= | ("-" | "+")* ValueExpr
|
|
[46] | ValueExpr |
::= |
SimpleMapExpr
|
A subtraction operator must be preceded by whitespace if
it could otherwise be interpreted as part of the previous token. For
example, a-b
will be interpreted as a
name, but a - b
and a -b
will be interpreted as arithmetic expressions. (See A.3.5 Whitespace Rules for further details on whitespace handling.)
The arithmetic operator symbols *
and ×
(xD7) are interchangeable,
and denote multiplication.
The arithmetic operator symbols div
and ÷
(xF7) are interchangeable,
and denote division.
If an AdditiveExpr
contains more than two MultiplicativeExprs
,
they are grouped from left to right. So, for instance,
A - B + C - D
is equivalent to
((A - B) + C) - D
Similarly, the operands of a MultiplicativeExpr
are grouped from left to right.
The first step in evaluating an arithmetic expression is to evaluate its operands. The order in which the operands are evaluated is implementation-dependent.
If XPath 1.0 compatibility mode is true
, each operand is evaluated by applying the following steps, in order:
Atomization is applied to the operand. The result of this operation is called the atomized operand.
If the atomized operand is an empty sequence, the result of
the arithmetic expression is the xs:double
value NaN
, and the implementation
need not evaluate the other operand or apply the operator. However,
an implementation may choose to evaluate the other operand in order
to determine whether it raises an error.
If the atomized operand is a sequence of length greater than one, any items after the first item in the sequence are discarded.
If the atomized operand is now an instance of type xs:boolean
, xs:string
,
xs:decimal
(including xs:integer
), xs:float
, or xs:untypedAtomic
, then it
is converted to the type xs:double
by applying the fn:number
function. (Note that fn:number
returns the value NaN
if its operand cannot be converted to a number.)
If XPath 1.0 compatibility mode is false
, each
operand is evaluated by applying the following steps, in order:
Atomization is applied to the operand. The result of this operation is called the atomized operand.
If the atomized operand is an empty sequence, the result of the arithmetic expression is an empty sequence, and the implementation need not evaluate the other operand or apply the operator. However, an implementation may choose to evaluate the other operand in order to determine whether it raises an error.
If the atomized operand is a sequence of length greater than one, a type error is raised [err:XPTY0004].
If the atomized operand is of type xs:untypedAtomic
, it is cast to xs:double
. If
the cast fails, a dynamic
error is raised. [err:FORG0001]FO40
After evaluation of the operands, if the types of the operands are a valid combination for the given arithmetic operator, the operator is applied to the operands, resulting in an atomic value or a dynamic error (for example, an error might result from dividing by zero). The combinations of atomic types that are accepted by the various arithmetic operators, and their respective result types, are listed in B.2 Operator Mapping together with the operator functions that define the semantics of the operator for each type combination, including the dynamic errors that can be raised by the operator. The definitions of the operator functions are found in [XQuery and XPath Functions and Operators 4.0].
If the types of the operands, after evaluation, are not a valid combination for the given operator, according to the rules in B.2 Operator Mapping, a type error is raised [err:XPTY0004].
XPath 4.0 provides three division operators:
The div
and ÷
operators are synonyms, and implement
numeric division as well as division of duration values; the semantics are defined in
Section 4.2.4 op:numeric-divideFO40
The idiv
operator implements integer division; the semantics are defined
in Section 4.2.5 op:numeric-integer-divideFO40
Here are some examples of arithmetic expressions:
The first expression below returns the xs:decimal
value -1.5
, and the second expression returns the xs:integer
value -1
:
-3 div 2 -3 idiv 2
Subtraction of two date values results in a value of type xs:dayTimeDuration
:
$emp/hiredate - $emp/birthdate
This example illustrates the difference between a subtraction operator and a hyphen:
$unit-price - $unit-discount
Unary operators have higher precedence than binary operators (other than !
, /
, and []
), subject of
course to the use of parentheses. Therefore, the following two examples have different meanings:
-$bellcost + $whistlecost -($bellcost + $whistlecost)
Note:
Multiple consecutive unary arithmetic operators are permitted.
This section describes several ways of constructing strings.
[34] | StringConcatExpr |
::= |
RangeExpr ( "||" RangeExpr )* |
String concatenation expressions allow the string representations of values to be
concatenated. In XPath 4.0, $a || $b
is equivalent to
fn:concat($a, $b)
.
The following expression evaluates to the string concatenate
:
() || "con" || ("cat", "enate")
[106] | StringTemplate |
::= | "`" (StringTemplateFixedPart | StringTemplateVariablePart)* "`" |
/* ws: explicit */ |
[107] | StringTemplateFixedPart |
::= | ((Char - ('{' | '}' | '`')) | "{{" | "}}" | "``")* |
/* ws: explicit */ |
[108] | StringTemplateVariablePart |
::= |
EnclosedExpr
|
/* ws: explicit */ |
[9] | EnclosedExpr |
::= | "{" Expr? "}" |
String templates provide an alternative way of constructing strings. For example,
the expression `Pi is {round(math:pi(), 4)}`
returns the string "Pi is 3.1416"
.
A string template starts and ends with a grave accent (x60), popularly known as a back-tick. Between the back-ticks is a string consisting of an sequence of fixed parts and variable parts:
A variable part consists of an optional XPath expression enclosed in curly brackets ({}
):
more specifically, a string conforming
to the XPath production Expr?
.
Note:
An expression within a variable part may contain an unescaped curly bracket within a StringLiteral or within a comment.
Currently no XPath expression starts with an opening curly
bracket, so the use of {{
creates no ambiguity. If an enclosed
expression ends with a closing curly bracket, no whitespace is required between
this and the closing delimiter.
The fact that the expression is optional means that the string contained between the curly brackets may be zero-length, may comprise whitespace only, or may contain XPath comments. The effective value in this case is a zero-length string, which is equivalent to omitting the variable part entirely, together with its curly-bracket delimiters.
A fixed part may contain any characters, except that:
a left curly bracket must
be written as {{
a right curly bracket must be
written as }}
.
a back-tick must be
written as ``
.
The result of evaluating a string template is the string obtained by concatenating the expansions of the fixed and variable parts:
The expansion of a fixed part is obtained by replacing any double curly
brackets ({{
or }}
) by the corresponding single curly
bracket, and replacing doubled back-ticks (``
) by a single back-tick.
The expansion of a variable part containing an expression is as follows:
Atomization is applied to the value of the enclosed expression, converting it to a sequence of atomic values.
If the result of atomization is an empty sequence, the result is the zero-length string. Otherwise, each atomic value in the atomized sequence is cast into a string.
The individual strings resulting from the previous step are merged into a single string by concatenating them with a single space character between each pair.
The expansion of an empty variable part (one that contains no expression) is a zero-length string.
For example:
let $greeting := "Hello", $planet := "Mars" return `{$greeting}, {$planet}!`
returns "Hello, Mars!"
.
The expression:
let $longMonths := (1, 3, 5, 7, 8, 10, 12) return `The months with 31 days are: {$longMonths}.`
returns "The months with 31 days are: 1 3 5 7 8 10 12."
.
Note:
The rules for processing an enclosed expression are identical to the rules for attributes in XQuery direct element constructors. These rules differ slightly from the rules in XSLT attribute value templates, where adjacent text nodes are concatenated with no separator, prior to atomization.
Note:
A string template containing no variable parts is effectively just another
way of writing a string literal: "Goethe"
, 'Goethe'
, and `Goethe`
are interchangeable. This means that back-ticks can sometimes be a useful way of delimiting a string
that contains both single and double quotes: `He said: "I didn't."`
.
It is sometimes useful to use string templates in conjunction with the fn:char
function
to build strings containing special characters, for example `Chapter{fn:char("nbsp")}{$chapNr}`
.
Note:
String literals containing an ampersand behave differently between XPath and XQuery: in XPath
(unless first expanded by an XML parser) the string literal "Bacon & Eggs"
represents a string containing an ampersand, while in XQuery
it is an error, because an ampersand is taken as introducing a character reference. This difference
does not arise for string templates, since neither XPath nor XQuery recognizes entity or character references
in a string template.
This means that back-tick delimited strings (such as `Bacon & Eggs`
)
may be useful in contexts where an XPath expression
is required to have the same effect whether it is evaluated using an XPath or an XQuery processor.
In XQuery, the token ``[
is recognized as the start of a
StringConstructor
,
under the “longest token” rule (see A.3 Lexical structure). This means that the construct
``[1]
is not recognized as a StringTemplate followed by a predicate.
Although
the token ``[
is not used in XPath, it is reserved for compatibility reasons, and must
be rejected as syntactically invalid. In the unlikely event that an empty StringTemplate
followed by a predicate is wanted, whitespace or parentheses can be used to avoid the tokenization problem.
Comparison expressions allow two values to be compared. XPath 4.0 provides three kinds of comparison expressions, called value comparisons, general comparisons, and node comparisons.
[32] | ComparisonExpr |
::= |
OtherwiseExpr ( (ValueComp
|
|
[52] | ValueComp |
::= | "eq" | "ne" | "lt" | "le" | "gt" | "ge" |
|
[51] | GeneralComp |
::= | "=" | "!=" | "<" | "<=" | ">" | ">=" |
|
[53] | NodeComp |
::= | "is" | "<<" | ">>" |
Note:
When an XPath expression is written
within an XML document, the XML escaping rules for special characters
must be followed; thus <
must be written as
<
.
For a summary of the differences between different ways of comparing atomic values in XPath 4.0, see H Atomic Comparisons: An Overview.
The value comparison operators are eq
, ne
, lt
, le
, gt
, and ge
. Value comparisons are used for comparing single values.
The first step in evaluating a value comparison is to evaluate its operands. The order in which the operands are evaluated is implementation-dependent. Each operand is evaluated by applying the following steps, in order:
Atomization is applied to each operand. The result of this operation is called the atomized operand.
If an atomized operand is an empty sequence, the result of the value comparison is an empty sequence, and the implementation need not evaluate the other operand or apply the operator. However, an implementation may choose to evaluate the other operand in order to determine whether it raises an error.
If an atomized operand is a sequence of length greater than one, a type error is raised [err:XPTY0004].
If an atomized operand is of type
xs:untypedAtomic
, it is cast to
xs:string
.
Note:
The purpose of this rule is to
make value comparisons transitive. Users should be aware that the
general comparison operators have a different rule for casting of
xs:untypedAtomic
operands. Users should also be aware
that transitivity of value comparisons may be compromised by loss of
precision during type conversion (for example, two
xs:integer
values that differ slightly may both be
considered equal to the same xs:float
value because
xs:float
has less precision than
xs:integer
).
