XQuery and XPath Data Model 4.0

1 Introduction

Changes in 4.0 ⬇

Use the arrows to browse significant changes since the 3.1 version of this specification.
Sections with significant changes are marked Δ in the table of contents.

This document defines the XQuery and XPath Data Model 4.0, which is the data model of [XML Path Language (XPath) 4.0], [XSL Transformations (XSLT) Version 4.0], and [XQuery 4.0: An XML Query Language].

The XQuery and XPath Data Model 4.0 (henceforth “data model”) serves two purposes. First, it defines the information contained in the input to an XSLT or XQuery processor. Second, it defines all permissible values of expressions in the XSLT, XQuery, and XPath languages. A language is closed with respect to a data model if the value of every expression in the language is guaranteed to be in the data model. XSLT 4.0, XQuery 4.0, and XPath 4.0 are all closed with respect to the data model.

The data model describes items similar to those of the [Infoset] (henceforth “Infoset”). It is written to provide a data model suitable for XPath, XQuery and XSLT, which was not a goal of the Infoset, and this leads to a number of differences, some of which are:

Support for XML Schema types. The XML Schema recommendations define features, such as structures ([Schema Part 1]) and simple data types ([Schema Part 2]), that extend the Infoset with precise type information.
Representation of collections of documents and of complex values.
Support for typed atomic items.
Support for ordered, heterogeneous sequences.

As with the Infoset, the XQuery and XPath Data Model 4.0 specifies what information in the documents is accessible but does not specify the programming-language interfaces or bindings used to represent or access the data.

The data model can represent various values, including not only the input and the output of a stylesheet or query but all values of expressions used during the intermediate calculations. Examples include the input document or document repository (represented as a document node or a sequence of document nodes), the result of a path expression (represented as a sequence of nodes), the result of an arithmetic or a logical expression (represented as an atomic item), a sequence expression resulting in a sequence of items, etc.

This document provides a precise definition of the properties of nodes in the XQuery and XPath Data Model 4.0, how they are accessed, and how they relate to values in the Infoset and PSVI.

2 Terminology and Concepts

This section outlines a number of general concepts that apply throughout this specification.

In this document, examples and material labeled as “Note” are provided for explanatory purposes and are not normative.

2.1 Terminology

For a full glossary of terms, see C Glossary.

In this specification the phrases must, must not, should, should not, may, required, and recommended, when used in normative text and rendered in small capitals, are to be interpreted as described in [RFC 2119].

[Definition: Implementation-defined indicates an aspect that may differ between implementations, but must be specified by the implementer 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 implementer for any particular implementation.]

In all cases where this specification leaves the behavior implementation-defined or implementation-dependent, the implementation has the option of providing mechanisms that allow the user to influence the behavior.

This specification distinguishes between the data model as a general concept and specific items (documents, elements, atomic items, etc.) that are concrete examples of the data model by identifying all concrete examples as instances of the data model.

Sometimes it is necessary to distinguish the case where a particular property has no value in the data model. The canonical example of such a case is the namespace URI property of an expanded QName that is not in any namespace. For such properties, it is convenient to be able to speak of “the state of having no value”. [Definition: When a property has no value, we say that it is absent.]

Within this specification, the term URI refers to a Uniform Resource Identifier as defined in [RFC 3986] and extended in [RFC 3987] with the new name Internationalized Reference Identifier, 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.

2.2 Basic Concepts

Changes in 4.0 ⬇ ⬆

Clarified the terminology concerning atomic types and type annotations. [Issue 225 PR 232 5 November 2022]
The term atomic value has been replaced by atomic item. [Issue 1337 PR 1361 2 August 2024]

[Definition: Every instance of the data model is a sequence.]

[Definition: A sequence is an ordered collection of zero or more items.] A sequence cannot be a member of a sequence. A single item appearing on its own is modeled as a sequence containing one item. Sequences are defined in 2.6 Sequences.

[Definition: Because every value is a sequence, the term value is used synonymously with sequence.]

[Definition: An item is either a node, a function, or an atomic item.]

[Definition: An item type represents a class of items.] An item is said to be an instance of an item type (or to match the item type) if it is a member of that class. Items generally belong to more than one item type, and the membership of different item types is overlapping.

Every node is one of the seven kinds of nodes defined in Section 5 Nodes. Nodes form a tree. Each node has at most one parent (reachable via the dm:parent accessor) and zero or more descendant nodes that are reachable directly or indirectly via the dm:children, dm:attributes, and dm:namespace-nodes accessors.

[Definition: The root node is the topmost node of a tree, the node with no parent.] Every tree has exactly one root node and every other node can be reached from exactly one root node.

Note:

The term “root node” is merely a designator, based on position, for one of the nodes in the tree without implying what kind of a node it is. In the XPath 1.0 datamodel the root node was a kind of node.

Note:

Generally, the term tree is used to refer to a complete tree rooted at a parentless node. On occasions, which should be clear from the context, the same term is used to refer to a subtree, that is, a tree forming part of a larger tree.

[Definition: A tree whose root node is a document node is referred to as a document.]

[Definition: A tree whose root node is not a document node is referred to as a fragment.]

[Definition: An atomic item is a pair (T, D) where T (the type annotation) is an atomic type, and D (the datum) is a point in the value space of T.]

[Definition: The datum of an atomic item is a point in the value space of its type, which is also a point in the value space of the primitive type from which that type is derived.] There are 20 primitive atomic types (19 defined in XSD, plus xs:untypedAtomic), and these have non-overlapping value spaces, so each datum belongs to exactly one primitive atomic type.

Note:

The term value space is defined in [Schema 1.1 Part 2] as a set of values. The term datum is used here in preference to value, because value has a different meaning in this data model.

[Definition: An atomic type is either a primitive simple typewith variety atomic, or a type derived by restriction from another atomic type.] (Types derived by list or union are not atomic.)

Note:

Atomic types include the 19 primitive atomic types defined in XSD (such as xs:string, xs:boolean, and xs:decimal), the built-in non-primitive types defined in XSD (such as xs:integer, and xs:NCName, and xs:dayTimeDuration), atomic types derived from these in a user-defined schema, and the special type xs:untypedAtomic.

[Definition: The primitive simple types are the types defined in 2.2.1 Types adopted from XML Schema.]

[Definition: The term type annotation has two slightly different meanings. For an atomic item, the type annotation of the value is the most specific atomic type that it is an instance of (it is also an instance of every type from which that type is derived). For an element or attribute node, the type annotation is the schema type (a simple or complex type) against which the node has been validated, defaulting to xs:untypedAtomic for unvalidated attribute nodes, and xs:untyped for unvalidated element nodes.]

Named types are identified in the data model by an expanded QName. A schema may also contain anonymous types, and these may be used as type annotations on nodes and atomic items; anonymous types, however, cannot be referenced explicitly in programs.

[Definition: An expanded QName is a triple consisting of a possibly absent prefix, a possibly absent namespace URI, and a local name.] See 3.3.3 QNames and NOTATIONS.

