XSL Transformations (XSLT) Version 2.0

1.1 What is XSLT?

This specification defines the syntax and semantics of the XSLT 2.0 language.

[Definition: A transformation in the XSLT language is expressed in the form of a stylesheet, whose syntax is well-formed XML [XML 1.0] conforming to the Namespaces in XML Recommendation [Namespaces in XML 1.0].]

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 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 [Extensible Stylesheet Language (XSL)]), 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 zero or more source trees into one or more result trees. The structure of these trees is described in [Data Model]. The transformation is achieved by a set of template rules. A template rule associates a pattern, which 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. This mechanism allows a stylesheet to be applicable to a wide class of documents that have similar source tree structures.

[Definition: A stylesheet may consist of several stylesheet modules, contained in different XML documents. For a given transformation, one of these functions as the principal stylesheet module. The complete stylesheet is assembled by finding the stylesheet modules referenced directly or indirectly from the principal stylesheet module using xsl:include and xsl:import elements: see 3.10.2 Stylesheet Inclusion and 3.10.3 Stylesheet Import.]

1.2 What's New in XSLT 2.0?

XSLT 1.0 was published in November 1999, and version 2.0 represents a significant increase in the capability of the language. A detailed list of changes is included in J Changes from XSLT 1.0. XSLT 2.0 has been developed in parallel with XPath 2.0 (see [XPath 2.0]), so the changes to XPath must be considered alongside the changes to XSLT.

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 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 final output of a transformation. Once created, the contents of a final result tree are not accessible within the stylesheet itself.] The xsl:result-document instruction always creates a final result tree, and a final result tree may also be created implicitly by the initial template. The conditions under which this happens are described in 2.4 Executing a Transformation. A final result tree may be serialized as described in 20 Serialization.

[Definition: The term source tree means any tree provided as input to the transformation. This includes the document containing the initial context node if any, documents containing nodes supplied as the values of stylesheet parameters, documents obtained from the results of functions such as document, doc ^FO, and collection^FO, 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.

In this specification the phrases must, must not, should, should not, may, required, and recommended 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 21 Conformance.

Where the phrase must, must not, or required relates to a stylesheet, then the processor must enforce this constraint on stylesheets by reporting 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 2.0] or the XDM specification [Data Model]. Particular attention is drawn to the following:

• [Definition: The term atomization is defined in Section 2.4.2 Atomization^XP. It is a process that takes as input a sequence of nodes and atomic values, and returns a sequence of atomic values, in which the nodes are replaced by their typed values as defined in [Data Model].] For some nodes (for example, elements with element-only content), atomization generates a dynamic error.

• [Definition: The term typed value is defined in Section 5.15 typed-value Accessor^DM. Every node except an element defined in the schema with 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^DM. 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 2.1.1 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 2.0 and XPath 1.0.]

[Definition: The term core function means a function that is specified in [Functions and Operators] and that is in the standard function namespace.]

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 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.

  In all cases where this specification states that the value of an attribute must be one of a limited set of values, 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.

• 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 similar way 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, other than xsl:import, 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 only affects whether it is allowed in the content of elements that allow a sequence constructor or other-declarations.

[ERR XTSE0010] A static error is signaled 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.

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 non-recoverable 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 signal 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 3.9 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.] All constructs that are deprecated in this specification are also (as it happens) optional features that implementations are not required to provide.

XSLT defines a set of standard functions which are additional to those defined in [Functions and Operators]. The signatures of these functions are described using the same notation as used in [Functions and Operators]. The names of these functions are all 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.

Implementations may allow a transformation to run as two or more phases, for example parsing, compilation and execution. Such a distinction is outside the scope of this specification, which treats transformation as a single process controlled using a set of stylesheet modules, supplied in the form of XML documents.

The following information is supplied to execute a transformation:

• The stylesheet module that is to act as the principal stylesheet module for the transformation. The complete stylesheet is assembled by recursively expanding the xsl:import and xsl:include declarations in the principal stylesheet module, as described in 3.10.2 Stylesheet Inclusion and 3.10.3 Stylesheet Import.

• A set (possibly empty) of values for stylesheet parameters (see 9.5 Global Variables and Parameters). These values are available for use within expressions in the stylesheet.

• [Definition: A node that acts as the initial context node for the transformation. This node is accessible within the stylesheet as the initial value of the XPath expressions . (dot) and self::node(), as described in 5.4.3.1 Maintaining Position: the Focus].

  If no initial context node is supplied, then the context item, context position, and context size will initially be undefined, and the evaluation of any expression that references these values will result in a dynamic error. (Note that the initial context size and context position will always be 1 (one) when an initial context node is supplied, and will be undefined if no initial context node is supplied).

• Optionally, the name of a named template which is to be executed as the entry point to the transformation. This template must exist within the stylesheet. If no named template is supplied, then the transformation starts with the template rule that best matches the initial context node, according to the rules defined in 6.4 Conflict Resolution for Template Rules. Either a named template, or an initial context node, or both, must be supplied.

• Optionally, an initial mode. This must either be the default mode, or a mode that is explicitly named in the mode attribute of an xsl:template declaration within the stylesheet. If an initial mode is supplied, then in searching for the template rule that best matches the initial context node, the processor considers only those rules that apply to the initial mode. If no initial mode is supplied, the default mode is used.

• A base output URI. [Definition:  The base output URI is a URI to be used as the base URI when resolving a relative URI allocated to a final result tree. If the transformation generates more than one final result tree, then typically each one will be allocated a URI relative to this base URI. ] The way in which a base output URI is established is implementation-defined.

• A mechanism for obtaining a document node and a media type, given an absolute URI. The total set of available documents (modeled as a mapping from URIs to document nodes) forms part of the context for evaluating XPath expressions, specifically the doc ^FO function. The XSLT document function additionally requires the media type of the resource representation, for use in interpreting any fragment identifier present within a URI Reference.

  Note:

  The set of documents that are available to the stylesheet is implementation-dependent, as is the processing that is carried out to construct a tree representing the resource retrieved using a given URI. Some possible ways of constructing a document (specifically, rules for constructing a document from an Infoset or from a PSVI) are described in [Data Model].

[ERR XTDE0040] It is a non-recoverable dynamic error if the invocation of the stylesheet specifies a template name that does not match the expanded-QName of a named template defined in the stylesheet.

[ERR XTDE0045] It is a non-recoverable dynamic error if the invocation of the stylesheet specifies an initial mode (other than the default mode) that does not match the expanded-QName in the mode attribute of any template defined in the stylesheet.

[ERR XTDE0047] It is a non-recoverable dynamic error if the invocation of the stylesheet specifies both an initial mode and an initial template.

[ERR XTDE0050] It is a non-recoverable dynamic error if the stylesheet that is invoked declares a visible stylesheet parameter with required="yes" and no value for this parameter is supplied during the invocation of the stylesheet. A stylesheet parameter is visible if it is not masked by another global variable or parameter with the same name and higher import precedence.

[Definition: The transformation is performed by evaluating an initial template. If a named template is supplied when the transformation is initiated, then this is the initial template; otherwise, the initial template is the template rule selected according to the rules of the xsl:apply-templates instruction for processing the initial context node in the initial mode.]