If the two operands are instances of different primitive types (meaning the 19 primitive types defined in Section 3.2 Primitive datatypesXS2), then:
If each operand is an instance of one of the types xs:string
or xs:anyURI
, then both operands are cast to type xs:string
.
If each operand is an instance of one of the types xs:decimal
or xs:float
, then both operands are cast to type xs:float
.
If each operand is an instance of one of the types xs:decimal
, xs:float
, or xs:double
, then both operands are cast to type xs:double
.
Otherwise, a type error is raised [err:XPTY0004].
Note:
The primitive type of an xs:integer
value for this purpose is xs:decimal
.
Finally, if the types of the operands are a valid combination for the given operator, the operator is applied to the operands.
The combinations of atomic types that are accepted by the various value comparison operators, and their respective result types, are listed in B.2 Operator Mapping together with the operator functions that define the semantics of the operator for each type combination. The definitions of the operator functions are found in [XQuery and XPath Functions and Operators 4.0].
Informally, if both atomized operands consist of exactly one atomic
value, then the result of the comparison is true
if the value of the
first operand is (equal, not equal, less than, less than or equal,
greater than, greater than or equal) to the value of the second
operand; otherwise the result of the comparison is false
.
If the types of the operands, after evaluation, are not a valid combination for the given operator, according to the rules in B.2 Operator Mapping, a type error is raised [err:XPTY0004].
Here are some examples of value comparisons:
The following comparison atomizes the node(s) that are returned by the expression $book/author
. The comparison is true only if the result of atomization is the value "Kennedy" as an instance of xs:string
or xs:untypedAtomic
. If the result of atomization is an empty sequence, the result of the comparison is an empty sequence. If the result of atomization is a sequence containing more than one value, a type error is raised [err:XPTY0004].
$book1/author eq "Kennedy"
The following comparison is true
because atomization converts an array to its member sequence:
[ "Kennedy" ] eq "Kennedy"
The following path expression contains a predicate that selects products whose weight is greater than 100. For any product that does not have a weight
subelement, the value of the predicate is the empty sequence, and the product is not selected. This example assumes that weight
is a validated element with a numeric type.
//product[weight gt 100]
The following comparison is true if my:hatsize
and my:shoesize
are both user-defined types that are derived by restriction from a primitive numeric type:
my:hatsize(5) eq my:shoesize(5)
The following comparison is true. The eq
operator compares two QNames by performing codepoint-comparisons of their namespace URIs and their local names, ignoring their namespace prefixes.
QName("http://example.com/ns1", "this:color") eq QName("http://example.com/ns1", "that:color")
The general comparison operators are =
, !=
, <
, <=
, >
, and >=
. General comparisons are existentially quantified comparisons that may be applied to operand sequences of any length. The result of a general comparison that does not raise an error is
always true
or false
.
If XPath 1.0 compatibility mode is true
, a general comparison is evaluated by applying the following rules, in order:
If either operand is a single atomic value that is an instance of
xs:boolean
, then the other operand is converted to xs:boolean
by taking its
effective boolean value.
Atomization is applied to each operand. After atomization, each operand is a sequence of atomic values.
If the comparison operator is <
, <=
, >
, or >=
, then each item in both of the
operand sequences is converted to the type xs:double
by applying the
fn:number
function. (Note that fn:number
returns the value NaN
if its operand cannot be converted to a number.)
The result of the comparison is true
if and only if there is a pair of
atomic values, one in the first operand sequence and the other in the second operand sequence, that have the required
magnitude relationship. Otherwise the result of the comparison is
false
or an error. The magnitude relationship between two atomic values is determined by
applying the following rules. If a cast
operation called for by these rules is not successful, a dynamic error is raised. [err:FORG0001]FO40
If at least one of the two atomic values is an instance of a numeric type, then both atomic values are converted to the type xs:double
by
applying the fn:number
function.
If at least one of the two atomic values is an instance of xs:string
,
or if both atomic values are instances of xs:untypedAtomic
, then both
atomic values are cast to the type xs:string
.
If one of the atomic values is an instance of xs:untypedAtomic
and the other is not an instance of xs:string
, xs:untypedAtomic
, or any numeric type, then the xs:untypedAtomic
value is
cast to the dynamic type of the other value.
After performing the conversions described above, the atomic values are
compared using one of the value comparison operators eq
, ne
, lt
, le
, gt
, or
ge
, depending on whether the general comparison operator was =
, !=
, <
, <=
,
>
, or >=
. The values have the required magnitude relationship if and only if the result
of this value comparison is true
.
If XPath 1.0 compatibility mode is false
, a
general comparison is evaluated by applying the following rules, in order:
Atomization is applied to each operand. After atomization, each operand is a sequence of atomic values.
The result of the comparison is true
if and only if there is a pair of
atomic values, one in the first operand sequence and the other in the second operand sequence, that have the required
magnitude relationship. Otherwise the result of the comparison is
false
or an error. The magnitude relationship between two atomic values is determined by
applying the following rules. If a cast
operation called for by these rules is not successful, a dynamic error is raised. [err:FORG0001]FO40
If both atomic values are instances of xs:untypedAtomic
,
then the values are cast to the type xs:string
.
If exactly one of the atomic values is an instance of
xs:untypedAtomic
, it is cast to a type depending on
the other value’s dynamic type T according to the following rules,
in which V denotes the value to be cast:
If T is a numeric type or is derived from a numeric type,
then V is cast to xs:double
.
If T is xs:dayTimeDuration
or is derived from
xs:dayTimeDuration
,
then V is cast to xs:dayTimeDuration
.
If T is xs:yearMonthDuration
or is derived from
xs:yearMonthDuration
,
then V is cast to xs:yearMonthDuration
.
In all other cases, V is cast to the primitive base type of T.
Note:
The special treatment of the duration types is required to avoid
errors that may arise when comparing the primitive type
xs:duration
with any duration type.
After performing the conversions described above, the atomic values are
compared using one of the value comparison operators eq
, ne
, lt
, le
, gt
, or
ge
, depending on whether the general comparison operator was =
, !=
, <
, <=
,
>
, or >=
. The values have the required magnitude relationship if and only if the result
of this value comparison is true
.
When evaluating a general comparison in which either operand is a sequence of items, an implementation may return true
as soon as it finds an item in the first operand and an item in the second operand that have the required magnitude relationship. Similarly, a general comparison may raise a dynamic error as soon as it encounters an error in evaluating either operand, or in comparing a pair of items from the two operands. As a result of these rules, the result of a general comparison is not deterministic in the presence of errors.
Here are some examples of general comparisons:
The following comparison is true if the typed value of any
author
subelement of $book1
is "Kennedy" as an instance of xs:string
or xs:untypedAtomic
:
$book1/author = "Kennedy"
The following comparison is true
because atomization converts an array to its member sequence:
[ "Obama", "Nixon", "Kennedy" ] = "Kennedy"
The following example contains three general comparisons. The value of the first two comparisons is true
, and the value of the third comparison is false
. This example illustrates the fact that general comparisons are not transitive.
(1, 2) = (2, 3) (2, 3) = (3, 4) (1, 2) = (3, 4)
The following example contains two general comparisons, both of which are true
. This example illustrates the fact that the =
and !=
operators are not inverses of each other.
(1, 2) = (2, 3) (1, 2) != (2, 3)
Suppose that $a
, $b
, and $c
are bound to element nodes with type annotation xs:untypedAtomic
, with string values
"1"
, "2"
, and "2.0"
respectively. Then ($a, $b) = ($c, 3.0)
returns false
, because $b
and $c
are compared as strings. However, ($a, $b) = ($c, 2.0)
returns true
, because $b
and 2.0
are compared as numbers.
Node comparisons are used to compare two nodes, by their identity or by their document order. The result of a node comparison is defined by the following rules:
The operands of a node comparison are evaluated in implementation-dependent order.
If either operand is an empty sequence, the result of the comparison is an empty sequence, and the implementation need not evaluate the other operand or apply the operator. However, an implementation may choose to evaluate the other operand in order to determine whether it raises an error.
Each operand must be either a single node or an empty sequence; otherwise a type error is raised [err:XPTY0004].
A comparison with the is
operator is true
if the two operand nodes are the same node; otherwise it
is false
. See [XQuery and XPath Data Model (XDM) 4.0] for the definition of node identity.
A comparison with the <<
operator returns true
if the left operand node precedes the right operand node in
document order; otherwise it returns false
.
A comparison with the >>
operator returns true
if the left operand node follows the right operand node in
document order; otherwise it returns false
.
Here are some examples of node comparisons:
The following comparison is true only if the left and right sides each evaluate to exactly the same single node:
/books/book[isbn="1558604820"] is /books/book[call="QA76.9 C3845"]
The following comparison is true only if the node identified by the left side occurs before the node identified by the right side in document order:
/transactions/purchase[parcel="28-451"] << /transactions/sale[parcel="33-870"]
A logical expression is either an and-expression or
an or-expression. If a logical expression does not raise an error, its value is always one
of the boolean values true
or false
.
[30] | OrExpr |
::= |
AndExpr ( "or" AndExpr )* |
|
[31] | AndExpr |
::= |
ComparisonExpr ( "and" ComparisonExpr )* |
The first step in evaluating a logical expression is to find the effective boolean value of each of its operands (see 2.5.3 Effective Boolean Value).
The value of an and-expression is determined by the effective boolean values (EBVs) of its operands, as shown in the following table:
AND: | EBV2 =
true
|
EBV2 = false
|
error in EBV2 |
---|---|---|---|
EBV1 =
true
|
true
|
false
|
error |
EBV1
= false
|
false
|
false
|
if XPath 1.0 compatibility mode is true , then false ; otherwise either false or error.
|
error in EBV1 | error |
if XPath 1.0 compatibility mode is true , then error; otherwise either false or error.
|
error |
The value of an or-expression is determined by the effective boolean values (EBVs) of its operands, as shown in the following table:
OR: | EBV2 =
true
|
EBV2 = false
|
error in EBV2 |
---|---|---|---|
EBV1 =
true
|
true
|
true
|
if XPath 1.0 compatibility mode is true , then true ; otherwise either true or error.
|
EBV1 =
false
|
true
|
false
|
error |
error in EBV1 |
if XPath 1.0 compatibility mode is true , then error; otherwise either true or error.
|
error | error |
If XPath 1.0 compatibility mode is true
, the order in which the operands of a logical expression are evaluated is effectively prescribed. Specifically, it is defined that when there is no
need to evaluate the second operand in order to determine the result, then
no error can occur as a result of evaluating the second operand.