2.2.1 Types adopted from XML Schema

[Definition: A schema type corresponds to a type definition component as defined in XSD.] Schema types are either complex types or simple types; simple types are either atomic types, list types, or union types.

The data model adopts the following schema types:

The 19 primitive atomic types defined in Section 3.2 Primitive datatypes^XS2 of [Schema Part 2].
Three built-in list types: xs:NMTOKENS, xs:IDREFS, and xs:ENTITIES.
The following types, which were originally defined in [XQuery 1.0 and XPath 2.0 Data Model (XDM)] and were subsequently adopted by [Schema 1.1 Part 2]: xs:anyAtomicType, xs:dayTimeDuration, xs:yearMonthDuration.
In the case of a processor that supports [Schema 1.1 Part 2], the new union type xs:error (a type with no instances) and the new derived type xs:dateTimeStamp.
The following types, which use the xs: namespace and are defined here in the data model but not in XML Schema: xs:untypedAtomic, and xs:numeric, a union type whose members are xs:double, xs:float and xs:decimal.

Schema types fulfill a role different from item types. Schema types other than atomic types arise in the data model only as type annotations on element and attribute nodes. Nodes are not instances of schema types in the sense of the XPath instance of operator; but an element or attribute node may be an instance of the item type element(*, S) or attribute(*, S) where S is a schema type. The node matches this item type if its type annotation is S, or a type derived from S, which will be the case if the node has been validated against type S in the course of schema validation.

Schema types and item types form overlapping categories:

Atomic types belong to both categories.
Node types and function types are item types, but they are not schema types.
Complex types, list types, and union types are schema types, but they are not item types.

2.3 Notation

To explain the data model, this specification uses both prose and a defined set of accessor functions. The accessors are shown with the prefix dm:. This prefix is always shown in italics to emphasize that these functions are abstract; they exist to explain the interface between the data model and specifications that rely on the data model: they are not accessible directly from the host language.

Several prefixes are used throughout this document for notational convenience. The following bindings are assumed.

xs: bound to http://www.w3.org/2001/XMLSchema
xsi: bound to http://www.w3.org/2001/XMLSchema-instance
fn: bound to http://www.w3.org/2005/xpath-functions

In practice, any prefix that is bound to the appropriate URI may be used.

The signature of accessor functions is shown using the same style as [XQuery and XPath Functions and Operators 4.0], described in Section 1.5 Function signatures and descriptions^FO40.

This document relies on the [Infoset] and Post-Schema-Validation Infoset (PSVI). Information items and properties are indicated by the styles information item and [infoset property], respectively.

Some aspects of type assignment rely on the ability to access properties of the schema components. Such properties are indicated by curly brackets, e.g., {component property}. Note that this does not mean a lightweight schema processor cannot be used, it only means that the application must have some mechanism to access the necessary properties.

2.4 Node Identity

Each node has a unique identity. The identity of a node is distinct from its value or other intrinsic properties; nodes may be distinct even when they have the same values for all intrinsic properties other than their identity. (The identity of atomic items, by contrast, is determined solely by their intrinsic properties. No two distinct integers, for example, have the same value; every instance of the value “5” as an integer is identical to every other instance of the value “5” as an integer.)

Note:

The concept of node identity should not be confused with the concept of a unique ID, which is a unique name assigned to an element by the author to represent references using ID/IDREF correlation.

2.5 Document Order

[Definition: A document order is defined among all the nodes accessible during a given query or transformation. Document order is a total ordering, although the relative order of some nodes is implementation-dependent. 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 query or transformation, even if this order is implementation-dependent.]

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, if any; otherwise they immediately follow 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, occurs before any node in a different tree, T2, then all nodes in T1 are before all nodes in T2.

2.6 Sequences

An important characteristic of the data model is that there is no distinction between an item (a node, function, or atomic item) and a singleton sequence containing that item. An item is equivalent to a singleton sequence containing that item and vice versa.

A sequence may contain any mixture of nodes, functions, and atomic items. When a node is added to a sequence its identity remains the same. Consequently a node may occur in more than one sequence and a sequence may contain duplicate items.

Sequences never contain other sequences; if sequences are combined, the result is always a “flattened” sequence. In other words, appending “(d e)” to “(a b c)” produces a sequence of length 5: “(a b c d e)”. It does not produce a sequence of length 4: “(a b c (d e))”; such a nested sequence never occurs.

Note:

Sequences replace node-sets from XPath 1.0. In XPath 1.0, node-sets do not contain duplicates. In generalizing node-sets to sequences, duplicate removal is provided by functions on node sequences.

Note:

Arrays and maps are function items and therefore can also be contained within sequences.

The following constructor and accessor functions are defined for sequences; these provide a formal underpinning for user-visible functions, operators, and language constructs.

2.6.1 `empty-sequence` Constructor

dm:empty-sequence() as item()*

The dm:empty-sequence constructor returns a sequence containing no items.

The function is exposed in XPath as an empty sequence expression, written ().

2.6.2 `sequence-concat` Constructor

dm:sequence-concat($input1 as item()*, $input2 as item()*) as item()*

The dm:sequence-concat constructor returns a sequence by concatenating two supplied sequences.

The returned sequence contains the items in $input1 (retaining order), followed by the items in $input2 (also retaining order).

The function is exposed in XPath through the comma operator ,.

2.6.3 `count` Accessor

dm:count($input as item()*) as xs:integer

The dm:count accessor function returns the number of items in $input.

The function is exposed in XPath through the fn:count function.

2.6.4 `iterate-sequence` Accessor

`dm:iterate-sequence`(	`$input`	`as` `array(*)`,
`dm:iterate-sequence`(	`$action`	`as` `function(item(), xs:integer) as item()`) `as` `item()`

The dm:iterate-sequence accessor calls the supplied $action function once for each item in $input, in order, and returns the sequence concenation of the results. The $action function is called with two arguments. The first argument is an item in $input, and the second is the 1-based ordinal position of the item within $input.

The function is exposed in XPath most directly through the function for-each, as well as for expressions in XPath, for clauses in FLWOR expressions in XQuery, and the xsl:for-each instruction in XSLT. It also underpins all other functions that manipulate sequences, such as fn:count and fn:filter.

2.7 Namespace Names

The specifications [Namespaces in XML] and [Namespaces in XML 1.1] introduce the concept of a namespace name. In [Namespaces in XML] a namespace name is required to be a URI; in [Namespaces in XML 1.1] it is required to be an IRI; but both specifications explicitly do not require a processor to check that namespace names appearing in an instance document are in fact valid URIs or IRIs.

[Definition: This specification uses the term Namespace URI to refer to a namespace name, whether or not it is a valid URI or IRI]. Following the lead of [Namespaces in XML] and [Namespaces in XML 1.1], processors implementing this data model may raise an error if a namespace name is not a valid URI or IRI (depending on whether they support [Namespaces in XML] or [Namespaces in XML 1.1]), but they are not required to make any checks. Note that the use of a relative reference as a namespace name is deprecated and is defined to be meaningless, but it is not an error. Namespace names, whatever form they take, are treated as character strings and compared for equality using codepoint-by-codepoint comparison, subject only to whitespace normalization if they appear in a context (for example, within an attribute value) where this is appropriate.

