XSL Transformations (XSLT) Version 4.0

1.1 What is XSLT?

XSLT is a programming language designed principally to transform data.

A transformation in the XSLT language is expressed in the form of a stylesheet. A stylesheet is made up of one or more well-formed XML [XML 1.0] documents conforming to the Namespaces in XML Recommendation [Namespaces in XML].

A stylesheet generally includes elements that are defined by XSLT as well as elements that are not defined by XSLT. XSLT-defined elements are distinguished by use of the namespace http://www.w3.org/1999/XSL/Transform (see 3.1.1 XSLT Namespace), which is referred to in this specification as the XSLT namespace. Thus this specification is a definition of the syntax and semantics of the XSLT namespace.

The term stylesheet reflects the fact that one of the important roles of XSLT is to add styling information to an XML source document, by transforming it into a document consisting of XSL formatting objects (see [XSL-FO]), or into another presentation-oriented format such as HTML, XHTML, or SVG. However, XSLT is used for a wide range of transformation tasks, not exclusively for formatting and presentation applications.

A transformation expressed in XSLT describes rules for transforming input data into output data. The inputs and outputs will all be instances of the XDM data model, described in [XDM 3.0]. In the simplest and most common case, the input is an XML document referred to as the source document or source tree, and the output is an XML document referred to as the result tree. It is also possible to process multiple source documents, to generate multiple result documents, and to handle formats other than XML.

The transformation is achieved by a set of template rules. A template rule associates a pattern, which typically matches nodes in the source document, with a sequence constructor. In many cases, evaluating the sequence constructor will cause new nodes to be constructed, which can be used to produce part of a result tree. The structure of the result trees can be completely different from the structure of the source trees. In constructing a result tree, nodes from the source trees can be filtered and reordered, and arbitrary structure can be added. Because patterns can point to nodes according to typological features, a stylesheet can be applicable to a wide class of source documents that have similar tree structures.

Stylesheets are modular; they may contain several packages developed independently of each other, and each package may consist of several stylesheet modules.

[Definition: A stylesheet consists of one or more packages: specifically, one top-level package and zero or more library packages.]

[Definition: For a given transformation, one package functions as the top-level package. The complete stylesheet is assembled by finding the packages referenced directly or indirectly from the top-level package using xsl:use-package declarations: see 3.5.2 Dependencies between Packages.]

[Definition: Every package within a stylesheet, other than the top-level package, is referred to as a library package.]

[Definition: Within a package, one stylesheet module functions as the principal stylesheet module. The complete package is assembled by finding the stylesheet modules referenced directly or indirectly from the principal stylesheet module using xsl:include and xsl:import elements: see 3.11.2 Stylesheet Inclusion and 3.11.3 Stylesheet Import.]

1.2 What’s New in XSLT 4.0?

XSLT 4.0 is a revised version of the XSLT 3.0 Recommendation [XSLT 3.0] published on 8 June 2017.

The changes in this version of the language are relatively minor usability enhancements. There are no changes to the data model or processing model. Instead, the specification attempts to fill a number of gaps in functionality resulting from feedback from XSLT 3.0 users. The main areas covered are:

• Enhancements to the type system to allow more expressive constraints, especially for maps and atomic values.

• Additional functionality for processing arrays.

• Exploitation of the power afforded by first-class function items.

XSLT 4.0 also includes optional facilities to serialize the results of a transformation, by means of an interface to the serialization component described in [XSLT and XQuery Serialization 4.0].

XSLT 4.0 requires support for XPath 4.0.

A full list of changes is at I Changes since XSLT 3.0.

2.1 Terminology

For a full glossary of terms, see C Glossary.

[Definition: The software responsible for transforming source trees into result trees using an XSLT stylesheet is referred to as the processor. This is sometimes expanded to XSLT processor to avoid any confusion with other processors, for example an XML processor.]

[Definition: A specific product that performs the functions of an XSLT processor is referred to as an implementation.]

[Definition: The term tree is used (as in [XDM 3.0]) to refer to the aggregate consisting of a parentless node together with all its descendant nodes, plus all their attributes and namespaces.]

The output of a transformation consists of the following:

1. [Definition: A principal result: this can be any sequence of items (as defined in [XDM 3.0]).] The principal result is the value returned by the function or template in the stylesheet that is nominated as the entry point, as described in 2.3 Initiating a Transformation.

2. [Definition: Zero or more secondary results: each secondary result can be any sequence of items (as defined in [XDM 3.0]).] A secondary result is the value returned by evaluating the body of an xsl:result-document instruction.

3. Zero or more messages. Messages are generated by the xsl:message and xsl:assert instructions, and are described in 24.1 Messages and 24.2 Assertions.

4. Static or dynamic errors: see 2.14 Error Handling.

The principal result and the secondary results may be post-processed as described in 2.3.6 Post-processing the Raw Result.

[Definition: The term result tree is used to refer to any tree constructed by instructions in the stylesheet. A result tree is either a final result tree or a temporary tree.]

[Definition: A final result tree is a result tree that forms part of the output of a transformation: specifically, a tree built by post-processing the items in the principal result or in a secondary result. Once created, the contents of a final result tree are not accessible within the stylesheet itself.] Any final result tree may be serialized as described in 27 Serialization.

[Definition: The term source tree means any tree provided as input to the transformation. This includes the document containing the global context item if any, documents containing nodes present in the initial match selection, documents containing nodes supplied as the values of stylesheet parameters, documents obtained from the results of functions such as document, doc, and collection, documents read using the xsl:source-document instruction, and documents returned by extension functions or extension instructions. In the context of a particular XSLT instruction, the term source tree means any tree provided as input to that instruction; this may be a source tree of the transformation as a whole, or it may be a temporary tree produced during the course of the transformation.]

[Definition: The term temporary tree means any tree that is neither a source tree nor a final result tree.] Temporary trees are used to hold intermediate results during the execution of the transformation.

Unless otherwise stated, the term “tree” refers to a tree rooted at a parentless node: that is, the term does not include subtrees of larger trees. Every node therefore belongs to exactly one tree.

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 [RFC2119].

Where the phrase must, must not, or required relates to the behavior of the XSLT processor, then an implementation is not conformant unless it behaves as specified, subject to the more detailed rules in 28 Conformance.

Where the phrase must, must not, or required relates to a stylesheet then the processor must enforce this constraint on stylesheets by raising an error if the constraint is not satisfied.

Where the phrase should, should not, or recommended relates to a stylesheet then a processor may produce warning messages if the constraint is not satisfied, but must not treat this as an error.

[Definition: In this specification, the term implementation-defined refers to a feature where the implementation is allowed some flexibility, and where the choices made by the implementation must be described in documentation that accompanies any conformance claim.]

[Definition: The term implementation-dependent refers to a feature where the behavior may vary from one implementation to another, and where the vendor is not expected to provide a full specification of the behavior.] (This might apply, for example, to limits on the size of source documents that can be transformed.)

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.

A paragraph labeled as a Note or described as an example is non-normative.

Many terms used in this document are defined in the XPath specification [XPath 3.0] or the XDM specification [XDM 3.0]. Particular attention is drawn to the following:

• [Definition: The term atomization is defined in Section 2.5.3 Atomization^XP. It is a process that takes as input a sequence of items, and returns a sequence of atomic items, in which the nodes are replaced by their typed values as defined in [XDM 3.0]. Arrays (see 22 Arrays) are atomized by atomizing their members, recursively.] For some items (for example, elements with element-only content, function items, and maps, atomization raises a dynamic error.

• [Definition: The term typed value is defined in Section 5.15 typed-value Accessor ^DM30. Every node, other than an element whose type annotation identifies it as having element-only content, has a typed value. For example, the typed value of an attribute of type xs:IDREFS is a sequence of zero or more xs:IDREF values.]

• [Definition: The term string value is defined in Section 5.13 string-value Accessor ^DM30. Every node has a string value. For example, the string value of an element is the concatenation of the string values of all its descendant text nodes.]

• [Definition: The term XPath 1.0 compatibility mode is defined in Section 2.2.1 Static Context^XP. This is a setting in the static context of an XPath expression; it has two values, true and false. When the value is set to true, the semantics of function calls and certain other operations are adjusted to give a greater degree of backwards compatibility between XPath 4.0 and XPath 1.0.]

• [Definition: The term function definition is defined in Section 2.2.1 Static Context^XP. It is the definition of a function that can be called statically from within an XPath expression: in the case of XSLT it typically means either a stylesheet function, or a built-in function such as those defined in [Functions and Operators 4.0]]

• [Definition: A function definition has an arity range, which defines the minimum and maximum number of arguments that must be supplied in a call to the function. The static context can contain multiple function definitions with the same name, provided that their arity ranges do not overlap.]

2.2 Notation

[Definition: An XSLT element is an element in the XSLT namespace whose syntax and semantics are defined in this specification.] For a non-normative list of XSLT elements, see D Element Syntax Summary.

In this document the specification of each XSLT element is preceded by a summary of its syntax in the form of a model for elements of that element type. A full list of all these specifications can be found in D Element Syntax Summary. The meaning of the syntax summary notation is as follows:

• An attribute that is required is shown with its name in bold. An attribute that may be omitted is shown with a question mark following its name.

• An attribute that is deprecated is shown in a grayed font within square brackets.