Parameters passed to the transformation by the client application are matched against stylesheet parameters (see 9.5 Global Variables and Parameters), not against the template parameters declared within the initial template. All template parameters within the initial template to be executed will take their default values.

[ERR XTDE0060] It is a non-recoverable dynamic error if the initial template defines a template parameter that specifies required="yes".

A stylesheet can process further source documents in addition to those supplied when the transformation is invoked. These additional documents can be loaded using the functions document (see 16.1 Multiple Source Documents) or doc ^FO or collection^FO (see [Functions and Operators]), or they can be supplied as stylesheet parameters (see 9.5 Global Variables and Parameters), or as the result of an extension function (see 18.1 Extension Functions).

2.4 Executing a Transformation

[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 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.

A transformation as a whole is executed by evaluating the sequence constructor of the initial template as described in 5.7 Sequence Constructors.

If the initial template has an as attribute, then the result sequence of the initial template is checked against the required type in the same way as for any other template. If this result sequence is non-empty, then it is used to construct an implicit final result tree, following the rules described in 5.7.1 Constructing Complex Content: the effect is as if the initial template T were called by an implicit template of the form:

<xsl:template name="IMPLICIT">
  <xsl:result-document href="">
    <xsl:call-template name="T"/>
  </xsl:result-document>
</xsl:template>

An implicit result tree is also created when the result sequence is empty, provided that no xsl:result-document instruction has been evaluated during the course of the transformation. In this situation the implicit result tree will consist of a document node with no children.

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 18.2 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;

• 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:for-each, xsl:for-each-group;

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

• Instructions that declare variables: xsl:variable, xsl:param;

• other specialized instructions: xsl:number, xsl:analyze-string, xsl:message, xsl:result-document.

Often, a sequence constructor will include an xsl:apply-templates instruction, which selects a sequence of nodes to be processed. Each of the selected nodes is processed by searching the stylesheet for a matching template rule and evaluating the sequence constructor of that template rule. The resulting sequences of items are concatenated, in order, to give the result of the xsl:apply-templates instruction, as described in 6.3 Applying Template Rules; this sequence is often added to a result tree. Since the sequence constructors of the selected template rules may themselves contain xsl:apply-templates instructions, this results in a cycle of selecting nodes, identifying template rules, constructing sequences, and constructing result trees, that recurses through a source tree.

2.5 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 2.0 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 format-number) 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 (it may be a node or an atomic value) currently being processed. The value of the context item can be referenced within an XPath expression using the expression . (dot).

Full details of the static and dynamic context are provided in 5.4 The Static and Dynamic Context.

2.6 Parsing and Serialization

An XSLT stylesheet describes a process that constructs a set of final result trees from a set of source trees.

The stylesheet does not describe how a source tree is constructed. Some possible ways of constructing source trees are described in [Data Model]. 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 [Data Model], 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 20 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 report 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.

2.7 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 18.1 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 18.2 Extension Instructions. A stylesheet that uses extension instructions 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 18 Extensibility and Fallback.

The XSLT language can also be extended by the use of extension attributes (see 3.3 Extension Attributes), and by means of user-defined data elements (see 3.6.2 User-defined Data Elements).

2.8 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 the SequenceType ^XP syntax defined in [XPath 2.0].

[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 stylesheet are referred to as the in-scope schema components. This set is the same throughout all the modules of a stylesheet.]

The conformance rules for XSLT 2.0, defined in 21 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^FO function) and other constructs may produce different results depending on the data type (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 3.13 Built-in Types. The set of built-in types varies between a basic XSLT processor and a schema-aware XSLT processor.

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.

Further, 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:template match="document-node(schema-element(my:invoice))" priority="2">
. . .
</xsl:template>

<xsl:template match="document-node()" priority="1">
  <xsl:message terminate="yes">Source document is not an invoice</xsl:message>
</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 [Data Model]. The implementation may choose one of several strategies for dealing with this situation:

1. The processor may signal a non-recoverable 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 [Data Model], 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 2.9 Error Handling) type errors that are detected early may be reported 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 final result tree, or in temporary trees. This can be done in a number of ways.

• It is possible to request explicit validation of a complete document, that is, a tree rooted at a document node. This applies both to temporary trees constructed using the xsl:document (or xsl:copy) instruction and also to final result trees constructed using xsl:result-document. 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 19.2 Validation.

2.9 Error Handling

[Definition: An error that is 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.]

Errors classified in this specification as static errors must be signaled by all implementations: that is, the processor must indicate that the error is present. A static error must be signaled even if it occurs in a part of the stylesheet that is never evaluated. Static errors are never recoverable. After signaling a static error, a processor may continue for the purpose of signaling additional errors, but it must eventually terminate abnormally without producing any final result tree.

There is an exception to this rule when the stylesheet specifies forwards-compatible behavior (see 3.9 Forwards-Compatible Processing).

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.

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

[Definition: Some dynamic errors are classed as recoverable errors. When a recoverable error occurs, this specification allows the processor either to signal the error (by reporting the error condition and terminating execution) or to take a defined recovery action and continue processing.] It is implementation-defined whether the error is signaled or the recovery action is taken.

[Definition: If an implementation chooses to recover from a recoverable dynamic error, it must take the optional recovery action defined for that error condition in this specification.]

When the implementation makes the choice between signaling a dynamic error or recovering, it is not restricted in how it makes the choice; for example, it may provide options that can be set by the user. When an implementation chooses to recover from a dynamic error, it may also take other action, such as logging a warning message.

[Definition: A dynamic error that is not recoverable is referred to as a non-recoverable dynamic error. When a non-recoverable dynamic error occurs, the processor must signal 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 the final result trees 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 signal 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 signal any dynamic errors that would arise if the construct were evaluated.

An implementation may signal a dynamic error before any source document is available, but only if it can determine that the error would be signaled for every possible source document and every possible set of parameter values. For example, some circularity errors fall into this category: see 9.8 Circular Definitions.

The XPath specification states (see Section 2.3.1 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 reported as a static error. For XPath expressions used in an XSLT stylesheet, however, any such errors must not be reported as static errors in the stylesheet unless they would occur in every possible evaluation of that stylesheet; instead, they must be signaled as dynamic errors, and signaled 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 signaled in the same way as a non-recoverable dynamic error. Alternatively, an implementation may signal 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 signaled statically.

<xsl:if test="false()">
  <xsl:apply-templates select="42"/>
</xsl:if>

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

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

When a transformation signals 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 final result tree, it is still possible that errors may occur in serializing the result tree. For example, it may be impossible to serialize the result tree 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 20 Serialization, and in particular it must signal 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), RE (recoverable 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 reported 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). They are provided primarily for ease of reference. Implementations may use these codes when signaling errors, but they are not required to do so. An API specification, however, may require the use of error codes based on these QNames. 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.

Errors defined in the [XPath 2.0] and [Functions and Operators] specifications 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.

3.1 XSLT Namespace

[Definition: The XSLT namespace has the URI http://www.w3.org/1999/XSL/Transform. It is used to identify elements, attributes, and other names that have a special meaning defined in this specification.]

XSLT processors must use the XML namespaces mechanism [Namespaces in XML 1.0] to recognize elements and attributes from this namespace. Elements from the XSLT namespace are recognized only in the stylesheet and not in the source document. The complete list of XSLT-defined elements is specified in D Element Syntax Summary. Implementations must not extend the XSLT namespace with additional elements or attributes. Instead, any extension must be in a separate namespace. Any namespace that is used for additional instruction elements must be identified by means of the extension instruction mechanism specified in 18.2 Extension Instructions.