If XPath 1.0 compatibility mode is false
, the
order in which the operands of a logical expression are evaluated is
implementation-dependent. In this case,
an or-expression can return true
if the first
expression evaluated is true, and it can raise an error if evaluation
of the first expression raises an error. Similarly, an and-expression
can return false
if the first expression evaluated is
false, and it can raise an error if evaluation of the first expression
raises an error. As a result of these rules, a logical expression is
not deterministic in the presence of errors, as illustrated in the examples
below.
Here are some examples of logical expressions:
The following expressions return
true
:
1 eq 1 and 2 eq 2
1 eq 1 or 2 eq 3
The following
expression may return either false
or raise a dynamic error
(in XPath 1.0 compatibility mode, the result must be false
):
1 eq 2 and 3 idiv 0 = 1
The
following expression may return either true
or raise a
dynamic error
(in XPath 1.0 compatibility mode, the result must be true
):
1 eq 1 or 3 idiv 0 = 1
The following expression must raise a dynamic error:
1 eq 1 and 3 idiv 0 = 1
In addition to and- and or-expressions, XPath 4.0 provides a
function named fn:not
that takes a general sequence as
parameter and returns a boolean value. The fn:not
function
is defined in [XQuery and XPath Functions and Operators 4.0]. The
fn:not
function reduces its parameter to an effective boolean value. It then returns
true
if the effective boolean value of its parameter is
false
, and false
if the effective boolean
value of its parameter is true
. If an error is
encountered in finding the effective boolean value of its operand,
fn:not
raises the same error.
XPath provides an iteration facility called a for expression. It can be used to iterate over the items of a sequence, or the members of an array.
[14] | ForExpr |
::= |
SimpleForClause
SimpleReturnClause
|
|
[15] | SimpleForClause |
::= | "for" SimpleForBinding (("for" | ",") SimpleForBinding)* |
|
[16] | SimpleForBinding |
::= | "member"? "$" VarName "in" ExprSingle
|
|
[17] | SimpleReturnClause |
::= | "return" ExprSingle
|
A for
expression is evaluated as follows:
If the for
expression uses multiple variables,
it is first expanded to a set of nested for
expressions, each of which uses only one variable.
More specifically, every separating comma or for
keyword is replaced by return for
.
For example, the expression
for $x in X, $y in Y return $x + $y
is expanded to
for $x in X return for $y in Y return $x + $y
.
Similarly, the expression
for member $x in X for member $y in Y return $x + $y
is expanded to
for member $x in X return for member $y in Y return $x + $y
.
In a single-variable for
expression,
the variable is called the range variable,
the expression that follows the in
keyword is called the binding expression,
and the expression that follows the return
keyword is called the return expression.
[Definition: The result of evaluating
the binding expression in a for
expression is called the
binding collection
].
When the member
keyword is absent,
the result of the single-variable for
expression is obtained by evaluating the
return
expression once
for each item in the binding collection, with the range variable bound to that item.
The resulting sequences are concatenated (as if by the comma operator) in the order of the items in the binding collection from which they were derived.
When the member
keyword is present,
the value of the binding collection must be a single array.
Otherwise, a type error is raised: [err:XPTY0141].
The result of the single-variable for member
expression is obtained by evaluating the return
expression once
for each member of that array, with the range variable bound to that member (recall that this can be any sequence,
not necessarily a single item). The resulting sequences
are concatenated (as if by the comma operator) in the order of the members of the binding collection from
which they were derived.
Note that the result is a sequence, not an array.
The following example illustrates the use of a for
expression in restructuring an input document. The example is based on the following
input:
<bib> <book> <title>TCP/IP Illustrated</title> <author>Stevens</author> <publisher>Addison-Wesley</publisher> </book> <book> <title>Advanced Programming in the Unix Environment</title> <author>Stevens</author> <publisher>Addison-Wesley</publisher> </book> <book> <title>Data on the Web</title> <author>Abiteboul</author> <author>Buneman</author> <author>Suciu</author> </book> </bib>
The following example transforms the input document into a list in
which each author’s name appears only once, followed by a list of
titles of books written by that author. This example assumes that the
context value is the bib
element in the input
document.
for $a in distinct-values(book/author)
return ((book/author[. = $a])[1], book[author = $a]/title)
The result of the above expression consists of the following
sequence of elements. The titles of books written by a given author
are listed after the name of the author.
The ordering of author
elements in the result is implementation-dependent due to the semantics of the fn:distinct-values
function.
<author>Stevens</author> <title>TCP/IP Illustrated</title> <title>Advanced Programming in the Unix environment</title> <author>Abiteboul</author> <title>Data on the Web</title> <author>Buneman</author> <title>Data on the Web</title> <author>Suciu</author> <title>Data on the Web</title>
The following example illustrates a for
expression containing more than one variable:
for $i in (10, 20),
$j in (1, 2)
return ($i + $j)
The result of the above expression, expressed as a sequence of numbers, is as follows: 11, 12, 21, 22
The scope of a variable bound in a for
expression comprises all subexpressions of the for
expression
that appear after the variable binding. The scope does not
include the expression to which the variable is bound. The following example illustrates how a variable binding may reference another variable bound earlier in the same for
expression:
for $x in $z, $y in f($x)
return g($x, $y)
The following example illustrates processing of an array.
for member $map in parse-json('[{"x":1, "y":2}, {"x":10, "y":20}]') return $map!(?x+?y)
The result is the sequence (3, 30)
.
Note:
The focus for evaluation of the return
clause of a for
expression
is the same as the focus for evaluation of the for
expression itself. The
following example, which attempts to find the total value of a set of
order-items, is therefore incorrect:
sum(for $i in order-item return @price * @qty)
Instead, the expression must be written to use the variable bound in the for
clause:
sum(for $i in order-item return $i/@price * $i/@qty)
Note:
XPath 4.0 allows the format:
for $order in //orders for $line in $order/order-line return $line/value
primarily because it is familiar to XQuery users, some of whom may regard it as more
readable than the XPath 3.1 alternative which uses a comma in place of the second for
.
XPath allows a variable to be declared and bound to a value using a let expression.
[18] | LetExpr |
::= |
SimpleLetClause
SimpleReturnClause
|
|
[19] | SimpleLetClause |
::= | "let" SimpleLetBinding (("let" | ",") SimpleLetBinding)* |
|
[20] | SimpleLetBinding |
::= | "$" VarName ":=" ExprSingle
|
|
[17] | SimpleReturnClause |
::= | "return" ExprSingle
|
A let expression is evaluated as follows:
If the let expression uses multiple variables, it is first expanded to a
set of nested let expressions, each of which uses only one variable.
Specifically, any separating comma or let
keyword is replaced by return let
.
For example, the expression
let $x := 4, $y := 3 return $x + $y
is expanded to
let $x := 4 return let $y := 3 return $x + $y
.
Likewise, the expression
let $x := 4 let $y := 3 return $x + $y
is expanded to
let $x := 4 return let $y := 3 return $x + $y
.
In a single-variable let expression, the variable is called the range
variable, the value of the expression that follows the :=
symbol is called
the binding sequence, and the expression that follows the return keyword is
called the return expression. The result of the let expression is obtained
by evaluating the return expression with the range variable bound to the
binding sequence.
The scope of a variable bound in a let expression comprises all subexpressions of the let expression that appear after the variable binding. The scope does not include the expression to which the variable is bound. The following example illustrates how a variable binding may reference another variable bound earlier in the same let expression:
let $x := doc('a.xml')/*, $y := $x//* return $y[@value gt $x/@min]
Note:
It is not required that the variables should have distinct names. It is permitted, for example, to write:
let $x := "[A fine romance]" let $x := substring-after($x, "[") let $x := substring-before($x, "]") return upper-case($x)
which returns the result "A FINE ROMANCE"
. Note that this expression declares
three separate variables which happen to have the same name; it should not be read as declaring
a single variable and binding it successively to different values.
Note:
XPath 4.0 allows the form using multiple let
clauses, as illustrated in the previous example,
primarily because it is familiar to XQuery users, some of whom may regard it as more
readable than the XPath 3.1 alternative which uses a comma in place of the second and
subsequent let
keywords.
Most modern programming languages have support for collections of key/value pairs, which may be called maps, dictionaries, associative arrays, hash tables, keyed lists, or objects (these are not the same thing as objects in object-oriented systems). In XPath 4.0, we call these maps. Most modern programming languages also support ordered lists of values, which may be called arrays, vectors, or sequences. In XPath 4.0, we have both sequences and arrays. Unlike sequences, an array is an item, and can appear as an item in a sequence.
Note:
The XPath 4.0 specification focuses on syntax provided for maps and arrays, especially constructors and lookup.
Some of the functionality typically needed for maps and arrays is provided by functions defined in [TITLE OF FO40 SPEC, TITLE OF maps-and-arrays SECTION]FO40, including functions used to read JSON to create maps and arrays, serialize maps and arrays to JSON, combine maps to create a new map, remove map entries to create a new map, iterate over the keys of a map, convert an array to create a sequence, combine arrays to form a new array, and iterate over arrays in various ways.
[Definition: A map is a function that associates a set of keys with values, resulting in a collection of key / value pairs.] [Definition: Each key / value pair in a map is called an entry.] [Definition: The value associated with a given key is called the associated value of the key.]
A Map is created using a MapConstructor.
[99] | MapConstructor |
::= | "map" "{" (MapConstructorEntry ("," MapConstructorEntry)*)? "}" |
|
[100] | MapConstructorEntry |
::= |
MapKeyExpr ":" MapValueExpr
|
|
[101] | MapKeyExpr |
::= |
ExprSingle
|
|
[102] | MapValueExpr |
::= |
ExprSingle
|
Note:
In some circumstances, it is necessary to include whitespace before or after the colon of a MapConstructorEntry to ensure that it is parsed as intended.
For instance, consider the expression map{a:b}
.
Although it matches the EBNF for MapConstructor
(with a
matching MapKeyExpr and b
matching MapValueExpr),
the "longest possible match" rule requires that a:b
be parsed as a QName,
which results in a syntax error.
Changing the expression to map{a :b}
or map{a: b}
will prevent this, resulting in the intended parse.
Similarly, consider these three expressions:
map{a:b:c} map{a:*:c} map{*:b:c}
In each case, the expression matches the EBNF in two different ways,
but the “longest possible match” rule forces the parse in which
the MapKeyExpr is a:b
, a:*
, or *:b
(respectively)
and the MapValueExpr is c
.