• The string that occurs in the place of an attribute value specifies the allowed values of the attribute. If this is surrounded by curly brackets ({...}), then the attribute value is treated as an attribute value template, and the string occurring within curly brackets specifies the allowed values of the result of evaluating the attribute value template. Alternative allowed values are separated by |. A quoted string indicates a value equal to that specific string. An unquoted, italicized name specifies a particular type of value.

  The types used, and their meanings, are as follows:

  boolean

  One of the strings "yes", "true", or "1" to indicate the value true, or one of the strings "no", "false", or "0" to indicate the value false. Note: the values are synonyms; where this specification uses a phrase such as “If required='yes' is specified ...” this is to be interpreted as meaning “If the attribute named required is present, and has the value yes, true, or 1 (after stripping leading and trailing whitespace) ...”.

  string

  Any string.

  expression

  An XPath expression.

  pattern

  A pattern as described in 5.4 Patterns.

  item-type

  An ItemType^XP as defined in the XPath 4.0 specification.

  sequence-type

  A SequenceType^XP as defined in the XPath 4.0 specification.

  uri; uris

  A URI, for example a namespace URI or a collation URI; a whitespace-separated list of URIs.

  qname

  A lexical QName as defined in 5.1.1 Qualified Names.

  eqname; eqnames

  An EQName as defined in 5.1.1 Qualified Names; a whitespace-separated list of EQNames.

  token; tokens

  A string containing no significant whitespace; a whitespace-separated list of such strings.

  nmtoken; nmtokens

  A string conforming to the XML schema rules for the type xs:NMTOKEN; a whitespace-separated list of such strings.

  char

  A string comprising a single Unicode character.

  language

  A string in the value space of xs:language, or a zero-length string.

  integer

  An integer, that is, a string that is castable to the schema type xs:integer.

  decimal

  A decimal value, that is, a string that is castable to the schema type xs:decimal.

  ncname; ncnames

  An unprefixed name: a string that is castable to the schema type xs:NCName; a whitespace-separated list of such strings.

  prefix; prefixes

  An xs:NCName representing a namespace prefix, which must be in scope for the element on which it appears; a whitespace-separated list of such strings.

  id

  An xs:NCName used as a unique identifier for an element in the containing XML document.

  Except where the set of allowed values of an attribute is specified using the italicized name string or char, leading and trailing whitespace in the attribute value is ignored. In the case of an attribute value template, this applies to the effective value obtained when the attribute value template is expanded.

  XPath comments (delimited by (: ... :)) are permitted anywhere that inter-token whitespace is permitted in attributes whose type is given as expression, pattern, item-type, or sequence-type, and are not permitted in attributes of other types (other than within expressions enclosed by curly braces within an attribute value template).

• If an attribute has a simple default value, this is shown between tortoise-shell brackets (for example 〔'no'〕). Where no default is shown, the consequence of omitting the attribute is explained in the prose narrative. Default values shown in the summary apply only where the attribute itself is applicable; if an attribute is not permitted to appear in the particular context, then its default value should be ignored. (For example, the stable attribute of xsl:sort is shown as having a default value of 'yes', but the attribute is allowed only on the first of a sequence of adjacent xsl:sort elements.) The quotation marks around a default value are not part of the value.

• Unless the element is required to be empty, the model element contains a comment specifying the allowed content. The allowed content is specified in a way similar to an element type declaration in XML; sequence constructor means that any mixture of text nodes, literal result elements, extension instructions, and XSLT elements from the instruction category is allowed; other-declarations means that any mixture of XSLT elements from the declaration category is allowed, together with user-defined data elements.

• The element is prefaced by comments indicating if it belongs to the instruction category or declaration category or both. The category of an element affects only whether it is allowed in the content of elements that allow a sequence constructor or other-declarations.

[ERR XTSE0010] It is a static error if an XSLT-defined element is used in a context where it is not permitted, if a required attribute is omitted, or if the content of the element does not correspond to the content that is allowed for the element.

The rules in the element syntax summary (both for the element structure and for its attributes) apply to the stylesheet content after preprocessing as described in 3.13 Stylesheet Preprocessing.

[Definition: The effective value of an attribute or text node in the stylesheet is the value after any required expansion or normalization.]

More specifically, the effective value is the value after:

• Expanding shadow attributes as described in 3.13.4 Shadow Attributes;

• Expanding defaults (for example, if an xsl:message instruction has no terminate attribute, then the effective value of the terminate attribute is no);

• Stripping ignored whitespace (for example, the effective value of a boolean attribute written as terminate=" no " is no);

• Replacing synonyms (for example in boolean attributes, 1 and true are synonyms of yes);

• Expanding attribute value templates and text value templates.

• Applying rules from the static context: for example, the effective value of a collation attribute is the value after expanding a relative URI against the static base URI.

Attributes are validated as follows. These rules apply to the value of the attribute after removing leading and trailing whitespace.

• [ERR XTSE0020] It is a static error if an attribute (other than an attribute written using curly brackets in a position where an attribute value template is permitted) contains a value that is not one of the permitted values for that attribute.

• [ERR XTDE0030] It is a dynamic error if the effective value of an attribute written using curly brackets, in a position where an attribute value template is permitted, is a value that is not one of the permitted values for that attribute. If the processor is able to detect the error statically (for example, when any XPath expressions within the curly brackets can be evaluated statically), then the processor may optionally raise this as a static error.

Special rules apply if the construct appears in part of the stylesheet that is processed with forwards compatible behavior: see 3.10 Forwards Compatible Processing.

[Definition: Some constructs defined in this specification are described as being deprecated. The use of this term implies that stylesheet authors should not use the construct, and that the construct may be removed in a later version of this specification.]

XSLT defines a set of standard functions which are additional to those defined in [Functions and Operators 4.0]. A list of these functions appears in G.2 List of XSLT-defined functions. The signatures of these functions are described using the same notation as used in [Functions and Operators 4.0]. The names of many of these functions are in the standard function namespace.

2.3 Initiating a Transformation

This document does not specify any application programming interfaces or other interfaces for initiating a transformation. This section, however, describes the information that is supplied when a transformation is initiated. Except where otherwise indicated, the information is required.

The execution of a stylesheet necessarily involves two activities: static analysis and dynamic evaluation. Static analysis consists of those tasks that can be performed by inspection of the stylesheet alone, including the binding of static variables, the evaluation of [xsl:]use-when expressions (see 3.13.3 Conditional Element Inclusion) and shadow attributes (see 3.13.4 Shadow Attributes), and the detection of static errors. Dynamic evaluation consists of tasks which in general cannot be carried out until a source document is available.

Dynamic evaluation is further divided into two activities: priming the stylesheet, and invoking a selected component.

• Priming the stylesheet provides the dynamic context for evaluation, and supplies all the information needed to establish the values of global variables.

• Invoking a component (such as a template or function) causes evaluation of that template or function to produce a result, which is an arbitrary XDM value.

  [Definition: The result of invoking the selected component, after any required conversion to the declared result type of the component, is referred to as the raw result.]

  The raw result of the invocation is the immediate result of evaluating the sequence constructor contained in the target template or function, modified by applying the coercion rules to convert the immediate result to the type declared in the as attribute of the xsl:template or xsl:function declaration, if present.

  This raw result may optionally be post-processed to construct a result tree, to serialize the result, or both, as described in 2.3.6 Post-processing the Raw Result.

Implementations may allow static analysis and dynamic evaluation to be initiated independently, so that the cost of static analysis can be amortized over multiple transformations using the same stylesheet. Implementations may also allow priming of a stylesheet and invocation of components to be initiated independently, in which case a single act of priming the stylesheet may be followed by a series of independent component invocations. Although this specification does not require such a separation, this section distinguishes information that is needed before static analysis can proceed, information that is needed to prime the stylesheet, and information that is needed when invoking components.

The language is designed to allow the static analysis of each package to be performed independently of other packages, with only basic knowledge of the properties of components made available by used packages. Beyond this, the specification leaves it to implementations to decide how to organize this process. When packages are not used explicitly, the entire stylesheet is treated as a single package.

2.4 Instructions

The main executable components of a stylesheet are templates and functions. The body of a template or function is a sequence constructor, which is a sequence of elements and text nodes that can be evaluated to produce a result.

A sequence constructor is a sequence of sibling nodes in the stylesheet, each of which is either an XSLT instruction, a literal result element, a text node, or an extension instruction.

[Definition: An instruction is either an XSLT instruction or an extension instruction.]

[Definition: An XSLT instruction is an XSLT element whose syntax summary in this specification contains the annotation <!-- category: instruction -->.]

Extension instructions are described in 25.2 Extension Instructions.

The main categories of XSLT instruction are as follows:

• instructions that create new nodes: xsl:document, xsl:element, xsl:attribute, xsl:processing-instruction, xsl:comment, xsl:value-of, xsl:text, xsl:namespace;

• instructions that construct maps and arrays: xsl:array, xsl:array-member, xsl:map, xsl:map-entry;

• instructions that copy nodes: xsl:copy, xsl:copy-of;

• an instruction that returns an arbitrary sequence by evaluating an XPath expression: xsl:sequence;

• instructions that cause conditional or repeated evaluation of nested instructions: xsl:if, xsl:choose, xsl:switch, xsl:try, xsl:for-each, xsl:for-each-group, xsl:fork, xsl:iterate and its subordinate instructions xsl:next-iteration and xsl:break;

• instructions that generate output conditionally if elements are or are not empty: xsl:on-empty, xsl:on-non-empty, xsl:where-populated;

• instructions that invoke templates: xsl:apply-templates, xsl:apply-imports, xsl:call-template, xsl:next-match;

• Instructions that declare variables: xsl:variable;

• Instructions to assist debugging: xsl:message, xsl:assert;

• other specialized instructions: xsl:number, xsl:analyze-string, xsl:fork, xsl:result-document, xsl:source-document, xsl:perform-sort, xsl:merge.