In some interfaces, namespace names are held as values of type xs:anyURI. For example, the namespace part of an expanded QName is defined to be a value of type xs:anyURI. In [Schema Part 2], the type xs:anyURI imposes some restrictions on the value space, but there is latitude for implementers to decide exactly what these restrictions are. In [Schema 1.1 Part 2] there are no restrictions on the form of an xs:anyURI value, so any sequence of characters is acceptable within the value space. In this and related specifications, the use of the type xs:anyURI to hold a namespace name does not imply any restrictions on the value space beyond those described in this section: implementations may reject character strings that are not valid URIs or IRIs, but they are not required to do so.

2.8 Schema Information

The data model supports strongly typed languages such as [XML Path Language (XPath) 4.0] and [XQuery 4.0: An XML Query Language] that have a type system based on [Schema Part 1]. To achieve this, the data model includes (by reference) the Schema Component Model described in [Schema Part 1].

Note:

The Schema Component Model includes a number of kinds of component, such as type definitions and element and attribute declarations, and defines the properties and relationships of these components. Many of these components and properties are not used by the language specifications that rely on XDM, and where this is the case, there is no requirement for implementations to make them visible. However, this specification makes no attempt to define the minimal subset of the schema component model that is needed to support the semantics of XPath and XQuery processing.

There are two main areas where the language semantics depend on information in schema components:

Expressions are evaluated with respect to a static context, which includes schema components, specifically type definitions, element declarations, and attribute declarations. The names of such components may be used in language constructs only if the components are present in the static context.
Values including element and attribute nodes, and atomic items, have a property called a type annotation whose value is a type: this is a reference to a type definition in the Schema Component Model.

The diagram below illustrates the schema type system, in which all types are derived from xs:anyType.

XML Schema types (abstract)
- anyType (built-in complex)
  - Simple types (abstract)
    - anySimpleType (built-inlist)
      - Atomic types (abstract)
        anyAtomicType (built-in atomic)
      - list types (abstract)
        ENTITIES (built-in list)
        IDREFS (built-in list)
        NMTOKENS (built-in list)
        user-defined list types (user-defined)
      - union types (abstract)
        numeric (built-in complex)
        user-defined union types (user-defined)
    - complex types (complex)
      - untyped (built-in complex)
      - user-defined complex types (user-defined)

Legend:

Supertype
- subtype

Abstract types (abstract)
Built-in atomic types (built-in atomic)
Built-in complex types (built-in complex)
Built-in list types (built-in list)
User-defined types (user-defined)

2.8.1 Schema Consistency

[Definition: Following the terminology of [Schema Part 1], a schema is defined as set of schema components. Schema components include, for example, element declarations and type definitions.]

Note:

This contrasts with a schema document, which is an XML document containing an xs:schema element and other XML elements such as xs:element and xs:complexType. In popular usage, the term schema is often used inaccurately to refer to a schema document. A schema generally consists of the result of processing a set of schema documents linked using xs:include and xs:import declarations; it may also include built-in schema components that are known intrinsically to a processor, or schema components that are constructed programmatically.

It is possible for an application to use more than one schema. For example, a transformation may accept as input a source document validated against a schema X, and produce as output a result document validated against a different schema Y; the stylesheet or query might import a schema that is the union of X and Y. Different modules of a query, or different packages within a stylesheet, may import different schemas. When this happens, there is a requirement that the different schemas must be compatible with each other. This requirement is expanded in the following paragraphs.

[Definition: Two schemasX and Y are compatible if the union of X and Y is a valid schema.] This essentially means that there must be no schema components in X and Y that have the same name but different definitions.

An implication of this rule is that when one schema document uses xs:redefines or xs:override to modify the definitions found in another schema document, the resulting schemas will generally be incompatible.

The rule means that when a type T appears in both X and Y, the properties of the schema component corresponding to T, and the properties of all the schema components that it refers to, transitively, will be the same.

This will always be true (in the absence of xs:redefines and xs:override) if both schemas derive their definition of T from the same schema document. It may also be true if the definitions of T derive from different schema documents, but in this case processors may treat the definitions as incompatible without further analysis.

It will not always be the case that validating an element node N against type T using schema X has the same outcome as validating the same element node N against type T using schema Y. Cases where the outcome may be different include the following:

The content model of T includes an element particle E whose corresponding element declaration has different substitution group members in X and Y.
Schema X includes a type U that extends T, and U is not present in Y; element N has the attribute xsi:type="U", which references this extended type definition.
The content model of T includes a wildcard particle that specifies processContents="lax" or processContents="strict"; element N has a child C that is matched against this wildcard; a declaration for C is present in X but not in Y.
The content model of T (in XSD 1.1) includes a wildcard particle that specifies notQName="##defined; element N has a child C that is matched against this wildcard; a declaration for C is present in X but not in Y.

It is essential that nodes validated against one schema can be passed to a stylesheet or query that is using a different but compatible schema. This is necessary, for example, to ensure that the validated output of one stylesheet can form the validated input of another. The definitions for schema-based item types such as element(*, T) and schema-element(E) therefore allow for the possibility that the node being tested against the item type was validated using a different but compatible schema, and by implication, that revalidation using the local schema might not succeed.

Note:

It is potentially useful to prevent differences arising between different schemas that share schema components by:

Disallowing extensions to element declarations and types by using attributes such as blockDefault="#all"
Avoiding use of wildcards whose effect depends on the presence of unrelated schema components, for example processContents="lax" and processContents="strict".

Note:

This makes it the responsibility of the processor to ensure that the schema components used in the static context of a query or expression during static analysis are compatible with the schema components used to validate documents during query or expression evaluation. This specification does not say how this should be achieved.

It is also a constraint that the schema information available to the processor must contain at least the components and properties needed to correctly implement the semantics of the XPath and XQuery language. For example, this means that given an element node with a particular type annotation T, and a function that expects an argument of type element(*, S), there must be sufficient information available to the processor to establish whether or not T is derived from S. As with other consistency constraints described in this data model, it is a precondition that these constraints are satisfied; the specifications do not speculate on what happens if they are not.

2.8.2 Representation of Types

The data model uses expanded QNames to represent the names of schema types, which include the built-in types defined by [Schema Part 2] and the five additional types defined by this specification, and may include other user- or implementation-defined types.

For XML Schema types, the namespace name of the expanded QName is the {target namespace} property of the type definition, and its local name is the {name} property of the type definition.

The data model relies on the fact that an expanded QName uniquely identifies every named type. Although it is possible for different schemas to define different types with the same expanded QName, at most one of them can be used in any given validation episode. The data model cannot support environments where different types with the same expanded QName are available.

The scope over which the names of anonymous types must be meaningful and distinct depends on the processing context. It is the responsibility of the host language to define the size and scope of the processing context.

Note:

The type annotation of a schema-validated node, or of an atomic item extracted by atomizing a schema-validated node, may be an anonymous type. Queries and expressions cannot refer explicitly to anonymous types, but it is always possible to test whether such an item matches a named type from which the anonymous type is derived.