This specification uses a prefix of xsl: for referring to elements in the XSLT namespace. However, XSLT stylesheets are free to use any prefix, provided that there is a namespace declaration that binds the prefix to the URI of the XSLT namespace.

3.2 Reserved Namespaces

[Definition: The XSLT namespace, together with certain other namespaces recognized by an XSLT processor, are classified as reserved namespaces and must be used only as specified in this and related specifications.] The reserved namespaces are those listed below.

• The XSLT namespace, described in 3.1 XSLT Namespace, is reserved.

• [Definition: The standard function namespace http://www.w3.org/2005/xpath-functions is used for functions in the function library defined in [Functions and Operators] and standard functions defined in this specification.]

• [Definition: The XML namespace, defined in [Namespaces in XML 1.0] as http://www.w3.org/XML/1998/namespace, is used for attributes such as xml:lang, xml:space, and xml:id.]

• [Definition: The schema namespace http://www.w3.org/2001/XMLSchema is used as defined in [XML Schema Part 1] ]. In a stylesheet this namespace may be used to refer to built-in schema datatypes and to the constructor functions associated with those datatypes.

• [Definition: The schema instance namespace http://www.w3.org/2001/XMLSchema-instance is used as defined in [XML Schema Part 1] ]. Attributes in this namespace, if they appear in a stylesheet, are treated by the XSLT processor in the same way as any other attributes.

Reserved namespaces may be used without restriction to refer to the names of elements and attributes in source documents and result documents. As far as the XSLT processor is concerned, reserved namespaces other than the XSLT namespace may be used without restriction in the names of literal result elements and user-defined data elements, and in the names of attributes of literal result elements or of XSLT elements: but other processors may impose restrictions or attach special meaning to them. Reserved namespaces must not be used, however, in the names of stylesheet-defined objects such as variables and stylesheet functions.

[ERR XTSE0080] It is a static error to use a reserved namespace in the name of a named template, a mode, an attribute set, a key, a decimal-format, a variable or parameter, a stylesheet function, a named output definition, or a character map.

3.3 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 2.0]) 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 final result trees produced by the transformation to be different from the result trees that a conformant XSLT 2.0 processor might produce. They must not cause the processor to fail to signal an error that a conformant processor is required to signal. 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 xml, xhtml, html, or text described in 20 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.

3.4 XSLT Media Type

The media type application/xslt+xml will be registered for XSLT stylesheet modules.

The proposed definition of the media type is at B The 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.

3.5 Standard Attributes

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

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:version, xsl:exclude-result-prefixes, xsl:extension-element-prefixes, xsl:xpath-default-namespace, xsl:default-collation, or xsl:use-when.

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:]version, [xsl:]xpath-default-namespace, and [xsl:]default-collation, 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 3.12 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 and xsl:transform elements only. This reflects the fact that these attributes are often used on the xsl:stylesheet element only, in which case they apply to the entire stylesheet module.

Note that the effect of these attributes does not extend to stylesheet modules referenced by xsl:include or xsl:import declarations.

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

[xsl:]version

see 3.8 Backwards-Compatible Processing and 3.9 Forwards-Compatible Processing

[xsl:]xpath-default-namespace

see 5.2 Unprefixed QNames in Expressions and Patterns

[xsl:]exclude-result-prefixes

see 11.1.3 Namespace Nodes for Literal Result Elements

[xsl:]extension-element-prefixes

see 18.2 Extension Instructions

[xsl:]use-when

see 3.12 Conditional Element Inclusion

[xsl:]default-collation

see 3.6.1 The default-collation attribute

3.6 Stylesheet Element

<xsl:stylesheet
  id? = id
  extension-element-prefixes? = tokens
  exclude-result-prefixes? = tokens
  version = number
  xpath-default-namespace? = uri
  default-validation? = "preserve" | "strip"
  default-collation? = uri-list
  input-type-annotations? = "preserve" | "strip" | "unspecified">
  <!-- Content: (xsl:import*, other-declarations) -->
</xsl:stylesheet>

<xsl:transform
  id? = id
  extension-element-prefixes? = tokens
  exclude-result-prefixes? = tokens
  version = number
  xpath-default-namespace? = uri
  default-validation? = "preserve" | "strip"
  default-collation? = uri-list
  input-type-annotations? = "preserve" | "strip" | "unspecified">
  <!-- Content: (xsl:import*, other-declarations) -->
</xsl:transform>

A stylesheet module is represented by an xsl:stylesheet element in an XML document. xsl:transform is allowed as a synonym for xsl:stylesheet; everything this specification says about the xsl:stylesheet element applies equally to xsl:transform.

An xsl:stylesheet element must have a version attribute, indicating the version of XSLT that the stylesheet module requires.

[ERR XTSE0110] The value of the version attribute must be a number: specifically, it must be a a valid instance of the type xs:decimal as defined in [XML Schema Part 2]. For this version of XSLT, the value should normally be 2.0. A value of 1.0 indicates that the stylesheet module was written with the intention that it should be processed using an XSLT 1.0 processor.

If a stylesheet that specifies [xsl:]version="1.0" in the outermost element of the principal stylesheet module (that is, version="1.0" in the case of a standard stylesheet module, or xsl:version="1.0" in the case of a simplified stylesheet module) is submitted to an XSLT 2.0 processor, the processor should output a warning advising the user of possible incompatibilities, unless the user has requested otherwise. The processor must then process the stylesheet using the rules for backwards-compatible behavior. These rules require that if the processor does not support backwards-compatible behavior, it must signal an error and must not execute the transformation.

When the value of the version attribute is greater than 2.0, forwards-compatible behavior is enabled (see 3.9 Forwards-Compatible Processing).

The effect of the input-type-annotations attribute is described in 4.3 Stripping Type Annotations from a Source Tree.

The default-validation attribute defines the default value of the validation attribute of all xsl:document, xsl:element, xsl:attribute, xsl:copy, xsl:copy-of, and xsl:result-document instructions, and of the xsl:validation attribute of all literal result elements. It also determines the validation applied to the implicit final result tree created in the absence of an xsl:result-document instruction. This default applies within the stylesheet module: it does not extend to included or imported stylesheet modules. If the attribute is omitted, the default is strip. The permitted values are preserve and strip. For details of the effect of this attribute, see 19.2 Validation.

[ERR XTSE0120] An xsl:stylesheet element must not have any text node children. (This rule applies after stripping of whitespace text nodes as described in 4.2 Stripping Whitespace from the Stylesheet.)

[Definition: An element occurring as a child of an xsl:stylesheet element is called a top-level element.]

[Definition: Top-level elements fall into two categories: declarations, and user-defined data elements. Top-level elements whose names are in the XSLT namespace are declarations. Top-level elements in any other namespace are user-defined data elements (see 3.6.2 User-defined Data Elements)].