2.5 Rule-Based Processing

The classic method of executing an XSLT transformation is to apply template rules to the root node of an input document (see 2.3.3 Apply-Templates Invocation). The operation of applying templates to a node searches the stylesheet for the best matching template rule for that node. This template rule is then evaluated. A common coding pattern, especially when XSLT is used to convert XML documents into display formats such as HTML, is to have one template rule for each kind of element in the source document, and for that template rule to generate some appropriate markup elements, and to apply templates recursively to its own children. The effect is to perform a recursive traversal of the source tree, in which each node is processed using the best-fit template rule for that node. The final result of the transformation is then the tree produced by this recursive process. This result can then be optionally serialized (see 2.3.6 Post-processing the Raw Result).

<PERSONAE PLAY="OTHELLO">
    <TITLE>Dramatis Personae</TITLE>
    <PERSONA>DUKE OF VENICE</PERSONA>
    <PERSONA>BRABANTIO, a senator.</PERSONA>
    <PERSONA>Other Senators.</PERSONA>
    <PERSONA>GRATIANO, brother to Brabantio.</PERSONA>
    <PERSONA>LODOVICO, kinsman to Brabantio.</PERSONA>
    <PERSONA>OTHELLO, a noble Moor in the service of the Venetian state.</PERSONA>
    <PERSONA>CASSIO, his lieutenant.</PERSONA>
    <PERSONA>IAGO, his ancient.</PERSONA>
    <PERSONA>RODERIGO, a Venetian gentleman.</PERSONA>
    <PERSONA>MONTANO, Othello's predecessor in the government of Cyprus.</PERSONA>
    <PERSONA>Clown, servant to Othello. </PERSONA>
    <PERSONA>DESDEMONA, daughter to Brabantio and wife to Othello.</PERSONA>
    <PERSONA>EMILIA, wife to Iago.</PERSONA>
    <PERSONA>BIANCA, mistress to Cassio.</PERSONA>
    <PERSONA>Sailor, Messenger, Herald, Officers, 
             Gentlemen, Musicians, and Attendants.</PERSONA>
  </PERSONAE>

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    version="3.0"
    expand-text="yes">
    
 <xsl:strip-space elements="PERSONAE"/>   
    
 <xsl:template match="PERSONAE">
   <html>
     <head>
       <title>The Cast of {@PLAY}</title>
     </head>
     <body>
       <xsl:apply-templates/>
     </body>
   </html>
 </xsl:template>
 
 <xsl:template match="TITLE">
   <h1>{.}</h1>
 </xsl:template>
 
 <xsl:template match="PERSONA[count(tokenize(., ',')) = 2]">
   <p><b>{substring-before(., ',')}</b>: {substring-after(., ',')}</p>
 </xsl:template> 

 <xsl:template match="PERSONA">
   <p><b>{.}</b></p>
 </xsl:template>

</xsl:stylesheet>

[Definition: A stylesheet contains a set of template rules (see 6 Template Rules). A template rule has three parts: a pattern that is matched against selected items (often but not necessarily nodes), a (possibly empty) set of template parameters, and a sequence constructor that is evaluated to produce a sequence of items.] In many cases these items are newly constructed nodes, which are then written to a result tree.

2.6 The Evaluation Context

The results of some expressions and instructions in a stylesheet may depend on information provided contextually. This context information is divided into two categories: the static context, which is known during static analysis of the stylesheet, and the dynamic context, which is not known until the stylesheet is evaluated. Although information in the static context is known at analysis time, it is sometimes used during stylesheet evaluation.

Some context information can be set by means of declarations within the stylesheet itself. For example, the namespace bindings used for any XPath expression are determined by the namespace declarations present in containing elements in the stylesheet. Other information may be supplied externally or implicitly: an example is the current date and time.

The context information used in processing an XSLT stylesheet includes as a subset all the context information required when evaluating XPath expressions. The XPath 4.0 specification defines a static and dynamic context that the host language (in this case, XSLT) may initialize, which affects the results of XPath expressions used in that context. XSLT augments the context with additional information: this additional information is used firstly by XSLT constructs outside the scope of XPath (for example, the xsl:sort element), and secondly, by functions that are defined in the XSLT specification (such as key and current-group) that are available for use in XPath expressions appearing within a stylesheet.

The static context for an expression or other construct in a stylesheet is determined by the place in which it appears lexically. The details vary for different components of the static context, but in general, elements within a stylesheet module affect the static context for their descendant elements within the same stylesheet module.

The dynamic context is maintained as a stack. When an instruction or expression is evaluated, it may add dynamic context information to the stack; when evaluation is complete, the dynamic context reverts to its previous state. An expression that accesses information from the dynamic context always uses the value at the top of the stack.

The most commonly used component of the dynamic context is the context item. This is an implicit variable whose value is the item currently being processed (it may be a node, an atomic item, or a function item). The value of the context item can be referenced within an XPath expression using the expression . (dot).

XPath 4.0 generalizes the context item so it can be any value (not necessarily a single item), and it is now known as the context value^XP. In certain XPath expressions, the value of the expression . can now be an arbitrary value, rather than a single item. However, there is currently no construct in XSLT 4.0 that takes advantage of this; in the context supplied by XSLT to XPath (except when using the xsl:evaluate instruction), the context value is always either a single item, or absent. The XSLT specification therefore continues to refer to the context item rather than the context value^XP.

Full details of the static and dynamic context are provided in 5.3 The Static and Dynamic Context.

2.7 Parsing and Serialization

An XSLT stylesheet describes a process that constructs a set of results from a set of inputs. The inputs are the data provided at stylesheet invocation, as described in 2.3 Initiating a Transformation. The results include the principal result (an arbitrary sequence), which is the result of the initial component invocation, together with any secondary results produced using xsl:result-document instructions.

The stylesheet does not describe how a source tree is constructed. Some possible ways of constructing source trees are described in [XDM 3.0]. Frequently an implementation will operate in conjunction with an XML parser (or more strictly, in the terminology of [XML 1.0], an XML processor), to build a source tree from an input XML document. An implementation may also provide an application programming interface allowing the tree to be constructed directly, or allowing it to be supplied in the form of a DOM Document object (see [DOM Level 2]). This is outside the scope of this specification. Users should be aware, however, that since the input to the transformation is a tree conforming to the XDM data model as described in [XDM 3.0], constructs that might exist in the original XML document, or in the DOM, but which are not within the scope of the data model, cannot be processed by the stylesheet and cannot be guaranteed to remain unchanged in the transformation output. Such constructs include CDATA section boundaries, the use of entity references, and the DOCTYPE declaration and internal DTD subset.

[Definition: A frequent requirement is to output a final result tree as an XML document (or in other formats such as HTML). This process is referred to as serialization.]

Like parsing, serialization is not part of the transformation process, and it is not required that an XSLT processor must be able to perform serialization. However, for pragmatic reasons, this specification describes declarations (the xsl:output element and the xsl:character-map declarations, see 27 Serialization), and attributes on the xsl:result-document instruction, that allow a stylesheet to specify the desired properties of a serialized output file. When serialization is not being performed, either because the implementation does not support the serialization option, or because the user is executing the transformation in a way that does not invoke serialization, then the content of the xsl:output and xsl:character-map declarations has no effect. Under these circumstances the processor may raise any errors in an xsl:output or xsl:character-map declaration, or in the serialization attributes of xsl:result-document, but is not required to do so.

2.8 Packages and Modules

In XSLT 1.0 and 2.0 it was possible to structure a stylesheet as a collection of modules, using the xsl:include and xsl:import declarations to express the dependency of one module on others.

In XSLT 3.0 an additional layer of modularization of stylesheet code was enabled through the introduction of packages. A package is a collection of stylesheet modules with a controlled interface to the packages that use it: for example, it defines which functions and templates defined in the package are visible to callers, which are purely internal, and which are not only public but capable of being overridden by other functions and templates supplied by the using package.

Packages are introduced with several motivations, which broadly divide into two categories:

1. Software engineering benefits: greater re-use of code, greater robustness through ease of testing, controlled evolution of code in response to new requirements, ability to deliver code that users cannot see or modify.

2. Efficiency benefits: the ability to avoid compiling libraries repeatedly when they are used in multiple stylesheets, and to avoid holding multiple copies of the same library in memory simultaneously.

Packages are designed to allow separate compilation: that is, a package can be compiled independently of the packages that use it. This specification does not define a process model for compilation, or expand on what it means to compile different packages independently. Nor does it mandate that implementations offer any feature along these lines. It merely defines language features that are designed to make separate compilation of packages possible.

To achieve this, packages (unlike modules):

• Must not contain unresolved references to functions, templates, or variables declared in other packages;

• Have strict rules governing the ability to override declarations in a library package with declarations in a package that uses the library;

• Constrain the visibility of component names and of context declarations such as the declarations of keys and decimal formats;

• Can declare a mode (a collection of template rules) as final, which disallows the addition of new overriding template rules in a using package;

• Require explicit disambiguation where naming conflicts arise, for example when a package uses two other packages that both export like-named components;

• Allow multiple specializations of library components to coexist in the same application.