2.8.3 Predefined Types

The three atomic types xs:anyAtomicType, xs:dayTimeDuration, and xs:yearMonthDuration were first introduced in the 2.0 version of this specification, and were subsequently adopted by XSD 1.1. These types are always present in the XDM data model, with the definitions as given in XSD 1.1, whether or not the processor actually supports XSD 1.1.

Note:

The types xs:dayTimeDuration, and xs:yearMonthDuration have a special status in these specifications because many arithmetic operations (such as comparing two durations) are available in XPath only on these subtypes of xs:duration, not on the primitive type xs:duration itself.

The datatype xs:anyAtomicType is an atomic type that includes all atomic items (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.

No type may be derived from xs:anyAtomicType by restriction, union, or list.

The types xs:untyped and xs:untypedAtomic, although they have names in the XSD namespace, are defined in this XDM specification, and not in XSD.

The type xs:untypedAtomic denotes untyped atomic data, such as text that has not been assigned a more specific type. It is classified as an atomic type. An attribute that has not been validated (or 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 types are derived from xs:untypedAtomicand no such derivations are allowed.

The datatype xs:untyped is used as the type annotation of an element node that has not been validated, or that has been validated in skip mode. It is a classified as a complex type. The properties of xs:untyped are the same as the properties of xs:anyType except for the base type and name. The base type of xs:untyped is xs:anyType. No predefined types are derived from xs:untypedand no such derivations are allowed.

2.8.4 XML and XSD Versions

Changes in 4.0 ⬇ ⬆

Relaxed the rules regarding use of non-XML characters in instances of xs:string. [Issue 414 PR 546 25 July 2023]

Some of the types defined in XML Schema have differing definitions in XSD 1.0 and XSD 1.1; furthermore, some types are defined by reference to other specifications, including XML and XML Namespaces, and these too may vary from one version of the specification to the next.

As a general policy, implementations of data types should support the latest definitive version of any referenced specification, even if that is published after the date of this specification.

This means, for example, that the xs:string data type should support the set of characters defined by Unicode. Similarly, the xs:anyURI data type should support the definition used in XSD 1.1 (which allows any sequence of characters), and the xs:NCName data type should support the definition based on the syntax of a name as defined in both XML 1.1 Second Edition and XML 1.0 Fifth Edition (which provide the same definition).

In practice interoperability problems can arise both because specifications are not always in synchronization with each other (for example, XSD 1.0 contains references to dated versions of XML 1.0 other than the latest version), and also because implementations may use third-party components (such as XML parsers, serializers, and schema validators) that were built against different versions of the base specifications. For these reasons, use of the latest version of referenced specifications is generally recommended but not required. It is implementation-dependent how a processor handles any such conflicts.

[Definition: A string is a sequence of zero or more characters.]

[Definition: A character is any Unicode character.] Implementations may restrict characters to those Unicode characters allowed by the Char production in [XML]. Unpaired surrogates are always forbidden.

[Definition: A codepoint is a non-negative integer assigned to a character by the Unicode consortium, or reserved for future assignment to a character.]

The definitions of string and character in the data model allow an implementation to accept input that cannot occur in a well-formed XML document. For example, an implementation might allow the unparsed-text() function to return the content of a text file that includes the control character U+0007 (BEL) or might not restrict what codepoints-to-string() can return.

An implementation that allows a broader repertoire of characters to be consumed by the processor, must ensure that

Any characters serialized with the XML or XHTML output methods satisfy the well-formedness criteria of the selected version of XML.
Any schema validation carried out using an XML Schema 1.0 or 1.1 schema rejects any nodes or atomic items containing characters that do not satisfy the constraints of the selected version of XML.

Note:

The lexical space of type xs:duration is defined in XSD 1.1 part 2 (§3.3.6.2) in two different ways: with a BNF grammar starting with the production durationLexicalRep, and with a regular expression. The two definitions are inconsistent: the BNF allows a decimal point at the start or end of the seconds component, while the regular expression requires any decimal point to be preceded and followed by a digit. For the purposes of this specification, the regular expression is considered to be correct, and the BNF incorrect; this makes the representation identical to that in XSD 1.0, and is compatible with ISO 8601.

2.9 Type System

Every value manipulated by XPath, XQuery, or XSLT is a sequence comprising zero or more items.

[Definition: A sequence type constrains the set of permitted sequences, by defining the permitted item types and the permitted number of items in the sequence (exactly zero, exactly one, zero-or-more, one-or-more, zero-or-one).]

Every item is an instance of one or more item types:

All items are instances of the type item().
Every node is an instance of the type node(), and more specifically it is an instance of one of seven node kinds: document(), element(*), attribute(*), text(), comment(), processing-instruction(), or namespace(). Nodes may also be instances of more specific types characterized by the node name and type annotation.
Every atomic item is an instance of a specific atomic type determined by its type annotation; it is also an instance of every type from which that type is derived by restriction (directly or indirectly), and of every union type that includes that type as a member type.
Every function item is an instance of the generic type function(*), and also of a specific function type defining the types of the function's parameters and the type of the result.
A map item, as well as being a function, is also an instance of the generic map type map(*), of more specific map types map(K, V) defining the types of the keys and values, and perhaps of one or more record types that associate a type with specific key values.
An array item, as well as being a function, is also an instance of the generic array type array(*), and also of more specific array types array(M) defining the type of the array's members.

This section describes how item types relate to each other.

The diagrams below show how nodes, functions, primitive simple types, and user defined types fit together into a type system. In the diagrams, connecting lines represent relationships between derived types and the types from which they are derived; the latter are always higher and to the left of the latter.

The xs:IDREFS, xs:NMTOKENS, xs:ENTITIES types, and xs:numeric, and both the user-defined list types and user-defined union types are special types in that these types are lists or unions rather than types derived by extension or restriction.

The first diagram illustrates the relationship of various item types. Item types in the data model form a directed graph, rather than a hierarchy or lattice: in the relationship defined by the derived-from(A, B) function, some types are derived from more than one other type. Examples include functions (function(xs:string) as xs:int is substitutable for function(xs:NCName) as xs:int and also for function(xs:string) as xs:decimal), and union types (A is substitutable for the union type (A | B) and also for the union type (A | C)). In XDM, item types include node types, function types, and built-in atomic types. The list, which shows only hierarchic relationships, is therefore a simplification of the full model.

item (abstract)
- anyAtomicType (built-in atomic)
- node (node)
  - attribute (node)
    - user-defined attribute types (user-defined)
  - document (node)
    - user-defined document types (user-defined)
  - element (node)
    - user-defined element types (user-defined)
  - text (node)
  - comment (node)
  - processing-instruction (node)
  - namespace (node)
- function(*) (function item)
  - array(*) (function item)
  - map(*) (function item)

Legend:

Supertype
- subtype

Abstract types (abstract)
Built-in atomic types (built-in atomic)
Node types (node)
Function item types (function item)
User-defined types (user-defined)

The XPath Data Model is the abstraction over which XPath expressions are evaluated. Historically, all of the items in the data model could be derived directly (nodes) or indirectly (typed values, sequences) from an XML document. However, as the XPath expression language has matured, new features have been added which require additional types of items to appear in the data model. These items have no direct XML serialization, but they are never the less part of the data model.