The declaration elements permitted in the xsl:stylesheet element are:

xsl:import
xsl:include
xsl:attribute-set
xsl:character-map
xsl:decimal-format
xsl:function
xsl:import-schema
xsl:key
xsl:namespace-alias
xsl:output
xsl:param
xsl:preserve-space
xsl:strip-space
xsl:template
xsl:variable

Note that the xsl:variable and xsl:param elements can act either as declarations or as instructions. A global variable or parameter is defined using a declaration; a local variable or parameter using an instruction.

If there are xsl:import elements, these must come before any other elements. Apart from this, the child elements of the xsl:stylesheet element may appear in any order. The ordering of these elements does not affect the results of the transformation unless there are conflicting declarations (for example, two template rules with the same priority that match the same node). In general, it is an error for a stylesheet to contain such conflicting declarations, but in some cases the processor is allowed to recover from the error by choosing the declaration that appears last in the stylesheet.

3.7 Simplified Stylesheet Modules

A simplified syntax is allowed for a stylesheet module that defines only a single template rule for the document node. The stylesheet module may consist of just a literal result element (see 11.1 Literal Result Elements) together with its contents. The literal result element must have an xsl:version attribute (and it must therefore also declare the XSLT namespace). Such a stylesheet module is equivalent to a standard stylesheet module whose xsl:stylesheet element contains a template rule containing the literal result element, minus its xsl:version attribute; the template rule has a match pattern of /.

<html xsl:version="2.0"
      xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
      xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <title>Expense Report Summary</title>
  </head>
  <body>
    <p>Total Amount: <xsl:value-of select="expense-report/total"/></p>
  </body>
</html>

<xsl:stylesheet version="2.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns="http://www.w3.org/1999/xhtml">
<xsl:template match="/">
<html>
  <head>
    <title>Expense Report Summary</title>
  </head>
  <body>
    <p>Total Amount: <xsl:value-of select="expense-report/total"/></p>
  </body>
</html>
</xsl:template>
</xsl:stylesheet>

More formally, a simplified stylesheet module is equivalent to the standard stylesheet module that would be generated by applying the following transformation to the simplified stylesheet module, invoking the transformation by calling the named template expand, with the containing literal result element as the context node:

<xsl:stylesheet version="2.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

<xsl:template name="expand">
  <xsl:element name="xsl:stylesheet">
    <xsl:attribute name="version" select="@xsl:version"/>
    <xsl:element name="xsl:template">
      <xsl:attribute name="match">/</xsl:attribute>
      <xsl:copy-of select="."/>
    </xsl:element>
  </xsl:element>
</xsl:template>  

</xsl:stylesheet>

[ERR XTSE0150] A literal result element that is used as the outermost element of a simplified stylesheet module must have an xsl:version attribute. This indicates the version of XSLT that the stylesheet requires. For this version of XSLT, the value will normally be 2.0; the value must be a valid instance of the type xs:decimal as defined in [XML Schema Part 2].

Other literal result elements may also have an xsl:version attribute. When the xsl:version attribute is numerically less than 2.0, backwards-compatible processing behavior is enabled (see 3.8 Backwards-Compatible Processing). When the xsl:version attribute is numerically greater than 2.0, forwards-compatible behavior is enabled (see 3.9 Forwards-Compatible Processing).

The allowed content of a literal result element when used as a simplified stylesheet is the same as when it occurs within a sequence constructor. Thus, a literal result element used as the document element of a simplified stylesheet cannot contain declarations. Simplified stylesheets therefore cannot use global variables, stylesheet parameters, stylesheet functions, keys, attribute-sets, or output definitions. In turn this means that the only useful way to initiate the transformation is to supply a document node as the initial context node, to be matched by the implicit match="/" template rule using the default mode.

3.8 Backwards-Compatible Processing

[Definition: An element enables backwards-compatible behavior for itself, its attributes, its descendants and their attributes if it has an [xsl:]version attribute (see 3.5 Standard Attributes) whose value is less than 2.0.]

An element that has an [xsl:]version attribute whose value is greater than or equal to 2.0 disables backwards-compatible behavior for itself, its attributes, its descendants and their attributes. The compatibility behavior established by an element overrides any compatibility behavior established by an ancestor element.

If an attribute containing an XPath expression is processed with backwards-compatible behavior, then the expression is evaluated with XPath 1.0 compatibility mode set to true. For details of this mode, see Section 2.1.1 Static Context^XP. Furthermore, in such an expression any function call for which no implementation is available (unless it uses the standard function namespace) is bound to a fallback error function whose effect when evaluated is to raise a dynamic error [see ERR XTDE1425] . The effect is that with backwards-compatible behavior enabled, calls on extension functions that are not available in a particular implementation do not cause an error unless the function call is actually evaluated. For further details, see 18.1 Extension Functions.

Certain XSLT constructs also produce different results when backwards-compatible behavior is enabled. This is described separately for each such construct.

These rules do not apply to the xsl:output element, whose version attribute has an entirely different purpose: it is used to define the version of the output method to be used for serialization.

It is implementation-defined whether a particular XSLT 2.0 implementation supports backwards-compatible behavior.

[ERR XTDE0160] If an implementation does not support backwards-compatible behavior, then it is a non-recoverable dynamic error if any element is evaluated that enables backwards-compatible behavior.

3.9 Forwards-Compatible Processing

The intent of forwards-compatible behavior is to make it possible to write a stylesheet that takes advantage of features introduced in some version of XSLT subsequent to XSLT 2.0, while retaining the ability to execute the stylesheet with an XSLT 2.0 processor using appropriate fallback behavior.

It is always possible to write conditional code to run under different XSLT versions by using the use-when feature described in 3.12 Conditional Element Inclusion. The rules for forwards-compatible behavior supplement this mechanism in two ways:

• certain constructs in the stylesheet that mean nothing to an XSLT 2.0 processor are ignored, rather than being treated as errors.

• explicit fallback behavior can be defined for instructions defined in a future XSLT release, using the xsl:fallback instruction.

The detailed rules follow.

[Definition: An element enables forwards-compatible behavior for itself, its attributes, its descendants and their attributes if it has an [xsl:]version attribute (see 3.5 Standard Attributes) whose value is greater than 2.0.]

An element that has an [xsl:]version attribute whose value is less than or equal to 2.0 disables forwards-compatible behavior for itself, its attributes, its descendants and their attributes. The compatibility behavior established by an element overrides any compatibility behavior established by an ancestor element.

These rules do not apply to the version attribute of the xsl:output element, which has an entirely different purpose: it is used to define the version of the output method to be used for serialization.

Within a section of a stylesheet where forwards-compatible behavior is enabled:

• if an element in the XSLT namespace appears as a child of the xsl:stylesheet element, and XSLT 2.0 does not allow such elements to occur as children of the xsl:stylesheet element, then the element and its content must be ignored.

• if an element has an attribute that XSLT 2.0 does not allow the element to have, then the attribute must be ignored.

• if an element in the XSLT namespace appears as part of a sequence constructor, and XSLT 2.0 does not allow such elements to appear as part of a sequence constructor, then:

  1. If the element has one or more xsl:fallback children, then no error is reported either statically or dynamically, and the result of evaluating the instruction is the concatenation of the sequences formed by evaluating the sequence constructors within its xsl:fallback children, in document order. Siblings of the xsl:fallback elements are ignored, even if they are valid XSLT 2.0 instructions.

  2. If the element has no xsl:fallback children, then a static error is reported in the same way as if forwards-compatible behavior were not enabled.