A package is defined in XSLT by means of an XML document whose outermost element is an xsl:package element. This is referred to as the package manifest. The xsl:package element has optional child elements xsl:use-package and xsl:expose describing properties of the package. The package manifest may refer to an external top-level stylesheet module using an xsl:include or xsl:import declaration, or it may contain the body of a stylesheet module inline (the two approaches can also be mixed).

Although this specification defines packages as constructs written using a defined XSLT syntax, implementations may provide mechanisms that allow packages to be written using other languages (for example, XQuery).

When no packages are explicitly defined, the entire stylesheet is treated as a single package; the effect is as if the xsl:stylesheet or xsl:transform element of the principal stylesheet module were replaced by an xsl:package element with no other information in the package manifest.

2.9 Extensibility

XSLT defines a number of features that allow the language to be extended by implementers, or, if implementers choose to provide the capability, by users. These features have been designed, so far as possible, so that they can be used without sacrificing interoperability. Extensions other than those explicitly defined in this specification are not permitted.

These features are all based on XML namespaces; namespaces are used to ensure that the extensions provided by one implementer do not clash with those of a different implementer.

The most common way of extending the language is by providing additional functions, which can be invoked from XPath expressions. These are known as extension functions, and are described in 25.1 Extension Functions.

It is also permissible to extend the language by providing new instructions. These are referred to as extension instructions, and are described in 25.2 Extension Instructions. A stylesheet that uses extension instructions in a particular namespace must declare that it is doing so by using the [xsl:]extension-element-prefixes attribute.

Extension instructions and extension functions defined according to these rules may be provided by the implementer of the XSLT processor, and the implementer may also provide facilities to allow users to create further extension instructions and extension functions.

This specification defines how extension instructions and extension functions are invoked, but the facilities for creating new extension instructions and extension functions are implementation-defined. For further details, see 25 Extensibility and Fallback.

The XSLT language can also be extended by the use of extension attributes (see 3.2 Extension Attributes), and by means of user-defined data elements (see 3.7.4 User-defined Data Elements).

2.10 Stylesheets and XML Schemas

An XSLT stylesheet can make use of information from a schema. An XSLT transformation can take place in the absence of a schema (and, indeed, in the absence of a DTD), but where the source document has undergone schema validity assessment, the XSLT processor has access to the type information associated with individual nodes, not merely to the untyped text.

Information from a schema can be used both statically (when the stylesheet is compiled), and dynamically (during evaluation of the stylesheet to transform a source document).

There are places within a stylesheet, and within XPath expressions and patterns in a stylesheet, where it is possible to refer to named type definitions in a schema, or to element and attribute declarations. For example, it is possible to declare the types expected for the parameters of a function. This is done using a SequenceType.

[Definition: A SequenceType constrains the type and number of items in a sequence. The term is used both to denote the concept, and to refer to the syntactic form in which sequence types are expressed in the XPath grammar: specifically SequenceType^XP in [XPath 3.0].]

[Definition: Type definitions and element and attribute declarations are referred to collectively as schema components.]

[Definition: The schema components that may be referenced by name in a package are referred to as the in-scope schema components.]

The set of in-scope schema components may vary between one package and another, but as explained in 3.15 Importing Schema Components, the schema components used in different packages must be consistent with each other.

The conformance rules for XSLT 4.0, defined in 28 Conformance, distinguish between a basic XSLT processor and a schema-aware XSLT processor. As the names suggest, a basic XSLT processor does not support the features of XSLT that require access to schema information, either statically or dynamically. A stylesheet that works with a basic XSLT processor will produce the same results with a schema-aware XSLT processor provided that the source documents are untyped (that is, they are not validated against a schema). However, if source documents are validated against a schema then the results may be different from the case where they are not validated. Some constructs that work on untyped data may fail with typed data (for example, an attribute of type xs:date cannot be used as an argument of the substring function) and other constructs may produce different results depending on the datatype (for example, given the element <product price="10.00" discount="2.00"/>, the expression @price gt @discount will return true if the attributes have type xs:decimal, but will return false if they are untyped).

There is a standard set of type definitions that are always available as in-scope schema components in every stylesheet. These are defined in 3.14 Built-in Types.

The remainder of this section describes facilities that are available only with a schema-aware XSLT processor.

Additional schema components (type definitions, element declarations, and attribute declarations) may be added to the in-scope schema components by means of the xsl:import-schema declaration in a stylesheet.

The xsl:import-schema declaration may reference an external schema document by means of a URI, or it may contain an inline xs:schema element.

It is only necessary to import a schema explicitly if one or more of its schema components are referenced explicitly by name in the stylesheet; it is not necessary to import a schema merely because the stylesheet is used to process a source document that has been assessed against that schema. It is possible to make use of the information resulting from schema assessment (for example, the fact that a particular attribute holds a date) even if no schema has been imported by the stylesheet.

Importing a schema does not of itself say anything about the type of the source document that the stylesheet is expected to process. The imported type definitions can be used for temporary nodes or for nodes on a result tree just as much as for nodes in source documents. It is possible to make assertions about the type of an input document by means of tests within the stylesheet. For example:

<xsl:mode typed="lax"/>
<xsl:global-context-item use="required"
            as="document-node(schema-element(my:invoice))"/>

It is also true that importing a schema does not of itself say anything about the structure of the result tree. It is possible to request validation of a result tree against the schema by using the xsl:result-document instruction, for example:

<xsl:template match="/">
  <xsl:result-document validation="strict">
    <xhtml:html>
      <xsl:apply-templates/>
    </xhtml:html>
  </xsl:result-document>
</xsl:template>

It is possible that a source document may contain nodes whose type annotation is not one of the types imported by the stylesheet. This creates a potential problem because in the case of an expression such as data(.) instance of xs:integer the system needs to know whether the type named in the type annotation of the context node is derived by restriction from the type xs:integer. This information is not explicitly available in an XDM tree, as defined in [XDM 3.0]. The implementation may choose one of several strategies for dealing with this situation:

1. The processor may raise a dynamic error if a source document is found to contain a type annotation that is not known to the processor.

2. The processor may maintain additional metadata, beyond that described in [XDM 3.0], that allows the source document to be processed as if all the necessary schema information had been imported using xsl:import-schema. Such metadata might be held in the data structure representing the source document itself, or it might be held in a system catalog or repository.

3. The processor may be configured to use a fixed set of schemas, which are automatically used to validate all source documents before they can be supplied as input to a transformation. In this case it is impossible for a source document to have a type annotation that the processor is not aware of.

4. The processor may be configured to treat the source document as if no schema processing had been performed, that is, effectively to strip all type annotations from elements and attributes on input, marking them instead as having type xs:untyped and xs:untypedAtomic respectively.

Where a stylesheet author chooses to make assertions about the types of nodes or of variables and parameters, it is possible for an XSLT processor to perform static analysis of the stylesheet (that is, analysis in the absence of any source document). Such analysis may reveal errors that would otherwise not be discovered until the transformation is actually executed. An XSLT processor is not required to perform such static type-checking. Under some circumstances (see 2.14 Error Handling) type errors that are detected early may be raised as static errors. In addition an implementation may report any condition found during static analysis as a warning, provided that this does not prevent the stylesheet being evaluated as described by this specification.

A stylesheet can also control the type annotations of nodes that it constructs in a result tree. This can be done in a number of ways.

• It is possible to request explicit validation of a complete document, that is, a result tree rooted at a document node. Validation is either strict or lax, as described in [XML Schema Part 1]. If validation of a result tree fails (strictly speaking, if the outcome of the validity assessment is invalid), then the transformation fails, but in all other cases, the element and attribute nodes of the tree will be annotated with the names of the types to which these nodes conform. These type annotations will be discarded if the result tree is serialized as an XML document, but they remain available when the result tree is passed to an application (perhaps another stylesheet) for further processing.

• It is also possible to validate individual element and attribute nodes as they are constructed. This is done using the type and validation attributes of the xsl:element, xsl:attribute, xsl:copy, and xsl:copy-of instructions, or the xsl:type and xsl:validation attributes of a literal result element.

• When elements, attributes, or document nodes are copied, either explicitly using the xsl:copy or xsl:copy-of instructions, or implicitly when nodes in a sequence are attached to a new parent node, the options validation="strip" and validation="preserve" are available, to control whether existing type annotations are to be retained or not.

When nodes in a temporary tree are validated, type information is available for use by operations carried out on the temporary tree, in the same way as for a source document that has undergone schema assessment.

For details of how validation of element and attribute nodes works, see 26.4 Validation.

2.11 Streaming

[Definition: The term streaming refers to a manner of processing in which XML documents (such as source and result documents) are not represented by a complete tree of nodes occupying memory proportional to document size, but instead are processed “on the fly” as a sequence of events, similar in concept to the stream of events notified by an XML parser to represent markup in lexical XML.]

[Definition: A streamed document is a source tree that is processed using streaming, that is, without constructing a complete tree of nodes in memory.]

[Definition: A streamed node is a node in a streamed document.]

Many processors implementing earlier versions of this specification adopted an architecture that allowed streaming of the result tree directly to a serializer, without first materializing the complete result tree in memory. Streaming of the source tree, however, has proved to be more difficult without subsetting the language. This has created a situation where documents exceeding the capacity of virtual memory could not be transformed. XSLT 3.0 therefore introduced facilities allowing stylesheets to be written in a way that makes streaming of source documents possible, without excessive reliance on processor-specific optimization techniques.