To achieve the alternative parse
(in which the MapKeyExpr is merely a
or *
),
insert whitespace before and/or after the first colon.
The value of the expression is a map whose entries correspond to the key-value pairs obtained by evaluating the successive MapKeyExpr and MapValueExpr expressions.
Each MapKeyExpr expression is evaluated
and atomized; a
type error
[err:XPTY0004] occurs if the result is
not a single atomic value.
The associated value is the
result of evaluating the corresponding
MapValueExpr.
If the MapValueExpr evaluates to a node, the
associated value is the node itself, not a new node with the same
values.
[Definition: Two atomic values K1
and
K2
have the same key value if
fn:atomic-equal(K1, K2)
returns true
, as specified in Section 14.2.1 fn:atomic-equalFO40
]
If two or more entries have the same key value then a dynamic
error is raised [err:XQDY0137].
Example:
The following expression constructs a map with seven entries:
map { "Su" : "Sunday", "Mo" : "Monday", "Tu" : "Tuesday", "We" : "Wednesday", "Th" : "Thursday", "Fr" : "Friday", "Sa" : "Saturday" }
Maps can nest, and can contain any XDM value. Here is an example of a nested map with values that can be string values, numeric values, or arrays:
Maps are function items, and
a dynamic function call can be used to look up
the value associated with a key in a map.
If $map
is a map and $key
is a key,
then $map($key)
is equivalent to map:get($map, $key)
.
The semantics of such a function call are formally defined in
Section 18.2.9 map:getFO40.
Examples:
$weekdays("Su")
returns the associated value of the key Su
.
$books("Green Eggs and Ham")
returns associated value of the key Green Eggs and Ham
.
Note:
XPath 4.0 also provides an alternate syntax for map and array lookup that is more terse, supports wildcards, and allows lookup to iterate over a sequence of maps or arrays. See 4.15.3 The Lookup Operators ("?" and "??") for Maps and Arrays for details.
Map lookups can be chained.
Examples: (These examples assume that $b
is bound to the books map from the previous section)
The expression $b("book")("title")
returns the string Data on the Web
.
The expression $b("book")("author")
returns the array of authors.
The expression $b("book")("author")(1)("last")
returns the string Abiteboul
.
(This example combines 4.15.2.2 Array Lookup using Function Call Syntax with map lookups.)
[Definition: An array is a function item that associates a set of positions, represented as positive integer keys, with values.] The first position in an array is associated with the integer 1. [Definition: The values of an array are called its members.] In the type hierarchy, array has a distinct type, which is derived from function. Atomization converts arrays to sequences (see Atomization).
An array is created using an ArrayConstructor.
[103] | ArrayConstructor |
::= |
SquareArrayConstructor | CurlyArrayConstructor
|
|
[104] | SquareArrayConstructor |
::= | "[" (ExprSingle ("," ExprSingle)*)? "]" |
|
[105] | CurlyArrayConstructor |
::= | "array" EnclosedExpr
|
If a member of an array is a node, its node identity is preserved. In both forms of an ArrayConstructor, if a member expression evaluates to a node, the associated value is the node itself, not a new node with the same values. If the member expression evaluates to a map or array, the associated value is a new map or array with the same values.
A SquareArrayConstructor consists of a comma-delimited set of argument expressions. It returns an array in which each member contains the value of the corresponding argument expression.
Examples:
[ 1, 2, 5, 7 ]
creates an array with four members: 1
, 2
, 5
, and 7
.
[ (), (27, 17, 0)]
creates an array with two members: ()
and the sequence (27, 17, 0)
.
[ $x, local:items(), <tautology>It is what it is.</tautology> ]
creates an array with three members: the value of $x, the result of evaluating the function call, and a tautology element.
A CurlyArrayConstructor can use any expression to create its members. It evaluates its operand expression to obtain a sequence of items and creates an array with these items as members. Unlike a SquareArrayConstructor, a comma in a CurlyArrayConstructor is the comma operator, not a delimiter.
Examples:
array { $x }
creates an array with one member for each item in the sequence to which $x is bound.
array { local:items() }
creates an array with one member for each item in the sequence to which local:items()
evaluates.
array { 1, 2, 5, 7 }
creates an array with four members: 1
, 2
, 5
, and 7
.
array { (), (27, 17, 0) }
creates an array with three members: 27
, 17
, and 0
.
array{ $x, local:items(), <tautology>It is what it is.</tautology> }
creates an array with the following members: the items to which $x
is bound, followed by the items to which local:items()
evaluates, followed by a tautology element.
Note:
XPath 4.0 does not provide explicit support for sparse arrays. Use integer-valued maps to represent sparse arrays,
for example: map { 27 : -1, 153 : 17 }
.
Arrays are function items,
and a dynamic function call can be used to look up
the value associated with position in an array.
If $array
is an array and $index
is an integer corresponding to a position in the array,
then $array($key)
is equivalent to array:get($array, $key)
.
The semantics of such a function call are formally defined in
Section 19.1.11 array:getFO40.
Examples:
[ 1, 2, 5, 7 ](4)
evaluates to 7
.
[ [1, 2, 3], [4, 5, 6]](2)
evaluates to [4, 5, 6]
.
[ [1, 2, 3], [4, 5, 6]](2)(2)
evaluates to 5
.
[ 'a', 123, <name>Robert Johnson</name> ](3)
evaluates to <name>Robert Johnson</name>
.
array { (), (27, 17, 0) }(1)
evaluates to 27
.
array { (), (27, 17, 0) }(2)
evaluates to 17
.
array { "licorice", "ginger" }(20)
raises a dynamic error [err:FOAY0001]FO40.
Note:
XPath 4.0 also provides an alternate syntax for map and array lookup that is more terse, supports wildcards, and allows lookup to iterate over a sequence of maps or arrays. See 4.15.3 The Lookup Operators ("?" and "??") for Maps and Arrays for details.
XPath 4.0 provides two lookup operators for maps and arrays. These provide a terse syntax for accessing the entries in a map or the members of an array.
The operator "?", known as the shallow lookup operator, returns values found immediately in the operand map or array. The operator "??", known as the deep lookup operator, also searches nested maps and arrays. The effect of the deep lookup operator "??" is explained in 4.15.3.3 Deep Lookup.
[80] | LookupExpr |
::= |
PostfixExpr
Lookup
|
|
[81] | Lookup |
::= | ("?" | "??") KeySpecifier
|
|
[82] | KeySpecifier |
::= |
NCName | IntegerLiteral | StringLiteral | VarRef | ParenthesizedExpr | LookupWildcard
|
|
[83] | LookupWildcard |
::= | "*" ("::" SequenceType)? |
The semantics of a postfix lookup expression E?KS
are defined as follows:
E is evaluated to produce a value $V
.
If $V
is not a singleton (that is if count($V) ne 1
),
then the result (by recursive application of these rules) is the value of
for $v in $V return $v?KS
.
If $V
is a singleton array (that is,
if $V instance of array(*)
) then:
If KS
is a ParenthesizedExpr
,
then it is evaluated to produce a value $K
and the result is:
for $k in data($K) return array:get($V, $k)
Note:
The focus for evaluating the key specifier expression is the
same as the focus for the Lookup
expression itself.
If the KeySpecifier is an IntegerLiteral with value $i
,
the result is array:get($V, $i)
If the KeySpecifier is an NCName
or StringLiteral
,
the expression raises a type error [err:XPTY0004].
If KS
is a wildcard
(*
) qualified by a SequenceType ST,
the result is given by the following expression:
for $k in 1 to array:size($V) let $m := array:get($V, $k) return if ($m instance of ST) {$m}
If no SequenceType is provided, it defaults to item()*
.
Note:
Note that array items are returned in order.
If $V is a singleton map (that is, if $V instance of map(*)
)
then:
If KS
is a ParenthesizedExpr
,
then it is evaluated to produce a value $K
and the result is:
for $k in data($K) return map:get($V, $k)
Note:
The focus for evaluating the key specifier expression is the
same as the focus for the Lookup
expression itself.
If KS
is an NCName
or a StringLiteral
,
the result is map:get($V, $k)
, where $k
is the value of the NCName
or StringLiteral
.
If KS
is an IntegerLiteral
,
the result is map:get($V, $k)
, where $k
is the value of the IntegerLiteral
.
If the KeySpecifier is a wildcard (*
) qualified by a SequenceType ST,
the result is given by the following expression:
map:for-each($V, function($k, $v){if ($v instance of ST) {$v}})
If no SequenceType is provided, it defaults to item()*
.
Note:
The order of entries in map:for-each
is implementation-dependent,
so the order of values in the result sequence is also implementation-dependent.
Otherwise (that is, if $V
is neither a map nor an array)
a type error is raised [err:XPTY0004].
Examples:
map { "first" : "Jenna", "last" : "Scott" }?first
evaluates to "Jenna"
map { "first name" : "Jenna", "last name" : "Scott" }?"first name"
evaluates to "Jenna"
[4, 5, 6]?2
evaluates to 5
.
(map {"first": "Tom"}, map {"first": "Dick"}, map {"first": "Harry"})?first
evaluates to the sequence ("Tom", "Dick", "Harry")
.
([1,2,3], [4,5,6])?2
evaluates to the sequence (2, 5)
.
["a","b"]?3
raises a dynamic error [err:FOAY0001]FO40
[109] | UnaryLookup |
::= | ("?" | "??") KeySpecifier
|
|
[82] | KeySpecifier |
::= |
NCName | IntegerLiteral | StringLiteral | VarRef | ParenthesizedExpr | LookupWildcard
|
|
[83] | LookupWildcard |
::= | "*" ("::" SequenceType)? |
Unary lookup is most commonly used in predicates (for example, $map[?name='Mike']
)
or with the simple map operator (for example, avg($maps ! (?price - ?discount))
).
The unary lookup expression ?KS
is defined to be equivalent to the postfix lookup
expression .?KS
which has the context value (.
) as the implicit first operand.
See 4.15.3.1 Postfix Lookup Expressions for the postfix lookup operator.
Examples:
?name
is equivalent to .("name")
, an appropriate lookup for a map.
?2
is equivalent to .(2)
, an appropriate lookup for an array or an integer-valued map.
If the context item is the result of parsing the JSON input:
{ "name": "John Smith", "address": {"street": "18 Acacia Avenue", "postcode": "MK12 2EX"}, "previous-address": {"street": "12 Seaview Road", "postcode": "EX8 9AA"} }
then ?*::record(street, postcode)?postcode
returns ("MK12 2EX", "EX8 9AA")
(or some permutation thereof).