<xsl:stylesheet version="17.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="/">
    <xsl:exciting-new-17.0-feature>
      <xsl:fly-to-the-moon/>
      <xsl:fallback>
        <html>
          <head>
            <title>XSLT 17.0 required</title>
          </head>
          <body>
            <p>Sorry, this stylesheet requires XSLT 17.0.</p>
          </body>
        </html>
      </xsl:fallback>
    </xsl:exciting-new-17.0-feature>
  </xsl:template>
</xsl:stylesheet>

<xsl:stylesheet version="18.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

  <xsl:important-new-17.0-declaration/>

  <xsl:template match="/">
    <xsl:choose>
      <xsl:when test="number(system-property('xsl:version')) lt 17.0">
        <xsl:message terminate="yes">
          <xsl:text>Sorry, this stylesheet requires XSLT 17.0.</xsl:text>
        </xsl:message>
      </xsl:when>
      <xsl:otherwise>
        ...
      </xsl:otherwise>
    </xsl:choose>
  </xsl:template>
  ...
</xsl:stylesheet>

3.10 Combining Stylesheet Modules

XSLT provides two mechanisms to construct a stylesheet from multiple stylesheet modules:

• an inclusion mechanism that allows stylesheet modules to be combined without changing the semantics of the modules being combined, and

• an import mechanism that allows stylesheet modules to override each other.

<xsl:stylesheet version="2.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:import href="article.xsl"/>
  <xsl:import href="bigfont.xsl"/>
  <xsl:attribute-set name="note-style">
    <xsl:attribute name="font-style">italic</xsl:attribute>
  </xsl:attribute-set>
</xsl:stylesheet>

         A
         |
     +---+---+
     |       |
     B       C
     |       |
     D       E

3.11 Embedded Stylesheet Modules

An embedded stylesheet module is a stylesheet module whose containing element is not the outermost element of the containing XML document. Both standard stylesheet modules and simplified stylesheet modules may be embedded in this way.

Two situations where embedded stylesheets may be useful are:

• The stylesheet may be embedded in the source document to be transformed.

• The stylesheet may be embedded in an XML document that describes a sequence of processing of which the XSLT transformation forms just one part.

The xsl:stylesheet element may have an id attribute to facilitate reference to the stylesheet module within the containing document.

<?xml-stylesheet type="application/xslt+xml" href="#style1"?>
<!DOCTYPE doc SYSTEM "doc.dtd">
<doc>
<head>
<xsl:stylesheet id="style1"
                version="2.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns:fo="http://www.w3.org/1999/XSL/Format">
<xsl:import href="doc.xsl"/>
<xsl:template match="id('foo')">
  <fo:block font-weight="bold"><xsl:apply-templates/></fo:block>
</xsl:template>
<xsl:template match="xsl:stylesheet">
  <!-- ignore -->
</xsl:template>
</xsl:stylesheet>
</head>
<body>
<para id="foo">
...
</para>
</body>
</doc>

3.12 Conditional Element Inclusion

Any element in the XSLT namespace may have a use-when attribute whose value is an XPath expression that can be evaluated statically. If the attribute is present and the effective boolean value^XP of the expression is false, then the element, together with all the nodes having that element as an ancestor, is effectively excluded from the stylesheet module. When a node is effectively excluded from a stylesheet module the stylesheet module has the same effect as if the node were not there. Among other things this means that no static or dynamic errors will be reported in respect of the element and its contents, other than errors in the use-when attribute itself.

A literal result element, or any other element within a stylesheet module that is not in the XSLT namespace, may similarly carry an xsl:use-when attribute.

If the xsl:stylesheet or xsl:transform element itself is effectively excluded, the effect is to exclude all the children of the xsl:stylesheet or xsl:transform element, but not the xsl:stylesheet or xsl:transform element or its attributes.

Conditional element exclusion happens after stripping of whitespace text nodes from the stylesheet, as described in 4.2 Stripping Whitespace from the Stylesheet.

There are no syntactic constraints on the XPath expression that can be used as the value of the use-when attribute. However, there are severe constraints on the information provided in its evaluation context. These constraints are designed to ensure that the expression can be evaluated at the earliest possible stage of stylesheet processing, without any dependency on information contained in the stylesheet itself or in any source document.

Specifically, the components of the static and dynamic context are defined by the following two tables:

Within a stylesheet module, all expressions contained in [xsl:]use-when attributes are evaluated in a single execution scope^FO. This need not be the same execution scope as that used for [xsl]:use-when expressions in other stylesheet modules, or as that used when evaluating XPath expressions appearing elsewhere in the stylesheet module. This means that a function such as current-date^FO will return the same result when called in different [xsl:]use-when expressions within the same stylesheet module, but will not necessarily return the same result as the same call in an [xsl:]use-when expression within a different stylesheet module, or as a call on the same function executed during the transformation proper.

The use of [xsl:]use-when is illustrated in the following examples.

<xsl:import-schema schema-location="http://example.com/schema"
              use-when="system-property('xsl:is-schema-aware')='yes'"/>

<xsl:template match="/" 
              use-when="system-property('xsl:is-schema-aware')='yes'" 
              priority="2">
  <xsl:result-document validation="strict">
    <xsl:apply-templates/>
  </xsl:result-document>
</xsl:template>

<xsl:template match="/">
  <xsl:apply-templates/>
</xsl:template>

<xsl:include href="module-A.xsl" 
     use-when="system-property('xsl:vendor')='vendor-A'"/>
<xsl:include href="module-B.xsl" 
     use-when="system-property('xsl:vendor')='vendor-B'"/>

3.13 Built-in Types

Every XSLT 2.0 processor includes the following named type definitions in the in-scope schema components:

• All the primitive atomic types defined in [XML Schema Part 2], with the exception of xs:NOTATION. That is: xs:string, xs:boolean, xs:decimal, xs:double, xs:float, xs:date, xs:time, xs:dateTime, xs:duration, xs:QName, xs:anyURI, xs:gDay, xs:gMonthDay, xs:gMonth, xs:gYearMonth, xs:gYear, xs:base64Binary, and xs:hexBinary.

• The derived atomic type xs:integer defined in [XML Schema Part 2].

• The types xs:anyType and xs:anySimpleType.

• The following types defined in [XPath 2.0]: xs:yearMonthDuration, xs:dayTimeDuration, xs:anyAtomicType, xs:untyped, and xs:untypedAtomic.

A schema-aware XSLT processor additionally supports:

• All other built-in types defined in [XML Schema Part 2]

• User-defined types, and element and attribute declarations, that are imported using an xsl:import-schema declaration as described in 3.14 Importing Schema Components. These may include both simple and complex types.

An implementation may define mechanisms that allow additional schema components to be added to the in-scope schema components for the stylesheet. For example, the mechanisms used to define extension functions (see 18.1 Extension Functions) may also be used to import the types used in the interface to such functions.

These schema components are the only ones that may be referenced in XPath expressions within the stylesheet, or in the [xsl:]type and as attributes of those elements that permit these attributes.