Streaming achieves two important objectives: it allows large documents to be transformed without requiring correspondingly large amounts of memory; and it allows the processor to start producing output before it has finished receiving its input, thus reducing latency.

This specification does not attempt to legislate precisely which implementation techniques fall under the definition of streaming, and which do not. A number of techniques are available that reduce memory requirements, while still requiring a degree of buffering, or allocation of memory to partial results. A stylesheet that requests streaming of a source document is indicating that the processor should avoid assuming that the entire source document will fit in memory; in return, the stylesheet must be written in a way that makes streaming possible. This specification does not attempt to describe the algorithms that the processor should actually use, or to impose quantitative constraints on the resources that these algorithms should consume.

Nothing in this specification, nor in its predecessors [XSLT 1.0] and [XSLT 2.0], prevents a processor using streaming whenever it sees an opportunity to do so. However, experience has shown that in order to achieve streaming, it is often necessary to write stylesheet code in such a way as to make this possible. Therefore, XSLT provides explicit constructs allowing the stylesheet author to request streaming, and defines explicit static constraints on the structure of the code which are designed to make streaming possible.

A processor that claims conformance with the streaming option offers a guarantee that when streaming is requested for a source document, and when the stylesheet conforms to the rules that make the processing guaranteed-streamable, then an algorithm will be adopted in which memory consumption is either completely independent of document size, or increases only very slowly as document size increases, allowing documents to be processed that are orders-of-magnitude larger than the physical memory available. A processor that does not claim conformance with the streaming option must still process a stylesheet and deliver the correct results, but is not required to use streaming algorithms, and may therefore fail with out-of-memory errors when presented with large source documents.

Apart from the fact that there are constructs to request streaming, and rules that must be followed to guarantee that streaming is possible, the language has been designed so there are as few differences as possible between streaming and non-streaming evaluation. The semantics of the language continue to be expressed in terms of the XDM data model, which is substantively unchanged; but readers must take care to observe that when terms like “node” and “axis” are used, the concepts are completely abstract and may have no direct representation in the run-time execution environment.

Streamed processing of a document can be initiated in one of three ways:

• The initial mode can be declared as a streamable mode. In this case the initial match selection will generally be a document node (or sequence of document nodes), supplied by the calling application in a form that allows streaming (that is, in some form other than a tree in memory; for example, as a reference to a push or pull XML parser primed to deliver a stream of events). The type of these nodes can be constrained by using the attribute on-no-match="fail" on the initial mode, and using this mode only for processing the top-level nodes.

• Streamed processing of any document can be initiated using the xsl:source-document instruction. This has an attribute href whose value is the URI of a document to be processed, and an attribute streamable that indicates whether it is to be processed using streaming; the actual processing to be applied is defined by the instructions written as children of the xsl:source-document instruction.

• Streamed merging of a set of input documents can be initiated using the xsl:merge instruction.

The rules for streamability, which are defined in detail in 19 Streamability, impose two main constraints:

• The only nodes reachable from the node that is currently being processed are its attributes and namespaces, its ancestors and their attributes and namespaces, and its descendants and their attributes and namespaces. The siblings of the node, and the siblings of its ancestors, are not reachable in the tree, and any attempt to use their values is a static error.

• When processing a given node in the tree, each descendant node can only be visited once. Essentially this allows two styles of processing: either visit each of the children once, and then process that child with the same restrictions applied; or process all the descendants in a single pass, in which case it is not possible while processing a descendant to make any further downward selection.

The second restriction, that only one visit to the children is allowed, means that XSLT code that was not designed with streaming in mind will often need to be rewritten to make it streamable. In many cases it is possible to do this using a technique sometimes called windowing or burst-mode streaming (note this is not quite the same meaning as windowing in XQuery 3.0). Many XML documents consist of a large number of elements, each of manageable size, representing transactions or business objects where each such element can be processed independently: in such cases, an effective design pattern is to write a streaming transformation that takes a snapshot of each element in turn, processing the snapshot using the full power of the XSLT language. Each snapshot is a tree built in memory and is therefore fully navigable. For details see the snapshot and copy-of functions.

The new facility of accumulators allows applications complete control over how much information is retained (and by implication, how much memory is required) in the course of a pass over a streamed document. An accumulator computes a value for every node in a streamed document: or more accurately, two values, one for the first visit to a node (before visiting its descendants), and a second value for the second visit to the node (after visiting the descendants). The computation is structured in such a way that the value for a given node can depend only on the value for the previous node in document order together with the data available when positioned at the current node (for example, the attribute values). Based on the well-established fold operation of functional programming languages, accumulators provide the convenience and economy of mutable variables while remaining within the constraints of a purely declarative processing model.

When streaming is initiated, for example using the xsl:source-document instruction, it is necessary to declare which accumulators are applicable to the streamed document.

Streaming applications often fall into one of the following categories:

• Aggregation applications, where a single aggregation operation (perhaps count, sum, exists, or distinct-values) is applied to a set of elements selected from the streamed source document by means of a path expression.

• Record-at-a-time applications, where the source document consists of a long sequence of elements with similar structure (“records”), and each “record” is processed using the same logic, independently of any other “records”. This kind of processing is facilitated using the snapshot and copy-of function mentioned earlier.

• Grouping applications, where the output follows the structure of the input, except that an extra layer of hierarchy is added. For example, the input might be a flat series of banking transactions in date/time order, and the output might contain the same transactions grouped by date.

• Accumulator applications, which are the same as record-at-a-time applications, except that the processing of one “record” might depend on data encountered earlier in the document. A classic example is processing a sequence of banking transactions in which the input transaction contains a debit or credit amount, and the output adds a running total (the account balance). The xsl:iterate instruction has been introduced to facilitate this style of processing.

• Isomorphic transformations, in which there is an ordered (often largely one-to-one) relationship between the nodes of the source tree and the nodes of the result tree: for example, transformations that involve only the renaming or selective deletion of nodes, or scalar manipulations of the values held in the leaf nodes. Such transformations are most conveniently expressed using recursive application of template rules. This is possible with a streamed input document only if all the template rules adhere to the constraints required for streamability. To enforce these rules, while still allowing unrestricted processing of other documents within the same transformation, all streaming evaluation must be carried out using a specific mode, which is declared to be a streaming mode by means of an xsl:mode declaration in the stylesheet.

There are important classes of application in which streaming is possible only if multiple streams can be processed in parallel. This specification therefore provides facilities:

1. allowing multiple sorted input sequences to be merged into one sorted output sequence (the xsl:merge instruction)

2. allowing multiple output sequences to be generated during a single pass of an input sequence (the xsl:fork instruction).

These facilities have been designed in such a way that they can readily be implemented using streaming, that is, without materializing the input or output sequences in memory.

2.12 Streamed Validation

Streaming can be combined with schema-aware processing: that is, the streamed input to a transformation can be subjected to on-the-fly validation, a process which typically accepts an input stream from the XML parser and delivers an output stream (of type-annotated nodes) to the transformation processor. The XSD specification is designed so that validation is, with one or two exceptions, a streamable process. The exceptions include:

• There may be a need to allocate memory to hold keys, in order to enforce uniqueness and referential integrity constraints (xs:unique, xs:key, xs:keyref).

• In XSD 1.1, assertions can be defined by means of XPath expressions. These are not constrained to be streamable; in the general case, any subtree of the document that is validated using an assertion may need to be buffered in memory while the assertion is processed.

Applications that need to run in finite memory may therefore need to avoid these XSD features, or to use them with care.

XSD is designed so that the intended type of an element (the “governing type”) can be determined as soon as the start tag of the element is encountered: the process of validation checks whether the content of the element actually conforms to this type, and by the time the end tag is encountered, the process will have established either that the element is valid against the governing type, or that it is invalid.

By default, dynamic errors occurring during streamed processing are fatal: they typically cause the transformation to fail immediately. XSLT 3.0 introduced the ability to catch dynamic errors and recover from them. Schema invalidity, however, is treated as a dynamic error of the instruction that processes the entire input stream, so after a validation failure, no further processing of that input stream is possible.

In consequence, a streamed validator that is running in tandem with a streamed transformation can present the transformer with element nodes that carry a provisional type annotation representing the type that the element will have if it turns out to be valid. As soon as a node is encountered that violates this assumption, the validator should stop the flow of data to the transformer, so that the transformer never sees invalid data. This allows the stylesheet code to be compiled with the assumption of type-safety: at run-time, all nodes seen by the transformation will conform to their XSLT-declared types (for example, a type declared implicitly using match="schema-element(invoice)" on an xsl:template element).

A streamed transformation that only accesses part of the input document (for example, a header at the start of a document) is not required to continue reading once the data it needs has been read. This means that XML well-formedness or validity errors occurring in the unread part of the input stream may go undetected.

2.13 Streaming of non-XML data

The facilities in this specification designed to enable large data sets to be processed in a streaming manner are oriented almost entirely to XML data. This does not mean that there is never a requirement to stream non-XML data, or that the Working Group has ignored this requirement; rather, the Working Group has concluded that for the most part, streaming of non-XML data can be achieved by implementations without the need for specific language features in XSLT.

To make streamed processing of unparsed text files easier, the function unparsed-text-lines has been introduced. This is not only more convenient for stylesheet authors than reading the entire input using the unparsed-text function and then tokenizing the result, it is also easier for implementations to optimize, allowing each line of text to be discarded from memory after it has been processed.