Note:
Writing ?*?postcode
would raise a type error, because the result of the initial
step ?*
includes an item (the string "John Smith"
) that is neither
a map nor an array.
?"first name"
is equivalent to .("first name")
?($a)
and ?$a
are
equivalent to for $k in $a return .($k)
, allowing keys for an array or map to be passed using a variable.
?(2 to 4)
is equivalent to for $k in (2,3,4) return .($k)
, a convenient way to return a range of values from an array.
?(3.5)
raises a type error if the context value is an array because the parameter must be an integer.
([1,2,3], [1,2,5], [1,2])[?3 = 5]
raises an error because ?3
on one of the
items in the sequence fails.
If $m
is bound to the weekdays map described in 4.15.1 Maps, then $m?*
returns the values ("Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday")
, in implementation-dependent order.
[1, 2, 5, 7]?*
evaluates to (1, 2, 5, 7)
.
[[1, 2, 3], [4, 5, 6]]?*
evaluates to ([1, 2, 3], [4, 5, 6])
[[1, 2, 3], 4, 5]?*::array(*)
evaluates to ([1, 2, 3])
[[1, 2, 3], [4, 5, 6], 7]?*::array(*)?2
evaluates to (2, 5)
[[1, 2, 3], 4, 5]?*::xs:integer
evaluates to (4, 5)
The deep lookup operator ??
has both unary and postfix forms. The unary form ??KS
(where KS is any KeySpecifier
) has the same effect as the binary form .??KS
.
The semantics are defined as follows.
First we define the recursive content of an item as follows:
declare function immediate-content($item as item()) as map(*)* { if ($item instance of map(*)) { map:entries($item) } else if ($item instance of array(*)) { for member $m at $p in $item return map:entry($p, $m) } }; declare function recursive-content($item as item()) as map(*)* { immediate-content($item) ! (., map:values(.) =!> recursive-content()) };
Note:
Explanation: the immediate content of a map is obtained by splitting it into a sequence of singleton maps, each representing one entry. The immediate content of an array is obtained by constructing a sequence of singleton maps, one for each array member, where the key of the map is the array index and the value is the corresponding member. The recursive content of an item contains the singleton maps in its immediate content, each followed by the recursive content obtained by expanding any maps or arrays in the immediate content.
In addition we define the function array-or-map
as follows:
declare function array-or-map($item as item()) { typeswitch ($item) { case array(*) | map(*) return $item default return error(xs:QName("err:XPTY0004")) } }
The result of the expression E??KS
, where E
is any expression
and KS
is any KeySpecifier
, is then:
(E =!> array-or-map() => recursive-content())?KS
Note:
This is best explained by considering examples.
Consider the expression let $value := [map{"first":"John", "last":"Smith"}, map{"first":"Mary", "last":"Evans"}]
.
The recursive content of this array is the sequence of six singleton maps:
map{1: map{"first":"John", "last":"Smith"}}
map{2: map{"first":"Mary", "last":"Evans"}}
map{"first":"John"}
map{"last":"Smith"}
map{"first":"Mary"}
map{"last":"Evans"}
The expression $value??*
returns the sequence
map{"first":"John", "last":"Smith"}, map{"first":"Mary", "last":"Evans"},
"John", "Smith", "Mary", "Evans"
.
The expression $value??first
returns the sequence "John", "Mary"
.
The expression $value??last
returns the sequence "Smith", "Evans"
.
The expression $value??1
returns the sequence map{"first":"John", "last":"Smith"}
.
Note:
The effect of evaluating all shallow lookups on maps rather than arrays is that no error arises
if an array subscript is out of bounds. In the above example, $value?3
would
return an empty sequence, it would not raise an error.
This also affects the way an xs:untypedAtomic
key value is handled.
Given the shallow lookup
expression $A?$x
, if $A
is an array and $x
(after atomization) is xs:untypedAtomic
then the value of $x
is converted to an integer (by virtue of the coercion rules applying to a call
on array:get
). With a deep lookup expression $A??$x
, by
contrast, the semantics are defined in terms of a map lookup, in which
xs:untypedAtomic
values are always treated as strings.
Note:
The definition of the recursive-content
function is such that items
in the top-level value that are not maps or arrays are ignored, whereas items that
are not themselves maps or arrays, but which appear in the content of a map or array
at the top level, are included. This means that E??X
mirrors the
behavior of E//X
, in that it includes all items that are one-or-more levels
deep in the tree.
Note:
The result of the deep lookup operator retains order when processing sequences and arrays, but not when processing maps.
Note:
An expression involving multiple deep lookup operators may return duplicates.
For example, the result of the expression
[[["a"],["b"]],[["c"],["d"]]] ?? 1 ?? 1
is (["a"], "a", "b", "a", "c")
. This is because the first ??
operator
selects members in position 1 at all three levels, that is it selects the arrays
[["a"],["b"]]
, ["a"]
, and ["c"]
as well
as each of the four string values. The second ??
operator
selects members in position 1 within each of these values, which results in the string
"a"
being selected twice.
Note:
A type error is raised if the value of the left-hand expression includes an item that is neither a map nor an array.
Consider the tree $tree
of maps and arrays that results from applying the fn:parse-json
function to the following input:
{ "desc" : "Distances between several cities, in kilometers.", "updated" : "2014-02-04T18:50:45", "uptodate": true, "author" : null, "cities" : { "Brussels": [ {"to": "London", "distance": 322}, {"to": "Paris", "distance": 265}, {"to": "Amsterdam", "distance": 173} ], "London": [ {"to": "Brussels", "distance": 322}, {"to": "Paris", "distance": 344}, {"to": "Amsterdam", "distance": 358} ], "Paris": [ {"to": "Brussels", "distance": 265}, {"to": "London", "distance": 344}, {"to": "Amsterdam", "distance": 431} ], "Amsterdam": [ {"to": "Brussels", "distance": 173}, {"to": "London", "distance": 358}, {"to": "Paris", "distance": 431} ] } }
Given two variables $from
and $to
containing the
names of two cities that are present in this table, the distance between the
two cities can be obtained with the expression:
$tree ??$from ??*::record(to, distance) [?to=$to] ?distance
The names of all pairs of cities whose distance is represented in the data can be obtained with the expression:
$tree ??$cities => map:for-each( fn($key, $val){$val ??to ! ($key || "-" || .)} )
This example provides XPath equivalents to some examples given in the JSONPath specification. [TODO: add a reference].
The examples query the result of parsing the following JSON value, representing a store whose stock consists of four books and a bicycle:
{ "store": { "book": [ { "category": "reference", "author": "Nigel Rees", "title": "Sayings of the Century", "price": 8.95 }, { "category": "fiction", "author": "Evelyn Waugh", "title": "Sword of Honour", "price": 12.99 }, { "category": "fiction", "author": "Herman Melville", "title": "Moby Dick", "isbn": "0-553-21311-3", "price": 8.99 }, { "category": "fiction", "author": "J. R. R. Tolkien", "title": "The Lord of the Rings", "isbn": "0-395-19395-8", "price": 22.99 } ], "bicycle": { "color": "red", "price": 399 } } }
The following table illustrates some queries on this data, expressed both in JSONPath and in XPath 4.0.
Query | JSONPath | XPath 4.0 |
---|---|---|
The authors of all books in the store |
$.store.book[*].author
|
$m?store?book??author
|
All authors |
$..author
|
$m??author
|
All things in store (four books and a red bicycle) |
$.store.*
|
$m?store?*
|
The prices of everything in the store |
$.store..price
|
$m?store??price
|
The third book |
$..book[2]
|
$m??book?3
|
The third book's author |
$..book[2].author
|
$m??book?3?author
|
The third book's publisher (empty result) |
$..book[2].publisher
|
$m??book?3?publisher
|
The last book (in order) |
$..book[-1]
|
$m??book => array:foot()
|
The first two books |
$..book[0,1]
|
$m??book?(1,2)
|
All books with an ISBN |
$..book[?@.isbn]
|
$m??book[?isbn]
|
All books cheaper than 10 |
$..book[?@.price<10]
|
$m??book[?price lt 10]
|
All member values and array elements contained in the input value |
$..*
|
$m??*
|
Under certain conditions a lookup expression that will never select anything is classified as implausible. During the static analysis phase, a processor may (subject to the rules in 2.4.6 Implausible Expressions) report a static error when such lookup expressions are encountered: [err:XPTY0145].
More specifically, a shallow unary or postfix lookup is classified as implausible if any of the following conditions applies:
The inferred type of the left-hand operand (or the context value, in the case
of a unary expression) is a record test (see 3.6.4.3 Record Test),
and the KeySpecifier
is an IntegerLiteral
.
The inferred type of the left-hand operand (or the context value, in the case
of a unary expression) is a record test (see 3.6.4.3 Record Test),
and the KeySpecifier
is an NCName
or StringLiteral
that cannot validly appear as a field name in the record.
The inferred type of the left-hand operand (or the context value, in the case
of a unary expression) is a map type,
and the inferred type of the KeySpecifier
, after coercion, is a type that
is disjoint with the key type of the map.
The inferred type of the left-hand operand (or the context value, in the case
of a unary expression) is an array type,
and the KeySpecifier
is the IntegerLiteral
0
(zero).
Note:
Other errors, such as using an NCName
KeySpecifier
for an array lookup, are handled under the general provisions for type errors.
Examples of implausible lookup expressions include the following:
parse-uri($uri)?3
: the declared result type of parse-uri
is a record
test, so the selector 3
will never select anything.
in-scope-namespaces($node)(current-date())
: the result type of
in-scope-namespaces
is a map with xs:string
keys, so the selector
current-date()
will never select anything.
array:subarray($a, 2, 5)?0
: the integer zero cannot select any member
of an array, because numbering starts at 1.
Unlike navigation within node trees derived from XML, navigation within a tree of maps and
arrays derived from JSON is normally “downwards only”: there is no equivalent of the parent
or ancestor axis. This means, for example, that having selected leav nodes in the tree
with an expression such as ??name
, there is no way of navigating from
the items in the result to any related items.
Pinned maps and arrays provide a solution to this problem; if a map or array
is pinned (by calling the fn:pin
function), then values found by navigating
within the map or array are labeled, which provides supplementary information
about their location within the containing tree structure.
For further information about pinned and labeled values see [TITLE OF DM40 SPEC, TITLE OF id-labeled-items SECTION]DM40.