For a Basic XSLT Processor, schema built-in types that are not included in the static context (for example, xs:NCName) are "unknown types" in the sense of Section 2.5.4 SequenceType Matching^XP. In the language of that section, a Basic XSLT Processor must be able to determine whether these unknown types are derived from known schema types such as xs:string. The purpose of this rule is to ensure that system functions such as local-name-from-QName^FO, which is defined to return an xs:NCName, behave correctly. A stylesheet that uses a Basic XSLT Processor will not be able to test whether the returned value is an xs:NCName, but it will be able to use it as if it were an xs:string.

3.14 Importing Schema Components

<!-- Category: declaration -->
<xsl:import-schema
  namespace? = uri-reference
  schema-location? = uri-reference>
  <!-- Content: xs:schema? -->
</xsl:import-schema>

The xsl:import-schema declaration is used to identify schema components (that is, top-level type definitions and top-level element and attribute declarations) that need to be available statically, that is, before any source document is available. Names of such components used statically within the stylesheet must refer to an in-scope schema component, which means they must either be built-in types as defined in 3.13 Built-in Types, or they must be imported using an xsl:import-schema declaration.

The xsl:import-schema declaration identifies a namespace containing the names of the components to be imported (or indicates that components whose names are in no namespace are to be imported). The effect is that the names of top-level element and attribute declarations and type definitions from this namespace (or non-namespace) become available for use within XPath expressions in the stylesheet, and within other stylesheet constructs such as the type and as attributes of various XSLT elements.

The same schema components are available in all stylesheet modules; importing components in one stylesheet module makes them available throughout the stylesheet.

The namespace and schema-location attributes are both optional.

If the xsl:import-schema element contains an xs:schema element, then the schema-location attribute must be absent, and the namespace attribute must either have the same value as the targetNamespace attribute of the xs:schema element (if present), or must be absent, in which case its effective value is that of the targetNamespace attribute of the xs:schema element if present or the zero-length string otherwise.

[ERR XTSE0215] It is a static error if an xsl:import-schema element that contains an xs:schema element has a schema-location attribute, or if it has a namespace attribute that conflicts with the target namespace of the contained schema.

If two xsl:import-schema declarations specify the same namespace, or if both specify no namespace, then only the one with highest import precedence is used. If this leaves more than one, then all the declarations at the highest import precedence are used (which may cause conflicts, as described below).

After discarding any xsl:import-schema declarations under the above rule, the effect of the remaining xsl:import-schema declarations is defined in terms of a hypothetical document called the synthetic schema document, which is constructed as follows. The synthetic schema document defines an arbitrary target namespace that is different from any namespace actually used by the application, and it contains xs:import elements corresponding one-for-one with the xsl:import-schema declarations in the stylesheet, with the following correspondence:

• The namespace attribute of the xs:import element is copied from the namespace attribute of the xsl:import-schema declaration if it is explicitly present, or is implied by the targetNamespace attribute of a contained xs:schema element, and is absent if it is absent.

• The schemaLocation attribute of the xs:import element is copied from the schema-location attribute of the xsl:import-schema declaration if present, and is absent if it is absent. If there is a contained xs:schema element, the effective value of the schemaLocation attribute is a URI referencing a document containing a copy of the xs:schema element.

• The base URI of the xs:import element is the same as the base URI of the xsl:import-schema declaration.

The schema components included in the in-scope schema components (that is, the components whose names are available for use within the stylesheet) are the top-level element and attribute declarations and type definitions that are available for reference within the synthetic schema document. See [XML Schema Part 1] (section 4.2.3, References to schema components across namespaces).

[ERR XTSE0220] It is a static error if the synthetic schema document does not satisfy the constraints described in [XML Schema Part 1] (section 5.1, Errors in Schema Construction and Structure). This includes, without loss of generality, conflicts such as multiple definitions of the same name.

The use of a namespace in an xsl:import-schema declaration does not by itself associate any namespace prefix with the namespace. If names from the namespace are used within the stylesheet module then a namespace declaration must be included in the stylesheet module, in the usual way.

<xsl:import-schema>
  <xs:schema targetNamespace="http://localhost/ns/yes-no"
             xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xs:simpleType name="local:yes-no">
      <xs:restriction base="xs:string">
        <xs:enumeration value="yes"/>
        <xs:enumeration value="no"/>
      </xs:restriction>
    </xs:simpleType>
  </xs:schema>
</xsl:import-schema>

<xs:variable name="condition" select="'yes'" as="local:yes-no"/>

4.1 XML Versions

The XDM data model defined in [Data Model] is capable of representing either an XML 1.0 document (conforming to [XML 1.0] and [Namespaces in XML 1.0]) or an XML 1.1 document (conforming to [XML 1.1] and [Namespaces in XML 1.1]), and it makes no distinction between the two. In principle, therefore, XSLT 2.0 can be used with either of these XML versions.

Construction of the XDM tree is outside the scope of this specification, so XSLT 2.0 places no formal requirements on an XSLT processor to accept input from either XML 1.0 documents or XML 1.1 documents or both. This specification does define a serialization capability (see 20 Serialization), though from a conformance point of view it is an optional feature. Although facilities are described for serializing the XDM tree as either XML 1.0 or XML 1.1 (and controlling the choice), there is again no formal requirement on an XSLT processor to support either or both of these XML versions as serialization targets.

Because the XDM tree is the same whether the original document was XML 1.0 or XML 1.1, the semantics of XSLT processing do not depend on the version of XML used by the original document. There is no reason in principle why all the input and output documents used in a single transformation must conform to the same version of XML.

Some of the syntactic constructs in XSLT 2.0 and XPath 2.0, for example the productions Char ^XML and NCName ^Names, are defined by reference to the XML and XML Namespaces specifications. There are slight variations between the XML 1.0 and XML 1.1 versions of these productions. Implementations may support either version; it is recommended that an XSLT 2.0 processor that implements the 1.1 versions should also provide a mode that supports the 1.0 versions. It is thus implementation-defined whether the XSLT processor supports XML 1.0 with XML Namespaces 1.0, or XML 1.1 with XML Namespaces 1.1, or supports both versions at user option.

At the time of writing there is no published version of [XML Schema Part 2] that references the XML 1.1 specifications. This means that data types such as xs:NCName and xs:ID are constrained by the XML 1.0 rules, and do not allow the full range of values permitted by XML 1.1. This situation will not be resolved until a new version of [XML Schema Part 2] becomes available; in the meantime, it is recommended that implementers wishing to support XML 1.1 should consult [XML Schema 1.0 and XML 1.1] for guidance. An XSLT 2.0 processor that supports XML 1.1 should implement the rules in later versions of [XML Schema Part 2] as they become available.

4.2 Stripping Whitespace from the Stylesheet

The tree representing the stylesheet is preprocessed as follows:

1. All comments and processing instructions are removed.

2. Any text nodes that are now adjacent to each other are merged.

3. Any whitespace text node that satisfies both the following conditions is removed from the tree:

   • The parent of the text node is not an xsl:text element

   • The text node does not have an ancestor element that has an xml:space attribute with a value of preserve, unless there is a closer ancestor element having an xml:space attribute with a value of default.

4. Any whitespace text node whose parent is one of the following elements is removed from the tree, regardless of any xml:space attributes:

   xsl:analyze-string
xsl:apply-imports
xsl:apply-templates
xsl:attribute-set
xsl:call-template
xsl:character-map
xsl:choose
xsl:next-match
xsl:stylesheet
xsl:transform

5. Any whitespace text node whose following-sibling node is an xsl:param or xsl:sort element is removed from the tree, regardless of any xml:space attributes.