The next diagram shows all of the atomic types, including the primitive simple types and the built-in types derived from the primitive simple types. This includes all the built-in datatypes defined in [Schema Part 2]. Atomic types act both as item types (meaning they can be used to declare the types of variables and function arguments), and as schema types (meaning they can be used as type annotations on nodes).

anyAtomicType
- anyURI
- base64Binary
- boolean
- date
- dateTime
  - dateTimeStamp
- decimal
  - integer
    - long
      - int
        short
        byte
    - nonNegativeInteger
      - positiveInteger
      - unsignedLong
        unsignedInt
        unsignedShort
        unsignedByte
    - nonPositiveInteger
      - negativeInteger
- double
- duration
  - dayTimeDuration
  - yearMonthDuration
- float
- gDay
- gMonth
- gMonthDay
- gYear
- gYearMonth
- hexBinary
- NOTATION
- QName
- string
  - normalizedString
    - token
      - NMTOKEN
      - Name
        NCName
        ENTITY
        ID
        IDREF
      - language
- time

Legend:

Supertype
- subtype

Built-in atomic types

2.9.1 Atomic Items

Changes in 4.0 ⬇ ⬆

The term atomic value has been replaced by atomic item. [Issue 1337 PR 1361 2 August 2024]

An atomic item can be constructed from a lexical representation. Given a string and an atomic type, the atomic item is constructed in such a way as to be consistent with schema validation. If the string does not represent a valid value of the type, an error is raised. When xs:untypedAtomic is specified as the type, no validation takes place. The details of the construction are described in Section 19 Constructor functions^FO40 and the related Section 20 Casting^FO40 section of [XQuery and XPath Functions and Operators 4.0].

2.9.2 String Values

A string value can be constructed from an atomic item. Such a value is constructed by converting the atomic item to its string representation as described in Section 20 Casting^FO40.

2.9.3 Negative Zero

The xs:float and xs:double data types in the data model have the same value space as in XML Schema 1.1 ([Schema 1.1 Part 2]). Specifically they include both negative and positive zero, and in this respect they differ from XML Schema 1.0.

To accommodate this difference, when converting from an xs:string to an xs:float or xs:double, it is implementation-defined whether the lexical value “-0” (and similar forms such as “-0.0”) convert to negative zero or to positive zero in the value space.

2.9.4 Function Items

Changes in 4.0 ⬇ ⬆

Introduced the concept of function identity. [Issue 520 PR 525 30 May 2023]

[Definition: A function item is an item that can be called. ] Function items have no serialization.

Note:

XDM 4.0 uses the term function item where XDM 3.1 used function. There is no distinction in meaning, but function item is preferred for clarity, because the unqualified term function has additional meanings in relation to function definitions in XSLT and XQuery.

A function item has the following properties:

name(xs:QName): An expanded QName, possibly absent.
identity: An abstract property that can be used to test whether two variables refer to the same function or to different functions. This property is exposed only for this purpose.
Note:
Currently, the concept of function identity is used for two purposes: firstly, when functions appear in the arguments supplied to the fn:deep-equal function; and secondly, in establishing whether the arguments and results of a function are "the same" when deciding whether the function is deterministic.
Note:
Function identity is not currently defined for maps and arrays, because in the circumstances where function identity would otherwise be used, maps and arrays are compared by examining their content.
parameter names(xs:QName*): A list of distinct names, one for each of the function’s parameters.
Note:
This property is currently unused. There is no way of discovering the parameter names of a function item, and there is no functionality that depends on the parameter names.
signature[Definition: A function signature represents the type of a function.] The signature of a function item comprises:
- The required types of its parameters (each one being a SequenceType^XP40)
- The required types of the function result (also a SequenceType^XP40)
- A sequence of zero or more function annotations. Each annotation consists of an annotation name (an instance of xs:QName) and an annotation value (an arbitrary sequence of atomic items). Annotations are ordered and it is permitted for two annotations to share the same name.
body The body of a function provides the logic to map the arguments supplied in a function call into an instance of the function’s result type.
The function body is generally one of the following:
- a user-written construct in XPath, XQuery, XSLT, or some other host language known to the processor.
- vendor-supplied logic internal to the processor.
- external logic written in some third-party programming language, to be invoked by the processor using implementation-dependent mechanisms.
These categories are not mutually exclusive; they may be used in combination.
Note:
The term “function body” replaces “function implementation”, to avoid confusion with the use of the term “implementation” in phrases such as “implementation-defined”.
captured context This includes a static and dynamic context for evaluation of the function body, as described in Section 2.2 Expression Context^XP40. In particular it includes a set of nonlocal variable bindings (a mapping from xs:QName to item()*), which provides a value for each of the function’s free variables (i.e., variables referenced by the function’s body, other than locals and parameters).
Note:
Where the function body is implemented in XPath, XQuery, or XSLT, the captured context includes the static context for the user-written code (for example, its in-scope namespaces) as well as any nonlocal variable bindings.
Functions implemented internally by the processor may capture specific parts of the static or dynamic context, for example fn:position#0 captures the value of the context position.

[Definition: The arity of a function item is the number of its parameters. ] The number of names in a function’s parameter names, and the number of parameter types in its signature, must equal the function’s arity.

All function items match the generic function type function(*), which is itself a subtype of item(). A function signature defines a more specific function type, which is always a subtype of function(*). A function signature function(T₁, T₂, T₃, ...) as T_R is a subtype of another function signature function(U₁, U₂, U₃, ...) as U_R if (a) the two signatures have the same arity, (b) the return type T_R is a subtype of U_R, and (c) for each pair of parameter types, T_n is a supertype of U_n. The rules are explained more fully in Section 3.3 Subtype Relationships^XP40. For example:

function(item()) as item() is a subtype of function(*)
function(item()) as xs:integer is a subtype of function(item()) as item()
function(item()) as item() is a subtype of function(xs:string) as item()

2.9.5 Map Items

Changes in 4.0 ⬇ ⬆

Constructors are added, and the single accessor function is now an iterator over the key/value pairs in the map. [Issue 1335 20 July 2024]

[Definition: A map item is a value that represents a map (in other languages this is sometimes called a hash, dictionary, or associative array).] A map is logically a collection of key/value pairs. Each key in the map is unique (there is no other key to which it is equal) and has associated with it a value that is a sequence of zero or more items. There is no uniqueness constraint on values. The semantics of equality are described in Section 13.2.1 fn:atomic-equal^FO40.

Note:

Maps have no intrinsic identity separate from their content. A map can be given a transient identity, represented by an id property in its label, by applying the fn:pin function. This property is expected to be used in defining operations for deep update of maps.

Constructor and accessor functions for maps are defined in the following sections.

2.9.5.1 `empty-map` Constructor

dm:empty-map() as map(*)

The dm:empty-map constructor returns a map containing no key/value pairs.

The function is exposed in XPath as an empty map constructor, which may be written {} or map{}.