More specifically, if a map $M
or an array $A
is pinned,
then any value returned by map:get($M, $key)
or array:get($A, $index)
will be a sequence of labeled items. The label can be obtained by calling the function
fn:label
, and the result will be a map having the following properties:
pinned
: set to true
. This means that any
further selections from this value (if it is itself a map or array) will
also deliver labeled items.
parent
: the containing map ($M
) or array
($A
).
key
: the key ($key
) or index ($index
)
that was used to select the item.
position
: in the general case the value returned by
map:get
or array:get
is a sequence, and each item in the
sequence is labeled with its 1-based position in that sequence.
ancestors
: a zero-arity function that delivers the item's parent (its
containing map or array), that item's parent, and so on, recursively, up to
the map or array that was the root of the selection. The sequence is in upwards
navigation order (the immediate parent comes first).
path
: a zero-arity function that delivers the sequence of
keys (in the case of maps) or integer indexes (in the case of arrays) by which the
item was reached. The sequence is in downwards navigation order (the immediate
key or index of the item comes last).
The formal model for the fn:pin
is that it returns a deep copy of the
supplied map or array in which all items in the recursive content have been labeled.
This is a useful model because it avoids the need to specify the effect of each individual
function and operator on the structure. For example, the rule has the consequence that the result of
pin([11,12,13,14]) => array:remove(3) => array:for-each(fn{label(.)?key})
is
[1,2,4]
. In a practical implementation, however, it is likely that labels
will be attached to items lazily, at the time they are retrieved. Such an implementation will need
to recognize pinned maps and arrays and treat them specially when operations such as
array:get
, array:remove
, array:for-each
,
array:subarray
, and their map counterparts, are evaluated.
Because maps and arrays selected from a pinned map or array are themselves pinned,
deep lookup operations (whether conducted using the deep lookup operator ??
,
or the map:find
function, or by user-written recursive code) will deliver
a labeled value whose parent
or ancestor
properties can
be used to navigate back up through the tree.
For example, given the example map shown in 4.15.1.1 Map Constructors,
the expression $map??last[. = "Suciu"]
selects the map entry with key
"last"
and value "Suciu"
, but by itself gives no information
about where this entry was found. By first pinning the map, this extra information
can be made available through the label on the result. For example you can select
all co-authors of "Suciu" by writing:
pin($map)??last[. = "Suciu"] => label()?ancestors()?author??last
Note:
When an entry in a map, or a member of an array, has the empty sequence
as its value, the value contains no items and is therefore unchanged in the pinned
version of the containing structure. In addition, the lookup operators ?
and ??
flatten their result to a single sequence, so any empty values
are effectively discarded from the result. For this reason, pinned arrays and maps
work best when all values in arrays and maps are singleton items. An option is therefore provided
on the fn:parse-json
and fn:json-doc
functions to change
the representation of JSON null
values (whose default is an empty
sequence, ()
) to a user-supplied value.
Editorial note | |
That note is anticipating a proposal in a separate PR. |
XPath 4.0 allows conditional expressions to be written in several different ways.
[23] | IfExpr |
::= | "if" "(" Expr ")" (UnbracedActions | BracedActions) |
|
[24] | UnbracedActions |
::= | "then" ExprSingle "else" ExprSingle
|
|
[25] | BracedActions |
::= |
ThenAction
ElseIfAction* ElseAction? |
|
[26] | ThenAction |
::= |
EnclosedExpr
|
|
[27] | ElseIfAction |
::= | "else" "if" "(" Expr ")" EnclosedExpr
|
|
[28] | ElseAction |
::= | "else" EnclosedExpr
|
|
[9] | EnclosedExpr |
::= | "{" Expr? "}" |
There are two formats with essentially the same semantics.
The unbraced expression if (C) then T else E
is equivalent to
the braced expression if (C) {T} else {E}
.
The value V of a conditional expression using the braced format is obtained by applying the following rules in order, finishing as soon as V has a value:
Let C be the effective boolean value of the test expression, as defined in 2.5.3 Effective Boolean Value.
If C is true, V is the value of the EnclosedExpr in the ThenAction.
The ElseIfActions (if any) are processed in order as follows:
Let C′ be the effective boolean value of the test expression, as defined in 2.5.3 Effective Boolean Value.
If C′ is true, V is the value of the EnclosedExpr in the ElseIfAction
If there is an ElseAction, then V is the value of its EnclosedExpr.
V is the empty sequence.
Conditional expressions have a special rule for propagating dynamic errors: expressions whose value is not needed for computing the result are guarded, as described in 2.4.5 Guarded Expressions, to prevent spurious dynamic errors.
Here are some examples of conditional expressions:
In this example, the test expression is a comparison expression:
if ($widget1/unit-cost < $widget2/unit-cost) then $widget1 else $widget2
In this example, the test expression tests for the existence of an attribute
named discounted
, independently of its value:
if ($part/@discounted) then $part/wholesale else $part/retail
The above expression can equivalently be written:
if ($part/@discounted) { $part/wholesale } else { $part/retail }
The following example returns the attribute node @discount
provided the value of @price
is greater than 100; otherwise it returns the empty sequence:
if (@price gt 100) {@discount}
The following example tests a number of conditions:
if (@code = 1) { "food" } else if (@code = 2) { "fashion" } else if (@code = 3) { "household" } else { "general" }
Note:
The “dangling else ambiguity” found in many other languages cannot arise:
In the unbraced format, both the then
and else
clauses
are mandatory.
In the braced format, an else
clause is always unambiguously
associated with the immediately containing IfExpr.
[33] | OtherwiseExpr |
::= |
StringConcatExpr ( "otherwise" StringConcatExpr )* |
The otherwise
expression returns the value of its first operand, unless this is an empty
sequence, in which case it returns the value of its second operand.
For example, @price - (@discount otherwise 0)
returns the value of @price - @discount
,
if the attribute @discount
exists, or the value of @price
if the @discount
attribute is absent.
To prevent spurious errors, the right hand operand is guarded: it cannot throw any dynamic error unless the left-hand operand returns an empty sequence.
Note:
The operator is associative (even under error conditions): A otherwise (B otherwise C)
returns
the same result as (A otherwise B) otherwise C
.
The otherwise
operator binds more tightly than comparison operators such as
=
, but less tightly than string concatenation (||
) or arithemetic
operators. The expression $a = @x otherwise @y + 1
parses as
$a = (@x otherwise (@y + 1))
.
Quantified expressions support existential and universal quantification. The
value of a quantified expression is always true
or false
.
[21] | QuantifiedExpr |
::= | ("some" | "every") QuantifierBinding ("," QuantifierBinding)* "satisfies" ExprSingle
|
|
[22] | QuantifierBinding |
::= | "$" VarName "in" ExprSingle
|
A quantified expression begins with
a quantifier, which is the keyword some
or every
,
followed by one or more in-clauses that are used to bind variables,
followed by the keyword satisfies
and a test expression. Each in-clause associates a variable with an
expression that returns a sequence of items, called the binding sequence for that variable.
The value of the quantified expression is defined by the following rules:
If the QuantifiedExpr contains
more than one QuantifierBinding, then it is equivalent
to the expression obtained by replacing each comma with satisfies some
or satisfies every
respectively. For example, the expression some $x in X, $y in Y satisfies $x = $y
is equivalent to some $x in X satisfies some $y in Y satisfies $x = $y
,
while the expression every $x in X, $y in Y satisfies $x lt $y
is equivalent to
every $x in X satisfies every $y in Y satisfies $x lt $y
If the quantifier is some
, the QuantifiedExpr returns true
if at least one evaluation of the test expression has the effective boolean value
true
; otherwise it returns false
. In consequence, if the binding sequence is empty,
the result of the QuantifiedExpr is false
.
If the quantifier is every
, the QuantifiedExpr returns true
if every evaluation of the test expression has the effective boolean value
true
; otherwise it returns false
. In consequence, if the binding sequence is empty,
the result of the QuantifiedExpr is true
.
The scope of a variable bound in a quantified expression comprises all subexpressions of the quantified expression that appear after the variable binding. The scope does not include the expression to which the variable is bound.
The order in which test expressions are evaluated
for the various items in the binding sequence is implementation-dependent. If the quantifier
is some
, an implementation may
return true
as soon as it finds one item for which the test expression has
an effective boolean value of true
, and it may raise a dynamic error as soon as it finds one item for
which the test expression raises an error. Similarly, if the quantifier is every
, an
implementation may return false
as soon as it finds one item for which the test expression has
an effective boolean value of false
, and it may raise a dynamic error as soon as it finds one item for
which the test expression raises an error. As a result of these rules, the
value of a quantified expression is not deterministic in the presence of
errors, as illustrated in the examples below.
Here are some examples of quantified expressions:
This expression is true
if every part
element has a discounted
attribute (regardless of the values of these attributes):
every $part in /parts/part satisfies $part/@discounted
This expression is true
if at least
one employee
element satisfies the given comparison expression:
some $emp in /emps/employee satisfies ($emp/bonus > 0.25 * $emp/salary)
This expression is true
if every
employee
element has at least one salary
child with the attribute current="true"
:
every $emp in /emps/employee satisfies some $sal in $emp/salary satisfies $sal/@current='true'
Note:
Like many quantified expressions, this can be simplified. This example can be written
every $emp in /emps/employee satisfies $emp/salary[@current='true']
, or even
more concisely as empty(/emps/employee[not(salary/@current='true')]
.
Another alternative in XPath 4.0 is to use the higher-order functions fn:some
and fn:every
.
This example can be written fn:every(/emps/employee, function{salary/@current='true'})
In the following examples, each quantified expression evaluates its test
expression over nine pairs of items, formed from the Cartesian
product of the sequences (1, 2, 3)
and (2, 3, 4)
.
The expression beginning with some
evaluates to true
,
and the expression beginning with every
evaluates to false
.
some $x in (1, 2, 3), $y in (2, 3, 4) satisfies $x + $y = 4
every $x in (1, 2, 3), $y in (2, 3, 4) satisfies $x + $y = 4
This quantified expression may either return true
or raise a type error, since its test expression returns true
for one item
and raises a type error for another:
some $x in (1, 2, "cat") satisfies $x * 2 = 4
This quantified expression may either return false
or raise a type error, since its test expression returns false
for one item and raises a type error for another:
every $x in (1, 2, "cat") satisfies $x * 2 = 4
This quantified expression returns true
, because the binding sequence
is empty, despite the fact that the condition can never be satisfied:
every $x in () satisfies ($x lt 0 and $x gt 0)
The instance
of
, cast
, castable
,
and treat
expressions are used to test whether a value
conforms to a given type or to convert it to an instance of a given
type.