For all functions that access external data, including document, doc, collection, unparsed-text, unparsed-text-lines, and json-doc, the requirements on determinism can now be relaxed using implementation-defined configuration options. This is significant because it means that when a transformation reads the same external resource more than once, it becomes legitimate for the contents of the resource to be different on different invocations, and this eliminates the need for the processor to cache the contents of the resource in memory.

In the XDM data model, every value is a sequence, and (as with most functional programming languages), processing of sequences of items is pervasive throughout the XSLT and XPath languages and their function library. Good performance of a functional programming language often depends on sequence-based operations being pipelined, and being evaluated in a lazy fashion (that is, many operations process items in a sequence one at a time, in order; and many operations can deliver a result without processing the entire sequence). The semantics of XSLT and XPath permit pipelined and lazy evaluation (for example, the error handling semantics are carefully written to ensure this), but they do not require it: the details are left to implementations. Pipelined processing of a sequence is not the same thing as streamed processing of a tree, and where the XSLT specification talks of operations being “guaranteed streamable”, this is always referring to processing of trees, not of sequences.

The facilities for streaming of XML trees include operations such as copy-of and snapshot which are able to take a sequence of streamed nodes as input, and produce a sequence of in-memory (unstreamed) nodes as output. It is also possible to generate a sequence of strings or other atomic items through the process of atomization. The actual memory usage of a streamed XSLT application may depend significantly on whether the processing of the resulting sequence of in-memory nodes or atomic items is pipelined or not. The specification, however, has nothing to say on this matter: it is considered an area where implementers can exercise their discretion and ingenuity.

Streaming of JSON input receives little attention in this specification. One can envisage an implementation of the json-to-xml function in which the XML delivered by the function consists of streamed nodes; but the Working Group has not researched the feasibility of such an implementation in any detail.

2.14 Error Handling

[Definition: An error that can be detected by examining a stylesheet before execution starts (that is, before the source document and values of stylesheet parameters are available) is referred to as a static error.]

Generally, errors in the structure of the stylesheet, or in the syntax of XPath expressions contained in the stylesheet, are classified as static errors. Where this specification states that an element in the stylesheet must or must not appear in a certain position, or that it must or must not have a particular attribute, or that an attribute must or must not have a value satisfying specified conditions, then any contravention of this rule is a static error unless otherwise specified.

A processor must provide a mode of operation that takes a (possibly erroneous) stylesheet package as input and enables the user to determine whether or not that package contains any static errors.

A processor may also provide a mode of operation in which static errors in parts of the stylesheet that are not evaluated can go unreported.

[Definition: An error that is not capable of detection until a source document is being transformed is referred to as a dynamic error.]

When a dynamic error occurs, and is not caught using xsl:catch, the processor must raise the error, and the transformation fails.

Because different implementations may optimize execution of the stylesheet in different ways, the detection of dynamic errors is to some degree implementation-dependent. In cases where an implementation is able to produce a principal result or secondary result without evaluating a particular construct, the implementation is never required to evaluate that construct solely in order to determine whether doing so causes a dynamic error. For example, if a variable is declared but never referenced, an implementation may choose whether or not to evaluate the variable declaration, which means that if evaluating the variable declaration causes a dynamic error, some implementations will raise this error and others will not.

There are some cases where this specification requires that a construct must not be evaluated: for example, the content of an xsl:if instruction must not be evaluated if the test condition is false. This means that an implementation must not raise any dynamic errors that would arise if the construct were evaluated.

An implementation may raise a dynamic error before any source document is available, but only if it can determine that the error would be raised for every possible source document and every possible set of parameter values. For example, some circularity errors fall into this category: see 9.11 Circular Definitions.

There are also some dynamic errors where the specification gives a processor license to raise the error during the analysis phase even if the construct might never be executed; an example is the use of an invalid QName as a literal argument to a function such as key, or the use of an invalid regular expression in the regex attribute of the xsl:analyze-string instruction.

A dynamic error is also raised during the static analysis phase if the error occurs during evaluation of a static expression.

The XPath specification states (see Section 2.4.1 Kinds of Errors^XP) that if any expression (at any level) can be evaluated during the analysis phase (because all its explicit operands are known and it has no dependencies on the dynamic context), then any error in performing this evaluation may be raised as a static error. For XPath expressions used in an XSLT stylesheet, however, any such errors must not be raised as static errors in the stylesheet unless they would occur in every possible evaluation of that stylesheet; instead, they must be raised as dynamic errors, and raised only if the XPath expression is actually evaluated.

<xsl:choose>
  <xsl:when test="system-property('xsl:version') = '1.0'">
    <xsl:value-of select="1 div 0"/>
  </xsl:when>
  <xsl:otherwise>
    <xsl:value-of select="xs:double('INF')"/>
  </xsl:otherwise>
</xsl:choose>

[Definition: Certain errors are classified as type errors. A type error occurs when the value supplied as input to an operation is of the wrong type for that operation, for example when an integer is supplied to an operation that expects a node.] If a type error occurs in an instruction that is actually evaluated, then it must be raised in the same way as a dynamic error. Alternatively, an implementation may raise a type error during the analysis phase in the same way as a static error, even if it occurs in part of the stylesheet that is never evaluated, provided it can establish that execution of a particular construct would never succeed.

It is implementation-defined whether type errors are raised statically.

<xsl:if test="false()">
  <xsl:number select="42"/>
</xsl:if>

<xsl:template match="para">
  <xsl:param name="p" as="item()"/>
  <xsl:number select="$p"/>
</xsl:template>

If more than one error arises, an implementation is not required to raise any errors other than the first one that it detects. It is implementation-dependent which of the several errors is raised. This applies both to static errors and to dynamic errors. An implementation is allowed to raise more than one error, but if any errors have been raised, it must not finish as if the transformation were successful.

When a transformation raises one or more dynamic errors, the final state of any persistent resources updated by the transformation is implementation-dependent. Implementations are not required to restore such resources to their initial state. In particular, where a transformation produces multiple result documents, it is possible that one or more serialized result documents may be written successfully before the transformation terminates, but the application cannot rely on this behavior.

Everything said above about error handling applies equally to errors in evaluating XSLT instructions, and errors in evaluating XPath expressions. Static errors and dynamic errors may occur in both cases.

[Definition: If a transformation has successfully produced a principal result or secondary result, it is still possible that errors may occur in serializing that result . For example, it may be impossible to serialize the result using the encoding selected by the user. Such an error is referred to as a serialization error.] If the processor performs serialization, then it must do so as specified in 27 Serialization, and in particular it must raise any serialization errors that occur.

Errors are identified by a QName. For errors defined in this specification, the namespace of the QName is always http://www.w3.org/2005/xqt-errors (and is therefore not given explicitly), while the local part is an 8-character code in the form PPSSNNNN. Here PP is always XT (meaning XSLT), and SS is one of SE (static error), DE (dynamic error), or TE (type error). Note that the allocation of an error to one of these categories is purely for convenience and carries no normative implications about the way the error is handled. Many errors, for example, can be raised either dynamically or statically. These error codes are used to label error conditions in this specification, and are summarized in E Summary of Error Conditions.

Errors defined in related specifications ([XPath 4.0], [Functions and Operators 4.0] [XSLT and XQuery Serialization]) use QNames with a similar structure, in the same namespace. When errors occur in processing XPath expressions, an XSLT processor should use the original error code reported by the XPath processor, unless a more specific XSLT error code is available.

Implementations must use the codes defined in these specifications when raising dynamic errors, to ensure that xsl:catch behaves in an interoperable way across implementations. Stylesheet authors should note, however, that there are many examples of errors where more than one rule in this specification is violated, and where the processor therefore has discretion in deciding which error code to associate with the condition: there is therefore no guarantee that different processors will always use the same error code for the same erroneous input.

Additional errors defined by an implementation (or by an application) may use QNames in an implementation-defined (or user-defined) namespace without risk of collision.

3.1 Namespaces

A stylesheet module is typically written as a namespace well-formed XML document: more formally, as described in 3.6 Stylesheet Modules, a stylesheet module takes the form of an XDM tree rooted at an element node.

The names of elements and attributes in source documents and result documents are namespace-qualified names. In addition, as described in 5.1.1 Qualified Names, XSLT uses namespace-qualified names to identify variables, functions, templates, and other components. These names generally use namespace prefixes that are resolved to namespace URIs using a set of prefix-uri namespace bindings.

There are two ways namespace bindings can be established in a stylesheet module:

• [Definition:  The native namespace bindings for any element in an XSLT stylesheet module are the prefix-uri mappings defined by the namespace nodes of that element, according to the rules in [XDM 3.0].]

  For example, a namespace declaration of the form xmlns:math="http://www.w3.org/2005/xpath-functions/math establishes a binding of the prefix math to the namespace URI http://www.w3.org/2005/xpath-functions/math, thereby enabling functions in that namespace to be invoked using an expression such as math:sin($theta)

• [Definition:  The fixed namespace bindings for a stylesheet module are established using the fixed-namespaces attribute on the xsl:stylesheet, xsl:transform, or xsl:package element enclosing the stylesheet module.]

  For example, the attribute fixed-namespaces="math map array" establishes bindings for the prefixes math, map, and array to the namespace URIs conventionally associated with these prefixes as described in 5.1.3 Reserved Namespaces.