2.9.5.2 `map-put` Constructor

dm:map-put($map as map(*), $key as xs:anyAtomicType, $value as item()*) as map(*)

The dm:map-put constructor returns a map based on the contents of a supplied map.

The key/value pairs in the returned map are as follows:

One key/value pair for every key/value pair present in $map whose key is not equal to $key; plus
One key/value pair whose key is $key and whose associated value is $value.

The function is exposed in XPath through the function map:put.

2.9.5.3 `iterate-map` Accessor

`dm:iterate-map`(	`$map`	`as` `map(*)`,
`dm:iterate-map`(	`$action`	`as` `function(xs:anyAtomicType, item()) as item()`) `as` `item()*`

The dm:iterate-map accessor calls the supplied $action function once for each key/value pair in $map, in implementation-dependent order, and returns the sequence concenation of the results.

The function is exposed in XPath most directly through the function map:for-each, but it also underpins all other functions giving access to maps, such as map:size, map:contains, and map:get.

2.9.6 Array Items

Changes in 4.0 ⬇ ⬆

Constructors are added, and the single accessor function is now an iterator over the members of the array. [Issue 1335 20 July 2024]

[Definition: An array item is a value that represents an array.] An array is an ordered list of values; these values are called the members of the array. Unlike sequences, a member of an array can be any value (including a sequence or an array). The number of members in an array is called its size, and they are referenced by their position, in the range 1 to the size of the array.

Note:

Arrays have no intrinsic identity separate from their content. An array can be given a transient identity, represented by an id property in its label, by applying the fn:pin function. This property is expected to be used in defining operations for deep update of arrays.

Constructor and accessor functions for arrays are defined in the following sections.

2.9.6.1 `empty-array` Constructor

dm:empty-array() as array(*)

The dm:empty-array constructor returns an array containing no members.

The function is exposed in XPath as an empty array constructor, written [] or array{}.

2.9.6.2 `array-append` Constructor

dm:array-append($array as array(*), $member as item()*) as array(*)

The dm:array-append constructor returns an array based on the contents of a supplied array.

The returned array contains:

One member for every member present in $array, at the same position; plus
One additional member, $member, as the last member in the returned array.

The function is exposed in XPath through the function array:append.

2.9.6.3 `iterate-array` Accessor

`dm:iterate-array`(	`$array`	`as` `array(*)`,
`dm:iterate-array`(	`$action`	`as` `function(item(), xs:integer) as item()`) `as` `item()*`

The dm:iterate-array accessor calls the supplied $action function once for each member in $array, in order, and returns the sequence concenation of the results. The $action function is called with two arguments. The first argument is the array member (an arbitrary sequence), and the second is its 1-based ordinal position within the array.

The function is exposed in XPath most directly through the function array:for-each (but note that array:for-each delivers an array rather than a sequence). It also underpins all other functions giving access to arrays, such as array:size and array:get.

2.10 Labeled Items

Changes in 4.0 ⬆

Introduced the concept of labeled items. [Issue 960 PR 988 27 February 2024]

[Definition: A labeled item is a pair (S, L) where S (called the subject) is any item, and L (called the label) is a map containing supplementary information about the item.].

The keys in the map are always instances of xs:string, and the associated values can be arbitrary values.

There are currently no constructs in the language that would cause either the subject or the label of a labeled item to itself be a labeled item. Nor is there anything that would cause the label to contain a labeled item.

Operations on labeled items fall into three categories:

Operations that select items which are present in their input return the labeled item unchanged (including its label). Examples are filter expressions, axis expressions, lookup expressions, and functions such as fn:head, fn:tail, and fn:subsequence.
Operations that construct new values from their input treat the labeled item exactly as if the operation were applied directly to the subject. That is, the label is ignored. Examples are arithmetic and comparison operators, operators such as is and instance of, cast expressions, atomization, and functions such as fn:avg or fn:index-of.
The fn:label function is a special case: it returns the label of a labeled item, as a map. If applied to an item that is not labeled, it returns an empty map.

Note:

Two items representing the same node (such that $n1 is $n2 returns true) may nevertheless have different labels. It is perhaps helpful to think of such items not as nodes, but as references to nodes, which are automatically dereferenced by the majority of operations.

Note:

A labeled item matches a type if its subject matches the type; labels are thus extraneous to the type system.

Labeled items are returned by certain operations, such as the lookup operators ? and ??. As a result, the lookup operation can be treated at one level as if it returned a simple value (an entry found in a map or array), but applications that need extra information about the value can find this by examining the label: for example, they can identify the associated key value.

The function fn:pin takes a map or array J as its argument, and returns a labeled map or array with J as its subject (or the subject of J, if J is itself a labeled item), and with a map M as its label, where M contains a single entry having the key "pinned" (as an xs:string) and an associated xs:boolean value true. An item $item satisfying $item[label(.)?pinned] is referred to as a pinned item.

3 Data Model Construction

This section describes the constraints on instances of the data model.

The data model supports well-formed XML documents conforming to [Namespaces in XML] or [Namespaces in XML 1.1]. Documents that are not well-formed are, by definition, not XML. XML documents that do not conform to [Namespaces in XML] or [Namespaces in XML 1.1] are not supported (nor are they supported by [Infoset]).

In other words, the data model supports the following classes of XML documents:

Well-formed documents conforming to [Namespaces in XML] or [Namespaces in XML 1.1].
DTD-valid documents conforming to [Namespaces in XML] or [Namespaces in XML 1.1], and
W3C XML Schema-validated documents.

This document describes how to construct an instance of the data model from an infoset ([Infoset]) or a Post Schema Validation Infoset (PSVI), the augmented infoset produced by an XML Schema validation episode.

An instance of the data model can also be constructed directly through application APIs, or from non-XML sources such as relational tables in a database. Data model construction from sources other than an Infoset or PSVI is implementation-defined. Regardless of how an instance of the data model is constructed, every node and atomic item in the data model must have a typed value that is consistent with its type.

The data model supports some kinds of values that are not supported by [Infoset]. Examples of these are document fragments and sequences of document nodes. The data model also supports values that are not nodes. Examples of these are sequences of atomic items, or sequences mixing nodes and atomic items. These are necessary to be able to represent the results of intermediate expressions in the data model during expression processing.

3.1 Direct Construction

Although this document describes construction of an instance of the data model in terms of infoset properties, an infoset is not a necessary precondition for building an instance of the data model.

There are no constraints on how an instance of the data model may be constructed directly, save that the resulting instance must satisfy all of the constraints described in this document.

3.2 Construction from an Infoset

An instance of the data model can be constructed from an infoset that satisfies the following general constraints:

All general and external parsed entities must be fully expanded. The Infoset must not contain any unexpanded entity reference information items.
The infoset must provide all of the properties identified as “required” in this document. The properties identified as “optional” may be used, if they are present. All other properties are ignored.

An instance of the data model constructed from an information set must be consistent with the description provided for each node kind.

Furthermore, construction of an instance of the data model from an Infoset is guaranteed to be well-defined only for those Infosets that could have been derived from a conforming XML document.