[ERR XTSE0260] Within an XSLT element that is required to be empty, any content other than comments or processing instructions, including any whitespace text node preserved using the xml:space="preserve" attribute, is a static error.

4.3 Stripping Type Annotations from a Source Tree

[Definition: The term type annotation is used in this specification to refer to the value returned by the dm:type-name accessor of a node: see Section 5.14 type-name Accessor^DM.]

There is sometimes a requirement to write stylesheets that produce the same results whether or not the source documents have been validated against a schema. To achieve this, an option is provided to remove any type annotations on element and attribute nodes in a source tree, replacing them with an annotation of xs:untyped in the case of element nodes, and xs:untypedAtomic in the case of attribute nodes.

Such stripping of type annotations can be requested by specifying input-type-annotations="strip" on the xsl:stylesheet element. This attribute has three permitted values: strip, preserve, and unspecified. The default value is unspecified. Stripping of type annotations takes place if at least one stylesheet module in the stylesheet specifies input-type-annotations="strip".

[ERR XTSE0265] It is a static error if there is a stylesheet module in the stylesheet that specifies input-type-annotations="strip" and another stylesheet module that specifies input-type-annotations="preserve".

The source trees to which this applies are the same as those affected by xsl:strip-space and xsl:preserve-space: see 4.4 Stripping Whitespace from a Source Tree.

When type annotations are stripped, the following changes are made to the source tree:

• The type annotation of every element node is changed to xs:untyped

• The type annotation of every attribute node is changed to xs:untypedAtomic

• The typed value of every element and attribute node is set to be the same as its string value, as an instance of xs:untypedAtomic.

• The is-nilled property of every element node is set to false.

The values of the is-id and is-idrefs properties are not changed.

4.4 Stripping Whitespace from a Source Tree

A source tree supplied as input to the transformation process may contain whitespace text nodes that are of no interest, and that do not need to be retained by the transformation. Conceptually, an XSLT processor makes a copy of the source tree from which unwanted whitespace text nodes have been removed. This process is referred to as whitespace stripping.

For the purposes of this section, the term source tree means the document containing the initial context node, and any document returned by the functions document, doc ^FO, or collection^FO. It does not include documents passed as the values of stylesheet parameters or returned from extension functions.

The stripping process takes as input a set of element names whose child whitespace text nodes are to be preserved. The way in which this set of element names is established using the xsl:strip-space and xsl:preserve-space declarations is described later in this section.

A whitespace text node is preserved if either of the following apply:

• The element name of the parent of the text node is in the set of whitespace-preserving element names.

• An ancestor element of the text node has an xml:space attribute with a value of preserve, and no closer ancestor element has xml:space with a value of default.

Otherwise, the whitespace text node is stripped.

The xml:space attributes are not removed from the tree.

<!-- Category: declaration -->
<xsl:strip-space
  elements = tokens />

<!-- Category: declaration -->
<xsl:preserve-space
  elements = tokens />

The set of whitespace-preserving element names is specified by xsl:strip-space and xsl:preserve-space declarations. Whether an element name is included in the set of whitespace-preserving names is determined by the best match among all the xsl:strip-space or xsl:preserve-space declarations: it is included if and only if there is no match or the best match is an xsl:preserve-space element. The xsl:strip-space and xsl:preserve-space elements each have an elements attribute whose value is a whitespace-separated list of NameTests ^XP; an element name matches an xsl:strip-space or xsl:preserve-space element if it matches one of the NameTests ^XP. An element matches a NameTest ^XP if and only if the NameTest ^XP would be true for the element as an XPath node test. When more than one xsl:strip-space and xsl:preserve-space element matches, the best matching element is determined by the best matching NameTest ^XP. This is determined in the same way as with template rules:

• First, any match with lower import precedence than another match is ignored.

• Next, any match that has a lower default priority than the default priority of another match is ignored.

[ERR XTRE0270] It is a recoverable dynamic error if this leaves more than one match, unless all the matched declarations are equivalent (that is, they are all xsl:strip-space or they are all xsl:preserve-space). The optional recovery action is to select, from the matches that are left, the one that occurs last in declaration order.

If an element in a source document has a type annotation that is a simple type or a complex type with simple content, then any whitespace text nodes among its children are preserved, regardless of any xsl:strip-space declarations. The reason for this is that stripping a whitespace text node from an element with simple content could make the element invalid: for example, it could cause the minLength facet to be violated.

Stripping of type annotations happens before stripping of whitespace text nodes, so this situation will not occur if input-type-annotations="strip" is specified.

4.5 Attribute Types and DTD Validation

The mapping from the Infoset to the XDM data model, described in [Data Model], does not retain attribute types. This means, for example, that an attribute described in the DTD as having attribute type NMTOKENS will be annotated in the XDM tree as xs:untypedAtomic rather than xs:NMTOKENS, and its typed value will consist of a single xs:untypedAtomic value rather than a sequence of xs:NMTOKEN values.

Attributes with a DTD-derived type of ID, IDREF, or IDREFS will be marked in the XDM tree as having the is-id or is-idrefs properties. It is these properties, rather than any type annotation, that are examined by the functions id ^FO and idref ^FO described in [Functions and Operators].

4.6 Limits

The XDM data model (see [Data Model]) leaves it to the host language to define limits. This section describes the limits that apply to XSLT.

Limits on some primitive data types are defined in [XML Schema Part 2]. Other limits, listed below, are implementation-defined. Note that this does not necessarily mean that each limit must be a simple constant: it may vary depending on environmental factors such as available resources.

The following limits are implementation-defined:

1. For the xs:decimal type, the maximum number of decimal digits (the totalDigits facet). This must be at least 18 digits. (Note, however, that support for the full value range of xs:unsignedLong requires 20 digits.)

2. For the types xs:date, xs:time, xs:dateTime, xs:gYear, and xs:gYearMonth: the range of values of the year component, which must be at least +0001 to +9999; and the maximum number of fractional second digits, which must be at least 3.

3. For the xs:duration type: the maximum absolute values of the years, months, days, hours, minutes, and seconds components.

4. For the xs:yearMonthDuration type: the maximum absolute value, expressed as an integer number of months.

5. For the xs:dayTimeDuration type: the maximum absolute value, expressed as a decimal number of seconds.

6. For the types xs:string, xs:hexBinary, xs:base64Binary, xs:QName, xs:anyURI, xs:NOTATION, and types derived from them: the maximum length of the value.

7. For sequences, the maximum number of items in a sequence.

4.7 Disable Output Escaping

For backwards compatibility reasons, XSLT 2.0 continues to support the disable-output-escaping feature introduced in XSLT 1.0. This is an optional feature and implementations are not required to support it. A new facility, that of named character maps (see 20.1 Character Maps) is introduced in XSLT 2.0. It provides similar capabilities to disable-output-escaping, but without distorting the data model.

If an implementation supports the disable-output-escaping attribute of xsl:text and xsl:value-of, (see 20.2 Disabling Output Escaping), then the data model for trees constructed by the processor is augmented with a boolean value representing the value of this property. This boolean value, however, can be set only within a final result tree that is being passed to the serializer.