[40] | InstanceofExpr |
::= |
TreatExpr ( "instance" "of" SequenceType )? |
The boolean
operator instance of
returns true
if the value of its first operand matches
the SequenceType in its second
operand, according to the rules for SequenceType
matching; otherwise it returns false
. For example:
5 instance of xs:integer
This example returns true
because the given value is an instance of the given type.
5 instance of xs:decimal
This example returns true
because the given value is an integer literal, and xs:integer
is derived by restriction from xs:decimal
.
(5, 6) instance of xs:integer+
This example returns true
because the given sequence contains two integers, and is a valid instance of the specified type.
. instance of element()
This example returns true
if the context value is a
single element node or false
if the context value is defined
but is not a single element node. If the context value is absentDM40, a type error
is raised [err:XPDY0002].
Note:
An instance of
test does not allow any kind of casting or coercion.
The results may therefore be counterintuitive. For example, the expression
3 instance of xs:positiveInteger
returns false
, because
the expression 3
evaluates to an instance of xs:integer
,
not xs:positiveInteger
. For similar reasons, "red" instance of
enum("red", "green", "blue")
returns false.
On such occasions, a castable as
test may be more appropriate:
see 4.19.3 Castable
[43] | CastExpr |
::= |
ArrowExpr ( "cast" "as" SingleType )? |
|
[110] | SingleType |
::= |
CastTarget "?"? |
|
[133] | TypeName |
::= |
EQName
|
|
[146] | LocalUnionType |
::= | "union" "(" ItemType ("," ItemType)* ")" |
|
[147] | EnumerationType |
::= | "enum" "(" StringLiteral ("," StringLiteral)* ")" |
Sometimes
it is necessary to convert a value to a specific datatype. For this
purpose, XPath 4.0 provides a cast
expression that
creates a new value of a specific type based on an existing value. A
cast
expression takes two operands: an input
expression and a target type. The type of the
atomized value of the input expression is called the input type.
The target type must be one of:
The name of an named item type defined in the static context, which in turn must refer to an item type in one of the following categories.
The name of a type defined in the in-scope schema types,
which must be a simple type [err:XQST0052].
In addition, the target type cannot be xs:NOTATION
, xs:anySimpleType
,
or xs:anyAtomicType
A LocalUnionType
such as union(xs:date, xs:dateTime)
.
An EnumerationType
such as enum("red", "green", "blue")
.
Otherwise, a static error is raised [err:XPST0080].
The optional occurrence indicator ?
denotes that an empty
sequence is permitted.
Casting a node to xs:QName
can cause surprises because it uses the static context of the cast expression to provide the namespace bindings for this operation.
Instead of casting to xs:QName
, it is generally preferable to use the fn:QName
function, which allows the namespace context to be taken from the document containing the QName.
The semantics of the cast
expression
are as follows:
The input expression is evaluated.
The result of the first step is atomized.
If the result of atomization is a sequence of more than one atomic value, a type error is raised [err:XPTY0004].
If the result of atomization is an empty sequence:
If
?
is specified after the target type, the result of the
cast
expression is an empty sequence.
If ?
is not specified after the target type, a type error is raised [err:XPTY0004].
If the result of atomization is a single atomic value, the result of the cast expression is determined by casting to the target type as described in Section 21 CastingFO40. When casting, an implementation may need to determine whether one type is derived by restriction from another. An implementation can determine this either by examining the in-scope schema definitions or by using an alternative, implementation-dependent mechanism such as a data dictionary. The result of a cast expression is one of the following:
A value of the target type (or, in the case of list types, a sequence of values that are instances of the item type of the list type).
A type error, if casting from the source type to the target type is not supported (for example attempting to convert an integer to a date).
A dynamic error, if the particular input value cannot be
converted to the target type (for example, attempting to convert
the string "three"
to an integer).
Note:
Casting to an enumeration type relies on the fact that an enumeration type
is a generalized atomic type. So cast $x as enum("red")
is equivalent
to casting to an anonymous atomic type derived from xs:string
whose enumeration facet restricts the value space to the single string "red"
,
while cast $x as enum("red", "green")
is equivalent to casting
to union(enum("red"), enum("green"))
.
[42] | CastableExpr |
::= |
CastExpr ( "castable" "as" SingleType )? |
|
[110] | SingleType |
::= |
CastTarget "?"? |
|
[133] | TypeName |
::= |
EQName
|
|
[146] | LocalUnionType |
::= | "union" "(" ItemType ("," ItemType)* ")" |
|
[147] | EnumerationType |
::= | "enum" "(" StringLiteral ("," StringLiteral)* ")" |
XPath 4.0 provides an expression that tests whether a given value is castable into a given target type. The target type must be one of:
The name of an named item type defined in the static context, which in turn must refer to an item type in one of the following categories.
The name of a type defined in the in-scope schema types,
which must be a simple type [err:XQST0052].
In addition, the target type cannot be xs:NOTATION
, xs:anySimpleType
,
or xs:anyAtomicType
A LocalUnionType
such as union(xs:date, xs:dateTime)
.
An EnumerationType
such as enum("red", "green", "blue")
.
The expression E castable as T
returns true
if the result of evaluating E
can be successfully cast into the target type T
by using a cast
expression;
otherwise it returns false
.
If evaluation of E
fails with a dynamic error or if the value of E
cannot be atomized,
the castable
expression as a whole fails.
The castable
expression can be used as a predicate to
avoid errors at evaluation time.
It can also be used to select an appropriate type for processing of a given value, as illustrated in
the following example:
if ($x castable as hatsize) then $x cast as hatsize else if ($x castable as IQ) then $x cast as IQ else $x cast as xs:string
Note:
The expression $x castable as enum("red", "green", "blue")
is for most practical purposes equivalent to $x = ("red", "green", "blue")
;
the main difference is that it uses the Unicode codepoint collation for comparing strings,
not the default collation from the static context.
For every simple type in the in-scope schema types (except xs:NOTATION
and
xs:anyAtomicType
, and xs:anySimpleType
, which
are not instantiable), a constructor function is implicitly defined.
In each case, the name of the constructor function is the same as the name of
its target type (including namespace). The signature of the constructor
function for a given type depends on the type that is being constructed,
and can be found in Section 20 Constructor functionsFO40.
There is also a constructor function for every named item type in the static context that expands either to a generalized atomic type or to a RecordTest .
All such constructor functions are classified as system functions.
Note:
The constructor function is present in the static context if and only if the corresponding type is present in the static context.
For XSLT, this means that a constructor function corresponding to an imported
schema type is private to the stylesheet package, and a constructor function
corresponding to an xsl:item-type
declaration has the same visibility
as the xsl:item-type
declaration.
For XQuery, this means that a constructor function corresponding to an imported
schema type is private to the query module, and a constructor function
corresponding to a named item type declaration is %public
or %private
according to the annotations on the item type declaration.
[Definition: The constructor function for a given simple type is used to convert instances of other simple types into the given type.
The semantics of the constructor function call T($arg)
are defined to be equivalent to the expression (($arg) cast as T?)
.]
The following examples illustrate the use of constructor functions:
This
example is equivalent to ("2000-01-01" cast as
xs:date?)
.
xs:date("2000-01-01")
This
example is equivalent to
(($floatvalue * 0.2E-5) cast as xs:decimal?)
.
xs:decimal($floatvalue * 0.2E-5)
This example returns an
xs:dayTimeDuration
value equal to 21 days. It is
equivalent to ("P21D" cast as xs:dayTimeDuration?)
.
xs:dayTimeDuration("P21D")
If
usa:zipcode
is a user-defined atomic type
in the in-scope schema types, then the
following expression is equivalent to the
expression ("12345" cast as
usa:zipcode?)
.
usa:zipcode("12345")
If my:chrono
is a named item type that expands to
union(xs:date, xs:time, xs:dateTime)
, then the result
of my:chrono("12:00:00Z")
is the xs:time
value 12:00:00Z
.
If my:location
is a named item type that expands
to record(latitude as xs:double, longitude as xs:double)
,
then the result of my:location(50.52, -3.02)
is
the map map{'latitude':50.52e0, 'longitude':-3.02e0}
.
Note:
An instance of an atomic type that is not in a namespace can be constructed by using a URIQualifiedName in either a cast expression or a constructor function call. Examples:
17 cast as Q{}apple
Q{}apple(17)
In either context, using an unqualified NCName might not work: in a cast expression, an unqualified name is it is interpreted according to the default namespace for elements and types, while an unqualified name in a constructor function call is resolved using the default function namespace which will often be inappropriate.
[41] | TreatExpr |
::= |
CastableExpr ( "treat" "as" SequenceType )? |
XPath 4.0 provides an
expression called treat
that can be used to modify the
static type of its
operand.
Like cast
, the treat
expression takes two operands: an expression and a SequenceType. Unlike
cast
, however, treat
does not change the
dynamic type or value of its operand. Instead, the purpose of
treat
is to ensure that an expression has an expected
dynamic type at evaluation time.
The semantics of
expr1
treat as
type1
are as
follows:
During static analysis:
The
static type of the
treat
expression is
type1
. This enables the
expression to be used as an argument of a function that requires a
parameter of
type1
.
During expression evaluation:
If
expr1
matches
type1
,
using the rules for SequenceType
matching,
the treat
expression returns the value of
expr1
; otherwise, it raises a dynamic error
[err:XPDY0050].
If the value of
expr1
is returned, the identity of any nodes in the value is
preserved. The treat
expression ensures that the value of
its expression operand conforms to the expected type at
run-time.
Example:
$myaddress treat as element(*, USAddress)
The
static type of
$myaddress
may be element(*, Address)
, a
less specific type than element(*, USAddress)
. However,
at run-time, the value of $myaddress
must match the type
element(*, USAddress)
using rules for SequenceType
matching;
otherwise a dynamic error is
raised [err:XPDY0050].
!
)[54] | SimpleMapExpr |
::= |
PathExpr ("!" PathExpr)* |
A mapping expression S!E
evaluates the
expression E
once for every item in the sequence
obtained by evaluating S
. The simple mapping operator
!
can be applied to any sequence, regardless of the
types of its items, and it can deliver a mixed sequence of nodes,
atomic values, and functions. Unlike the similar /
operator, it does not sort nodes into document order or eliminate
duplicates.
Each operation E1!E2
is evaluated as follows:
Expression E1
is evaluated to a sequence S
.