3.3 Construction from a PSVI

An instance of the data model can be constructed from a PSVI, whose element and attribute information items have been strictly assessed, laxly assessed, or have not been assessed. Constructing an instance of the data model from a PSVI must be consistent with the description provided in this section and with the description provided for each node kind.

Data model construction requires that the PSVI provide unique names for all anonymous schema types.

Note:

[Schema Part 1] does not require all schema processors to provide unique names for anonymous schema types. In order to build an instance of the data model from a PSVI produced by a processor that does not provide the names, some post-processing will be required in order to ensure that they are all uniquely identified before construction begins.

[Definition: An incompletely validated document is an XML document that has a corresponding schema but whose schema-validity assessment has resulted in one or more element or attribute information items being assigned values other than ‘valid’ for the [validity] property in the PSVI.]

The data model supports incompletely validated documents. Elements and attributes that are not valid are treated as having unknown types.

The most significant difference between Infoset construction and PSVI construction occurs in the area of schema type assignment. Other differences can also arise from schema processing: default attribute and element values may be provided, white space normalization of element content may occur, and the user-supplied lexical form of elements and attributes with atomic schema types may be lost.

3.3.1 Mapping PSVI Additions to Node Properties

A PSVI element or attribute information item may have a [validity] property. The [validity] property may be “valid ”, “invalid ”, or “notKnown ” and reflects the outcome of schema-validity assessment. In the data model, precise schema type information is exposed for element and attribute nodes that are “valid ”. Nodes that are not “valid ” are treated as if they were simply well-formed XML and only very general schema type information is associated with them.

3.3.1.1 Element and Attribute Node Types

The precise definition of the schema type of an element or attribute information item depends on the properties of the PSVI. In the PSVI, [Schema Part 1] defines a [type definition] property as well as the [type definition namespace], [type definition name] and [type definition anonymous] properties, which are effectively short-cut terms for properties of the type definition. Further, the [element declaration] and [attribute declaration] properties are defined for elements and attributes, respectively. These declarations in turn will identify the [type definition] declared for the element or attribute. To distinguish the [type definition] given in the PSVI for the element or attribute instance from the [type definition] associated with the declaration, the former is referred to below as the actual type and the latter as the declared type of the element or attribute instance in question.

The type depends on the declared type, the actual type, and the [validity] and [validation attempted] properties in the PSVI. If:

The [validity] and [validation attempted] properties exist and have the values “valid ” and “full ”, respectively, the schema type of an element or attribute information item is represented by an expanded QName whose namespace and local name correspond to the first applicable items in the following list:
- If the declared type exists and is a union and the actual type is (not the same as the declared type, and not a type derived from the declared type, but) one of the member types of the union, or derived from one of its member types:
  - If the {name} property of the declared type is present: the {target namespace} and {name} properties of the declared type.
  - If the {name} property of the declared type is absent: the namespace and local name of the anonymous type name supplied for the declared type.
- If there is no declared type, and the actual type is a union, then:
  - If the {name} property of the actual type is present: the {target namespace} and {name} properties of the actual type.
  - If the {name} property of the actual type is absent: the namespace and local name of the anonymous type name supplied for the actual type.
- Otherwise:
  - If [type definition anonymous] is false: the {target namespace} and {name} properties of the actual type.
  - If [type definition anonymous] is true: the namespace and local name of the anonymous type name supplied for the actual type.
The [validity] property exists and is “invalid ”, or the [validation attempted] property exists and is “partial ”, the schema type of an element is xs:anyType and the type of an attribute is xs:anySimpleType.
The [validity] property exists and is “notKnown ”, the schema type of an element is xs:anyType and the type of an attribute is xs:anySimpleType.
The [validity] or [validation attempted] properties do not exist, the schema type of an element is xs:untyped and the type of an attribute is xs:untypedAtomic.

The prefix associated with the type names is implementation-dependent.

3.3.1.2 Typed Value Determination

This section describes how the typed value of an element or attribute node is computed from an element or attribute PSVI information item, where the information item has either a simple type or a complex type with simple content. For other kinds of element nodes, see 5.2.4 Construction from a PSVI; for other kinds of attribute nodes, see 5.3.4 Construction from a PSVI.

The typed value of attribute nodes and some element nodes is a sequence of atomic items. The types of the items in the typed value of a node may differ from the type of the node itself. This section describes how the typed value of a node is derived from the properties of an information item in a PSVI.

The types of the items in the typed value of a node are determined as follows. The process begins with a type, T. If the schema type of the node itself, as represented in the PSVI, is a complex type with simple content, then T is the {content type} of the schema type of the node; otherwise, T is the schema type of the node itself. For each primitive or ordinary simple type T, the W3C XML Schema specification defines a function M mapping the lexical representation of a value onto the value itself.

Note:

For atomic and list types, the mapping is the “lexical mapping” defined for T in [Schema Part 2]; for union types, the mapping is the lexical mapping defined in [Schema Part 2] modified as appropriate by any applicable rules in [Schema Part 1]. The mapping, so modified, is a function (in the mathematical sense) which maps to a single value even in cases where the lexical mapping proper maps to multiple values.

The typed value is determined as follows:

If the nilled property of the node in question is true, then the typed value is the empty sequence.
If T is xs:anySimpleType or xs:anyAtomicType, the typed value is the [schema normalized value] as an instance of xs:untypedAtomic.
Otherwise, the typed value is the result of applying M to the string value as an instance of the appropriate value type, where the appropriate value type is the [member type definition] if T is a union type, otherwise it is simply T.

The typed value determination process is guaranteed to result in a sequence of atomic items, each having a well-defined atomic type. This sequence of atomic items, in turn, determines the typed-value property of the node in the data model.

3.3.1.3 Relationship Between Typed-Value and String-Value

Element and attribute nodes have both typed-value and string-value properties (the terms typed value and string value are defined at Section 2.5.2 Typed Value and String Value^XP40 of [XML Path Language (XPath) 4.0]). However, implementations are allowed some flexibility in how these properties are stored. An implementation may choose to store the string-value property only and derive the typed-value property from it, or to store the typed-value property only and derive the string-value property from it, or to store both the string-value property and the typed-value property.

To permit these various implementation strategies, some variations in the string value of a node are defined as insignificant. Implementations that store only the typed value of a node are permitted to return a string value that is different from the original lexical form of the node content. For example, consider the following element:

<offset xsi:type="xs:integer">0030</offset>

Assuming that the node is valid, it has a typed value of 30 as an xs:integer. An implementation may return either “30” or “0030” as the string value of the node. Any string that is a valid lexical representation of the typed value is acceptable. In this specification, we express this rule by saying that the relationship between the string value of a node and its typed value must be “consistent with schema validation.”