Prefixes used in element and attribute names in the stylesheet, because these are interpreted by the XML parser and not only by the XSLT processor, must be bound using native namespace bindings. In particular, the XSLT namespace is used in the names of XSLT elements, so it must be declared in every stylesheet module using a namespace declaration such as xmlns:xsl="http://www.w3.org/1999/XSL/Transform. (A different prefix can be used: some users prefer xslt, some favor the default namespace.) But namespace prefixes that are only used within the content of attribute and text nodes in the stylesheet (for example, select="math:sin($theta)") can be declared in fixed namespace bindings.

[Definition:  The applicable static namespaces for an element in a stylesheet module are the fixed namespace bindings for the module if the root element of the module has a fixed-namespaces attribute, or the native namespace bindings of the element otherwise. ]

The effect of declaring fixed namespace bindings is described in more detail in 3.7.1 The fixed-namespaces Attribute.

3.2 Extension Attributes

[Definition: An element from the XSLT namespace may have any attribute not from the XSLT namespace, provided that the expanded QName (see [XPath 3.0]) of the attribute has a non-null namespace URI. These attributes are referred to as extension attributes.] The presence of an extension attribute must not cause the principal result or any secondary result of the transformation to be different from the results that a conformant XSLT 4.0 processor might produce. They must not cause the processor to fail to raise an error that a conformant processor is required to raise. This means that an extension attribute must not change the effect of any instruction except to the extent that the effect is implementation-defined or implementation-dependent.

Furthermore, if serialization is performed using one of the serialization methods described in [XSLT and XQuery Serialization], the presence of an extension attribute must not cause the serializer to behave in a way that is inconsistent with the mandatory provisions of that specification.

<xsl:message abc:pause="yes"
    xmlns:abc="http://vendor.example.com/xslt/extensions">
       Phase 1 complete
</xsl:message>

[ERR XTSE0090] It is a static error for an element from the XSLT namespace to have an attribute whose namespace is either null (that is, an attribute with an unprefixed name) or the XSLT namespace, other than attributes defined for the element in this document.

3.3 XSLT Media Type

The media type application/xslt+xml has been registered for XSLT stylesheet modules.

The definition of the media type is at [XSLT Media Type].

This media type should be used for an XML document containing a standard stylesheet module at its top level, and it may also be used for a simplified stylesheet module. It should not be used for an XML document containing an embedded stylesheet module.

3.4 Standard Attributes

[Definition: There are a number of standard attributes that may appear on any XSLT element: specifically default-collation, default-mode, default-validation, exclude-result-prefixes, expand-text, extension-element-prefixes, use-when, version, and xpath-default-namespace.]

These attributes may also appear on a literal result element, but in this case, to distinguish them from user-defined attributes, the names of the attributes are in the XSLT namespace. They are thus typically written as xsl:default-collation, xsl:default-mode, xsl:default-validation, xsl:exclude-result-prefixes, xsl:expand-text, xsl:extension-element-prefixes, xsl:use-when, xsl:version, or xsl:xpath-default-namespace.

It is recommended that all these attributes should also be permitted on extension instructions, but this is at the discretion of the implementer of each extension instruction. They may also be permitted on user-defined data elements, though they will only have any useful effect in the case of data elements that are designed to behave like XSLT declarations or instructions.

In the following descriptions, these attributes are referred to generically as [xsl:]version, and so on.

These attributes all affect the element they appear on, together with any elements and attributes that have that element as an ancestor. The two forms with and without the XSLT namespace have the same effect; the XSLT namespace is used for the attribute if and only if its parent element is not in the XSLT namespace.

In the case of [xsl:]default-collation, [xsl:]expand-text, [xsl:]version, and [xsl:]xpath-default-namespace, the value can be overridden by a different value for the same attribute appearing on a descendant element. The effective value of the attribute for a particular stylesheet element is determined by the innermost ancestor-or-self element on which the attribute appears.

In an embedded stylesheet module, standard attributes appearing on ancestors of the outermost element of the stylesheet module have no effect.

In the case of [xsl:]exclude-result-prefixes and [xsl:]extension-element-prefixes the values are cumulative. For these attributes, the value is given as a whitespace-separated list of namespace prefixes, and the effective value for an element is the combined set of namespace URIs designated by the prefixes that appear in this attribute for that element and any of its ancestor elements. Again, the two forms with and without the XSLT namespace are equivalent.

The effect of the [xsl:]use-when attribute is described in 3.13.3 Conditional Element Inclusion.

Because these attributes may appear on any XSLT element, they are not listed in the syntax summary of each individual element. Instead they are listed and described in the entry for the xsl:stylesheet, xsl:transform, and xsl:package elements only. This reflects the fact that these attributes are often used on the outermost element of the stylesheet, in which case they apply to the entire stylesheet module or package manifest.

Note that the effect of these attributes does not extend to stylesheet modules referenced by xsl:include or xsl:import declarations, nor to packages referenced using xsl:use-package.

For the detailed effect of each attribute, see the following sections:

[xsl:]default-collation

see 3.7.2 The default-collation Attribute

[xsl:]default-mode

see 3.7.3 The default-mode Attribute

[xsl:]default-validation

see 26.4 Validation

[xsl:]exclude-result-prefixes

see 11.1.3 Namespace Nodes for Literal Result Elements

[xsl:]expand-text

see 5.7.2 Text Value Templates

[xsl:]extension-element-prefixes

see 25.2 Extension Instructions

[xsl:]use-when

see 3.13.3 Conditional Element Inclusion

[xsl:]version

see 3.9 Backwards Compatible Processing and 3.10 Forwards Compatible Processing

[xsl:]xpath-default-namespace

see 5.1.2 Unprefixed Lexical QNames in Expressions and Patterns

3.5 Packages

[Definition: An explicit package is represented by an xsl:package element, which will generally be the outermost element of an XML document. When the xsl:package element is not used explicitly, the entire stylesheet comprises a single implicit package.] (This specification does not preclude the xsl:package being embedded in another XML document, but it will never have any other XSLT element as an ancestor).

<xsl:package
  id? = id
  name? = uri
  package-version? = string〔'1'〕
  version = decimal
  input-type-annotations? = "preserve" | "strip" | "unspecified"〔'unspecified'〕
  declared-modes? = boolean〔'yes'〕
  default-mode? = eqname | "#unnamed"〔'#unnamed'〕
  default-validation? = "preserve" | "strip"〔'strip'〕
  default-collation? = uris
  extension-element-prefixes? = prefixes
  exclude-result-prefixes? = prefixes
  expand-text? = boolean〔'no'〕
  fixed-namespaces? = string
  use-when? = expression〔true()〕
  xpath-default-namespace? = uri >
  <!-- Content: ((xsl:expose | declarations)*) -->
</xsl:package>

[Definition: The content of the xsl:package element is referred to as the package manifest].

The version attribute indicates the version of the XSLT language specification to which the package manifest conforms. The value should normally be 4.0. If the value is numerically less than 4.0, the content of the xsl:package element is processed using the rules for backwards compatible behavior (see 3.9 Backwards Compatible Processing). If the value is numerically greater than 4.0, it is processed using the rules for forwards compatible behavior (see 3.10 Forwards Compatible Processing).

A package typically has a name, given in its name attribute, which must be an absolute URI. Unnamed packages are allowed, but they can only be used as the “top level” of an application; they cannot be the target of an xsl:use-package declaration in another package.

A package may have a version identifier, given in its package-version attribute. This is used to distinguish different versions of a package. The value of the version attribute, after trimming leading and trailing whitespace, must conform to the syntax given in 3.5.1 Versions of a Package. If no version number is specified for a package, version 1 is assumed.

The attributes default-collation, default-mode, default-validation, exclude-result-prefixes, expand-text, extension-element-prefixes, use-when, version, and xpath-default-namespace are standard attributes that can appear on any XSLT element, and potentially affect all descendant elements. Their meaning is described in 3.4 Standard Attributes.

The package manifest contains the following elements, arbitrarily ordered:

1. Zero or more xsl:expose declarations that define the interface offered by this package to the outside world. An xsl:expose declaration may appear only as a child of xsl:package.

2. Zero or more additional declarations. These are the same as the declarations permitted as children of xsl:stylesheet or xsl:transform.

   Some declarations of particular relevance to packages include:

   1. The xsl:use-package declaration, which declares the names and versions of the packages on which this package depends.

   2. The optional xsl:global-context-item element; if present this element defines constraints on the existence and type of the global context item.

   3. Zero or more xsl:include and xsl:import declarations, which define additional stylesheet modules to be incorporated into this package.

   4. Zero or more ordinary declarations, that is, elements that are permitted as children of xsl:stylesheet or xsl:transform. One possible coding style is to include in the package manifest just a single xsl:import or xsl:include declaration as a reference to the effective top-level stylesheet module; this approach is particularly suitable when writing code that is required to run under releases of XSLT earlier than 3.0. Another approach is to include the substance of the top-level stylesheet module inline within the package manifest.