Each item in S
then serves in turn to provide an inner focus
(the item as the context value, its position in S
as the
context position, the length of S
as the context size)
for an evaluation of E2
in the dynamic context. The sequences resulting from all the
evaluations of E2
are combined as follows: Every evaluation
of E2
returns a (possibly empty) sequence of items.
These sequences are concatenated and returned.
The returned sequence preserves the orderings within and among the subsequences generated by the evaluations of E2
.
Simple map operators have functionality similar to 4.7.3 Path operator (/). The following table summarizes the differences between these two operators
Operator | Path operator (E1 / E2 ) |
Simple map operator (E1 ! E2 ) |
---|---|---|
E1 | Any sequence of nodes | Any sequence of items |
E2 | Either a sequence of nodes or a sequence of non-node items | A sequence of items |
Additional processing | Duplicate elimination and document ordering | Simple sequence concatenation |
The following examples illustrate the use of simple map operators combined with path expressions.
child::div1 / child::para / string() ! concat("id-", .)
Selects the para
element children of the div1
element children of the context node; that is, the para
element grandchildren of the context node that have div1
parents. It then outputs the strings obtained by prepending "id-"
to each of the string values of these grandchildren.
$emp ! (@first, @middle, @last)
Returns the values of the attributes first
, middle
, and last
for each element in $emp
, in the order given. (The /
operator, if used here, would return the attributes in an unpredictable order.)
$docs ! ( //employee)
Returns all the employee
elements within all the documents identified by the variable $docs
, in document order within each document, but retaining the order of documents.
avg( //employee / salary ! translate(., '$', '') ! number(.))
Returns the average salary of the employees, having converted the salary to a number by removing any $
sign and then converting to a number. (The second occurrence of !
could not be written as /
because the left-hand operand of /
cannot be an atomic value.)
string-join((1 to $n)!"*")
Returns a string containing $n
asterisks.
$values!(.*.) => sum()
Returns the sum of the squares of a sequence of numbers.
string-join(ancestor::*!name(), '/')
Returns the names of ancestor elements, joined by /
characters, i.e., the path to the parent of the context.
Arrow expressions apply a function to a value, using the value of the left-hand expression as the first argument to the function.
[44] | ArrowExpr |
::= |
UnaryExpr ( (SequenceArrowTarget | MappingArrowTarget | LookupArrowTarget) )* |
|
[47] | SequenceArrowTarget |
::= | "=>" ArrowTarget
|
|
[48] | MappingArrowTarget |
::= | "=!>" ArrowTarget
|
|
[50] | LookupArrowTarget |
::= | "=?>" NCName
PositionalArgumentList
|
|
[49] | ArrowTarget |
::= | (ArrowStaticFunction
ArgumentList) | (ArrowDynamicFunction
PositionalArgumentList) |
|
[84] | ArrowStaticFunction |
::= |
EQName
|
|
[85] | ArrowDynamicFunction |
::= |
VarRef | InlineFunctionExpr | ParenthesizedExpr
|
|
[98] | InlineFunctionExpr |
::= | ("function" | "fn") FunctionSignature? FunctionBody
|
|
[73] | ArgumentList |
::= | "(" ((PositionalArguments ("," KeywordArguments)?) | KeywordArguments)? ")" |
|
[74] | PositionalArgumentList |
::= | "(" PositionalArguments? ")" |
The arrow syntax is particularly helpful when applying multiple functions to a value in turn. For example, the following expression invites syntax errors due to misplaced parentheses:
tokenize((normalize-unicode(upper-case($string))),"\s+")
In the following reformulation, it is easier to see that the parentheses are balanced:
$string => upper-case() => normalize-unicode() => tokenize("\s+")
When the operator is written as =!>
, the function
is applied to each item in the sequence in turn.
Assuming that $string
is a single string, the above example could
equally be written:
$string =!> upper-case() =!> normalize-unicode() =!> tokenize("\s+")
The difference between the two operators is seen when the left-hand operand evaluates to a sequence:
(1, 2, 3) => avg()
returns a value of only one item, 2
, the average of all three items, whereas
(1, 2, 3) =!> avg()
returns the original sequence of three items, (1, 2, 3)
,
each item being the average of itself. The following example:
"The cat sat on the mat" => tokenize() =!> concat(".") =!> upper-case() => string-join(" ")
returns "THE. CAT. SAT. ON. THE. MAT."
. The first arrow
could be written either as =>
or =!>
because the operand is a singleton; the next two
arrows have to be =!>
because the function is applied to each item in the tokenized
sequence individually; the final arrow must be =>
because the string-join
function applies to the sequence as a whole.
Note:
It may be useful to think of this as a map/reduce pipeline. The functions
introduced by =!>
are mapping operations; the function introduced by =>
is a reduce operation.
The following example introduces an inline function to the pipeline:
(1 to 5) =!> xs:double() =!> math:sqrt() =!> fn($a){$a+1}() => sum()
This is equivalent to sum((1 to 5) ! (math:sqrt(xs:double(.))+1))
.
The same effect can be achieved using a focus function:
(1 to 5) =!> xs:double() =!> math:sqrt() =!> fn{.+1}() => sum()
Where the value of an expression is a map containing functions, simulating the behavior
of objects in object-oriented languages, then the lookup arrow operator
=?>
can be used to retrive a function from the map and to invoke the function with the map as its
first argument. For example, if my:rectangle
returns a map with entries width
,
height
, expand
, and area
, then it becomes possible to
write:
my:rectangle(3,5) =?> expand(2) =?> area()
Note:
The ArgumentList
may include PlaceHolders
,
though this is not especially useful. For example, the expression "$" => concat(?)
is equivalent
to concat("$", ?)
: its value is a function that prepends a supplied string with
a $
symbol.
Note:
The ArgumentList
may include keyword arguments if the
function is identified statically (that is, by name). For example,
the following is valid: $xml => xml-to-json(indent:=true()) => parse-json(escape:=false())
.
The sequence arrow operator thus applies the supplied function to the left-hand operand as a whole, while the mapping arrow operator applies the function to each item in the value of the left-hand operand individually. In the case where the result of the left-hand operand is a single item, the two operators have the same effect.
Note:
The mapping arrow symbol =!>
is intended to suggest a combination of
function application (=>
) and sequence mapping
(!
) combined in a single operation.
Similarly, the lookup arrow symbol =?>
is intended to suggest a combination
of function application (=>
) and map lookup (?
) in a single
operation.
[Definition:
The sequence arrow operator
=>
applies a function to a
supplied sequence.] It is defined as follows:
Given a UnaryExpr
U
, an ArrowStaticFunction
F
, and an ArgumentList
(A, B, C...)
, the expression U => F(A, B, C...)
is equivalent to the
expression F(U, A, B, C...)
.
Given a UnaryExpr
U
, an ArrowDynamicFunction
F
, and an PositionalArgumentList
(A, B, C...)
, the expression U => F(A, B, C...)
is equivalent to the
expression F(U, A, B, C...)
.
[Definition:
The mapping arrow operator
=!>
applies a function to each
item in a sequence.] It is defined as follows:
If the arrow is followed by an ArrowStaticFunction:
Given a UnaryExpr
U
, an ArrowStaticFunction
F
, and an ArgumentList
(A, B, C...)
, the expression U =!> F(A, B, C...)
is equivalent to the
expression (for $u in U return F($u, A, B, C...))
.
If the arrow is followed by an ArrowDynamicFunction:
Given a UnaryExpr
U
, an ArrowDynamicFunction
F
, and an PositionalArgumentList
(A, B, C...)
, the expression U =!> F(A, B, C...)
is equivalent to the
expression (for $u in U return F($u, A, B, C...))
.
The lookup arrow expression simulates the behavior of method invocations in object-oriented languages. It is useful for invoking functions that are contained as entries in maps.
For example, the expression
let $rectangle := map { "width": 20, "height": 12, "area": fn($this){$this?width * $this?height} } return $rectangle =?> area()
returns the value 240
.
An expression such as M =?> N(A, B, C)
is evaluated as follows:
The left-hand expression M is evaluated. If the value is an
empty sequence, then the result of the expression is an empty
sequence. If it is non-empty then it must be a single map: call it $m
.
The lookup expression $m?N
is evaluated. The result must be a single
function item: call it $f
.
The dynamic function call $f($m, A, B, C)
is evaluated, and the
result is returned.
Any of the above steps can lead to errors:
A type error [err:XPTY0004] is raised if the value of the left hand
expression does not match the type map(*)?
.
A type error [err:XPTY0004] is raised if the value of the lookup
expression $m?N
does not match the type function(*)
, or if the
arity of the function is not equal to the number of arguments in the argument list
plus one.
An error may occur in evaluating the dynamic function call, for example if the function does not expect a map to be supplied as the first argument.
This section defines the conformance criteria for an XPath 4.0 processor. In this section, the following terms are used to indicate the requirement levels defined in [RFC2119]. [Definition: MUST means that the item is an absolute requirement of the specification.] [Definition: MUST NOT means that the item is an absolute prohibition of the specification.] [Definition: MAY means that an item is truly optional.]
XPath is intended primarily as a component that can be used by other specifications. Therefore, XPath relies on specifications that use it (such as [XPointer] and [XSL Transformations (XSLT) Version 4.0]) to specify conformance criteria for XPath in their respective environments. Specifications that set conformance criteria for their use of XPath MUST NOT change the syntactic or semantic definitions of XPath as given in this specification, except by subsetting and/or compatible extensions.
If a language is described as an extension of XPath, then every expression that conforms to the XPath grammar MUST behave as described in this specification.
The grammar of XPath 4.0 uses the same simple Extended Backus-Naur Form (EBNF) notation as [XML 1.0] with the following minor differences.
All named symbols have a name that begins with an uppercase letter.
It adds a notation for referring to productions in external specifications.
Comments or extra-grammatical constraints on grammar productions are between '/*' and '*/' symbols.
A 'xgc:' prefix is an extra-grammatical constraint, the details of which are explained in A.1.2 Extra-grammatical Constraints
A 'ws:' prefix explains the whitespace rules for the production, the details of which are explained in A.3.5 Whitespace Rules
A 'gn:' prefix means a 'Grammar Note', and is meant as a clarification for parsing rules, and is explained in A.1.3 Grammar Notes. These notes are not normative.
The terminal symbols for this grammar include the quoted strings used in the production rules below, and the terminal symbols defined in section A.3.1 Terminal Symbols. The grammar is a little unusual in that parsing and tokenization are somewhat intertwined: for more details see A.3 Lexical structure.
The EBNF notation is described in more detail in A.1.1 Notation.