If an implementation stores only the string value of a node, the following considerations apply:

Where union types occur, the implementation must be able to deliver the typed value as an instance of the appropriate member type. For example, if the type of an element node is my:integer-or-string, which is defined as a union of xs:integer and xs:string, and the string value of the node is “47”, the implementation must be able to deliver the typed value of the node as either the integer 47 or the string "47", depending on which member type validated the element.
Where types of xs:QName, xs:NOTATION, or types derived from one of these types occur, the implementation must be able to deliver the typed value as a triple consisting of a local name, a namespace prefix, and a namespace URI, even though the namespace URI is not part of the string-value (see 3.3.3 QNames and NOTATIONS).
Where an element with a complex type and element-only content occurs, it is an error to attempt to access the typed-value of the element node.

If an implementation stores only the typed value of a node, it must be prepared to construct string values from not only the node, but in some cases also the descendants of that node. For example, an element with a complex type and element-only content has no typed value but does have a string value that is the concatenation of the string values of all its text node descendants in document order.

A further caveat applies if an implementation stores the typed value of a node. If a new data model is constructed by copying portions of another data model, and the copy operation does not preserve inherited namespaces, and the type is a union type that is sensitive to the namespace context, then the typed value may be different than what would be obtained by revalidating the node within its new namespace context. Although this may stretch the semantics of “consistent with schema validation”, we accept this possibility; it is not an error.

3.3.1.4 Pattern Facets

Creating a subtype by restriction generally reduces the value space of the original schema type. For example, expressing a hat size as a restriction of decimal with a minimum value of 6.5 and maximum value of 8.0 creates a schema type whose valid values are only those in the range 6.5 to 8.0.

The pattern facet is different because it restricts the lexical space of the schema type, not its value space. Expressing a three-digit number as a restriction of integer with the pattern facet “[0-9]{3}” creates a schema type whose valid values are only those with a lexical form consisting of three digits.

The pattern facet is not reversible in practice. A given point in the value space might have several lexical representations. In general, there is no practical way to determine which, if any, of these representations satisfies the pattern facet of the type.

As a consequence, pattern facets are not respected when mapping to an Infoset or during serialization, and values in the data model that were originally valid with respect to a schema that contains pattern-based restrictions may be invalid after serialization.

3.3.2 Dates and Times

The date and time types require special attention. This section applies to implementations that store the typed value of xs:dateTime, xs:date, xs:time, xs:gYearMonth, xs:gYear, xs:gMonthDay, xs:gMonth, xs:gDay, and types that are derived from them. These are known collectively as the date/time types in this specification.

The values of the date/time types are represented in the data model using seven components:

year: An xs:integer.
month: An xs:integer between 1 and 12, inclusive.
day: An xs:integer between 1 and 31, inclusive, possibly restricted further depending on the values of month and year.
hour: An xs:integer between 0 and 23, inclusive.
minute: An xs:integer between 0 and 59, inclusive.
second: An xs:decimal greater than or equal to zero and less than 60. Leap seconds are not supported.
timezone: An xs:dayTimeDuration between -PT14H00M and PT14H00M, inclusive. All timezone values must be an integral number of minutes.

Components that are intrinsic to the datatype (for example, day, month, and year in a xs:date) are required; components that can never be part of a datatype (for example, years in a xs:time) must be missing. Missing components are represented by the empty sequence. When a component is present, it contains the “local value” that has not been normalized in any way. The timezone component is optional for all the date/time datatypes.

Thus, the lexical xs:dateTime representation “2003-01-02T11:30:00-05:00” is stored as “{2003, 1, 2, 11, 30, 0.0, -PT05H00M}”. The value of the lexical representation “2003-01-16T16:30:00” is stored as “{2003, 1, 16, 16, 30, 0, ()}” because it has no timezone. The value of the lexical xs:gDay representation “---30+10:30” is stored as “{(), (), 30, (), (), (), PT10H30M}”.

The lexical form “24:00:00” is normalized in the component model. As a xs:time, it is stored as “{(), (), (), 0, 0, 0.0, ()}” and the xs:dateTime representation “1999-12-31T24:00:00” is stored as “{2000, 1, 1, 0, 0, 0.0, ()}”.

Note:

Implementations are permitted to store date/time values in any representation that is convenient for them, provided that the individual properties can be accessed and modified.

3.3.3 QNames and NOTATIONS

The QName and NOTATION data types require special attention. The following sections apply to xs:QName, xs:NOTATION, and types derived from them. These types are referred to collectively as “qualified names”.

As defined in XML Schema, the lexical space for qualified names includes a local name and an optional namespace prefix. The value space for qualified names contains a local name and an optional namespace URI. Therefore, it is not possible to derive a lexical value from the typed value, or vice versa, without access to some context that defines the namespace bindings.

When qualified names exist as values of nodes in a well-formed document, it is always possible to determine such a namespace context. However, the data model also allows qualified names to exist as freestanding atomic items, or as the name or value of a parentless attribute node, and in these cases no namespace context is available.

In this Data Model, therefore, the value space for qualified names contains a local-name, an optional namespace URI, and an optional prefix. The prefix is used only when producing a lexical representation of the value, that is, when casting the value to a string. The prefix plays no part in other operations involving qualified names: in particular, two qualified names are equal if their local names and namespace URIs match, regardless whether they have the same prefix.

The following consistency constraints apply:

If the namespace URI of a qualified name is absent, then the prefix must also be absent.
For every element node whose name has a prefix, the prefix must be one that has a binding to the namespace URI of the element name in the namespaces property of the element.
For every element node whose name has no prefix, the element must have a binding for the empty prefix to the namespace URI of the element name, or must have no binding for the empty prefix in the case where the name of the element has no namespace URI.
For every attribute node whose name has a prefix, the attribute node must either be parentless, or the prefix must be one that has a binding to the namespace URI of the attribute name in the namespaces property of the parent element.
For every qualified name that contains a prefix and that is included in the typed value of an element node, or of an attribute node that has an element node as its parent, the prefix must be one that is bound to the namespace URI of the qualified name in the namespaces property of that element.
For every qualified name that contains a namespace URI and no prefix, and that is included in the typed value of an element node, or of an attribute node that has an element node as its parent, that element node must have a binding for the empty prefix to that namespace URI in its namespace property.
For every qualified name that contains neither a namespace URI nor a prefix, and that is included in the typed value of an element node, or of an attribute node that has an element node as its parent, that node must not have a binding for the empty prefix.
No qualified name that contains a prefix may be included in the typed value of an attribute node that has no parent.

4 Accessors

A set of accessors is defined on nodes in the data model. For consistency, all the accessors are defined on every kind of node, although several accessors return a constant empty sequence on some kinds of nodes.

In order for processors to be able to operate on instances of the data model, the model must expose the properties of the items it contains. The data model does this by defining a family of accessor functions. These are not functions in the literal sense; they are not available for users or applications to call directly. Rather they are descriptions of the information that an implementation of the data model must expose to applications. Functions and operators available to end users are described in [XQuery and XPath Functions and Operators 4.0].

Some typed values in the data model are absent. Attempting to access an absent typed value is an error. Behavior in these cases is implementation defined and the host language is responsible for determining the result.

4.1 `attributes` Accessor

dm:attributes($n as node()) as attribute()*

The dm:attributes accessor returns the attributes of a node as a sequence containing zero or more attribute nodes. The order of attribute nodes is stable but implementation dependent.