<xsl:package
  name="http://example.org/complex-arithmetic.xsl"
  package-version="1.0"
  version="3.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:f="http://example.org/complex-arithmetic.xsl">
  
  <xsl:function name="f:complex-number" 
                as="map(xs:integer, xs:double)" visibility="public">
    <xsl:param name="real" as="xs:double"/>
    <xsl:param name="imaginary" as="xs:double"/>
    <xsl:sequence select="{ 0: $real, 1: $imaginary }"/>
  </xsl:function>
  
  <xsl:function name="f:real" 
                as="xs:double" visibility="public">
    <xsl:param name="complex" as="map(xs:integer, xs:double)"/>
    <xsl:sequence select="$complex(0)"/>
  </xsl:function>
  
  <xsl:function name="f:imag" 
                as="xs:double" visibility="public">
    <xsl:param name="complex" as="map(xs:integer, xs:double)"/>
    <xsl:sequence select="$complex(1)"/>
  </xsl:function>
  
  <xsl:function name="f:add" 
                as="map(xs:integer, xs:double)" visibility="public">
    <xsl:param name="x" as="map(xs:integer, xs:double)"/>
    <xsl:param name="y" as="map(xs:integer, xs:double)"/>
    <xsl:sequence select="
         f:complex-number(
           f:real($x) + f:real($y), 
           f:imag($x) + f:imag($y))"/>
  </xsl:function>
  
  <xsl:function name="f:multiply" 
                as="map(xs:integer, xs:double)" visibility="public">
    <xsl:param name="x" as="map(xs:integer, xs:double)"/>
    <xsl:param name="y" as="map(xs:integer, xs:double)"/>
    <xsl:sequence select="
         f:complex-number(
           f:real($x)*f:real($y) - f:imag($x)*f:imag($y),
           f:real($x)*f:imag($y) + f:imag($x)*f:real($y))"/>
  </xsl:function>
  
  <!-- etc. -->
  
</xsl:package>

A package that does not itself expose any components may be written using a simplified syntax: the xsl:package element is omitted, and the xsl:stylesheet or xsl:transform element is now the outermost element of the stylesheet module. For compatibility reasons, all the named templates and modes declared in the package are made public. More formally, the principal stylesheet module of the top-level package may be expressed as an xsl:stylesheet or xsl:transform element, which is equivalent to the package represented by the output of the following transformation, preserving the base URI of the source:

<xsl:transform version="3.0" 
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:t="http://www.w3.org/1999/XSL/TransformAlias">
 
    <xsl:namespace-alias stylesheet-prefix="t" result-prefix="xsl"/>
    
    <xsl:template match="xsl:stylesheet|xsl:transform">
      <t:package declared-modes="no">
        <xsl:copy-of select="@*"/>
        <t:expose component="mode" names="*" visibility="public"/>
        <t:expose component="template" names="*" visibility="public"/>
        <xsl:copy-of select="node()"/>
      </t:package>
    </xsl:template>
 </xsl:transform>

The effect of the input-type-annotations attribute is defined in 4.3.1 Stripping Type Annotations from a Source Tree.

A more extensive example of a package, illustrating how components in a package can be overridden in a client package, is given in 3.5.7 Worked Example of a Library Package.

        PackageVersion   ::= NumericPart ( "-" NamePart )?
        NumericPart      ::= IntegerLiteral ( "." IntegerLiteral )*
        NamePart         ::= NCName

0-rc1 < 0-rc2 < 0 < 1 = 1.0 < 1.0.2 
   < 1.0.3-rc1 < 1.0.3 < 1.0.3.2 < 1.0.10

        PackageVersionRange    ::=  AnyVersion | VersionRanges
        AnyVersion             ::=  "*"
        VersionRanges          ::=  VersionRange (S? "," S? VersionRange)*
        VersionRange           ::=  PackageVersion | VersionPrefix | 
                                      VersionFrom | VersionTo | VersionFromTo
        VersionPrefix          ::=  PackageVersion ".*"
        VersionFrom            ::=  PackageVersion "+"
        VersionTo              ::=  "to" S (PackageVersion | VersionPrefix)
        VersionFromTo          ::=  PackageVersion S "to" S (PackageVersion | VersionPrefix)

<xsl:package name="Q"
        version="3.0"                
        xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:variable name="A" visibility="final" select="$B + 1"/>
  <xsl:variable name="B" visibility="private" select="$C * 2"/>
  <xsl:variable name="C" visibility="public" select="22"/>
</xsl:package>

<xsl:package name="P"
        version="3.0"                
        xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:use-package name="Q">
    <xsl:override>
      <xsl:variable name="C" visibility="private" select="$xsl:original + 3"/>
    </xsl:override>
  </xsl:use-package>
  
  <xsl:template name="T" visibility="public">
    <xsl:value-of select="$A"/>
  </xsl:template>
</xsl:package>

<xsl:variable name="v" as="xs:integer" visibility="public" select="3"/>

<xsl:function name="f:factory" as="fn(*)" visibility="final">
  <xsl:sequence select="fn() { $v }"/>
</xsl:function>

<xsl:use-package name="P">
  <xsl:override>
    <xsl:variable name="v" as="xs:integer" select="4"/>
  </xsl:override>
</xsl:use-package>

<xsl:template name="xsl:initial-template">
  <v value="{f:factory()()}"/>
</xsl:template>

<xsl:variable name="get-order" select="key('orders-key', ?, ?)"/>

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
   xmlns:xs="http://www.w3.org/2001/XMLSchema"
   xmlns:csv="http://example.com/csv"
   exclude-result-prefixes="xs csv"
   version="3.0">

   <xsl:output indent="yes"/>

   <xsl:use-package name="http://example.com/csv-parser" 
                    package-version="*"/>

   <!-- example input "file"  -->
   <xsl:variable name="input" as="xs:string">
       name,id,postal code
       "Abel Braaksma",34291,1210 KA
       "Anders Berglund",473892,9843 ZD
   </xsl:variable>

   <!-- entry point -->
   <xsl:template name="xsl:initial-template">
       <xsl:copy-of select="csv:parse($input)"/>
   </xsl:template>

</xsl:stylesheet>

<csv>
  <row>
    <field quoted="no">name</field>
    <field quoted="no">id</field>
    <field quoted="no">postal code</field>
  </row>
  <row>
    <field quoted="yes">Abel Braaksma</field>
    <field quoted="no">34291</field>
    <field quoted="no">1210 KA</field>
  </row>
  <row>
    <field quoted="yes">Anders Berglund</field>
    <field quoted="no">473892</field>
    <field quoted="no">9843 ZD</field>
  </row>
</csv>

<?xml version="1.0" encoding="UTF-8"?>
<xsl:package
   name="http://example.com/csv-parser"
   package-version="1.0.0"
   xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
   xmlns:xs="http://www.w3.org/2001/XMLSchema"
   xmlns:csv="http://example.com/csv"
   exclude-result-prefixes="xs csv"
   declared-modes="yes"
   version="3.0">

   <!--* Mode declarations ... *-->
   <!--* Variable declarations ... *-->
   <!--* Attribute-set declaration ... *-->
   <!--* Function declarations ... *-->
   <!--* Templates ... *-->

</xsl:package>

<xsl:function name="csv:parse" visibility="final">
    <xsl:param name="input" as="xs:string"/>   
    <xsl:variable name="result" as="element()">
        <csv>
            <xsl:apply-templates 
                select="(tokenize($input, $csv:line-separator) 
                        ! csv:preprocess-line(.))" 
                mode="csv:parse-line"/>
        </csv>
    </xsl:variable>
    <xsl:apply-templates select="$result" 
                         mode="csv:post-process"/>
</xsl:function>

<xsl:variable name="csv:line-separator" 
              as="xs:string" 
              select="'\r\n?|\n\r?'" 
              visibility="public"/>

<xsl:function name="csv:preprocess-line" 
                 as="xs:string?" 
                 visibility="public">
    <xsl:param name="line" as="xs:string"/>
    <xsl:sequence select="normalize-space($line)"/>
</xsl:function>

<xsl:mode name="csv:parse-line" visibility="public"/>

<xsl:template match="." mode="csv:parse-line">
    <row>
        <xsl:apply-templates 
            select="tokenize(., $csv:field-separator)" 
            mode="csv:parse-field"/>
    </row>
</xsl:template>

<xsl:variable name="csv:field-separator" 
              as="xs:string" 
              select="','" 
              visibility="public"/>

<xsl:template match="." 
              mode="csv:parse-field" 
              expand-text="yes">
    <xsl:variable name="string-body-pattern"
                  as="xs:string"
                  select="'([^' || $csv:validated-quote || ']*)'"/>
    <xsl:variable name="quoted-value"
                  as="xs:string"
                  select="$csv:validated-quote 
                          || $string-body-pattern 
                          || $csv:validated-quote"/>
    <xsl:variable name="unquoted-value"
                  as="xs:string"
                  select="'(.+)'"/>

    <field xsl:use-attribute-sets="csv:field-attributes">{
        csv:preprocess-field(
          replace(., 
                  $quoted-value || '|' || $unquoted-value, 
                  '$1$2'))
    }</field>
</xsl:template>

<xsl:attribute-set name="csv:field-attributes" 
                   visibility="public">
    <xsl:attribute name="quoted" 
                   select="if (starts-with(., $csv:validated-quote)) 
                           then 'yes' 
                           else 'no'"/>
</xsl:attribute-set>

<xsl:mode name="csv:parse-field"
          on-no-match="shallow-copy" 
          visibility="public"/>

<xsl:variable name="csv:quote" 
              as="xs:string" 
              select="'&quot;'" 
              visibility="public"/>

<xsl:variable name="csv:validated-quote" visibility="private"
   as="xs:string" select="
       if (string-length($csv:quote) ne 1) 
       then error(xs:QName('csv:ERR001'), 
                  'Incorrect length for $csv:quote, should be 1') 
       else $csv:quote"/>