Conceptually, each character in a text node on such a result tree has a boolean property indicating whether the serializer is to disable the normal rules for escaping of special characters (for example, outputting of & as &) in respect of this character or attribute node.

5.1 Qualified Names

The name of a stylesheet-defined object, specifically a named template, a mode, an attribute set, a key, a decimal-format, a variable or parameter, a stylesheet function, a named output definition, or a character map is specified as a QName using the syntax for QName ^Names as defined in [Namespaces in XML 1.0].

[Definition: A QName is always written in the form (NCName ":")? NCName, that is, a local name optionally preceded by a namespace prefix. When two QNames are compared, however, they are considered equal if the corresponding expanded-QNames are the same, as described below.]

Because an atomic value of type xs:QName is sometimes referred to loosely as a QName, this specification also uses the term lexical QName to emphasize that it is referring to a QName ^Names in its lexical form rather than its expanded form. This term is used especially when strings containing lexical QNames are manipulated as run-time values.

[Definition: A lexical QName is a string representing a QName in the form (NCName ":")? NCName, that is, a local name optionally preceded by a namespace prefix.]

[Definition: A string in the form of a lexical QName may occur as the value of an attribute node in a stylesheet module, or within an XPath expression contained in such an attribute node, or as the result of evaluating an XPath expression contained in such an attribute node. The element containing this attribute node is referred to as the defining element of the QName.]

[Definition: An expanded-QName contains a pair of values, namely a local name and an optional namespace URI. It may also contain a namespace prefix. Two expanded-QNames are equal if the namespace URIs are the same (or both absent) and the local names are the same. The prefix plays no part in the comparison, but is used only if the expanded-QName needs to be converted back to a string.]

If the QName has a prefix, then the prefix is expanded into a URI reference using the namespace declarations in effect on its defining element. The expanded-QName consisting of the local part of the name and the possibly null URI reference is used as the name of the object. The default namespace of the defining element (see Section 6.2 Element Nodes^DM) is not used for unprefixed names.

There are three cases where the default namespace of the defining element is used when expanding an unprefixed QName:

1. Where a QName is used to define the name of an element being constructed. This applies both to cases where the name is known statically (that is, the name of a literal result element) and to cases where it is computed dynamically (the value of the name attribute of the xsl:element instruction).

2. The default namespace is used when expanding the first argument of the function element-available.

3. The default namespace applies to any unqualified element names appearing in the cdata-section-elements attribute of xsl:output or xsl:result-document

In the case of an unprefixed QName used as a NameTest within an XPath expression (see 5.3 Expressions) , and in certain other contexts, the namespace to be used in expanding the QName may be specified by means of the [xsl:]xpath-default-namespace attribute, as specified in 5.2 Unprefixed QNames in Expressions and Patterns.

[ERR XTSE0280] In the case of a prefixed QName used as the value of an attribute in the stylesheet, or appearing within an XPath expression in the stylesheet, it is a static error if the defining element has no namespace node whose name matches the prefix of the QName.

[ERR XTDE0290] Where the result of evaluating an XPath expression (or an attribute value template) is required to be a lexical QName, then unless otherwise specified it is a non-recoverable dynamic error if the defining element has no namespace node whose name matches the prefix of the lexical QName. This error may be signaled as a static error if the value of the expression can be determined statically.

5.2 Unprefixed QNames in Expressions and Patterns

The attribute [xsl:]xpath-default-namespace (see 3.5 Standard Attributes) may be used on an element in the stylesheet to define the namespace that will be used for an unprefixed element name or type name within an XPath expression, and in certain other contexts listed below.

The value of the attribute is the namespace URI to be used.

For any element in the stylesheet, this attribute has an effective value, which is the value of the [xsl:]xpath-default-namespace on that element or on the innermost containing element that specifies such an attribute, or the zero-length string if no containing element specifies such an attribute.

For any element in the stylesheet, the effective value of this attribute determines the value of the default namespace for element and type names in the static context of any XPath expression contained in an attribute of that element (including XPath expressions in attribute value templates). The effect of this is specified in [XPath 2.0]; in summary, it determines the namespace used for any unprefixed type name in the SequenceType production, and for any element name appearing in a path expression or in the SequenceType production.

The effective value of this attribute similarly applies to any of the following constructs appearing within its scope:

• any unprefixed element name or type name used in a pattern

• any unprefixed element name used in the elements attribute of the xsl:strip-space or xsl:preserve-space instructions

• any unprefixed element name or type name used in the as attribute of an XSLT element

• any unprefixed type name used in the type attribute of an XSLT element

• any unprefixed type name used in the xsl:type attribute of a literal result element.

The [xsl:]xpath-default-namespace attribute must be in the XSLT namespace if and only if its parent element is not in the XSLT namespace.

If the effective value of the attribute is a zero-length string, which will be the case if it is explicitly set to a zero-length string or if it is not specified at all, then an unprefixed element name or type name refers to a name that is in no namespace. The default namespace of the parent element (see Section 6.2 Element Nodes^DM) is not used.

The attribute does not affect other names, for example function names, variable names, or template names, or strings that are interpreted as lexical QNames during stylesheet evaluation, such as the effective value of the name attribute of xsl:element or the string supplied as the first argument to the key function.

5.3 Expressions

XSLT uses the expression language defined by XPath 2.0 [XPath 2.0]. Expressions are used in XSLT for a variety of purposes including:

• selecting nodes for processing;

• specifying conditions for different ways of processing a node;

• generating text to be inserted in a result tree.

[Definition: Within this specification, the term XPath expression, or simply expression, means a string that matches the production Expr ^XP defined in [XPath 2.0].]

An XPath expression may occur as the value of certain attributes on XSLT-defined elements, and also within curly brackets in attribute value templates.

Except where forwards-compatible behavior is enabled (see 3.9 Forwards-Compatible Processing), it is a static error if the value of such an attribute, or the text between curly brackets in an attribute value template, does not match the XPath production Expr ^XP, or if it fails to satisfy other static constraints defined in the XPath specification, for example that all variable references must refer to variables that are in scope. Error codes are defined in [XPath 2.0].

The transformation fails with a non-recoverable dynamic error if any XPath expression is evaluated and raises a dynamic error. Error codes are defined in [XPath 2.0].

The transformation fails with a type error if an XPath expression raises a type error, or if the result of evaluating the XPath expression is evaluated and raises a type error, or if the XPath processor signals a type error during static analysis of an expression. Error codes are defined in [XPath 2.0].

[Definition: The context within a stylesheet where an XPath expression appears may specify the required type of the expression. The required type indicates the type of the value that the expression is expected to return.] If no required type is specified, the expression may return any value: in effect, the required type is then item()*.

[Definition: Except where otherwise indicated, the actual value of an expression is converted to the required type using the function conversion rules. These are the rules defined in [XPath 2.0] for converting the supplied argument of a function call to the required type of that argument, as defined in the function signature. The relevant rules are those that apply when XPath 1.0 compatibility mode is set to false.]

This specification also invokes the XPath 2.0 function conversion rules to convert the result of evaluating an XSLT sequence constructor to a required type (for example, the sequence constructor enclosed in an xsl:variable, xsl:template, or xsl:function element).

Any dynamic error or type error that occurs when applying the function conversion rules to convert a value to a required type results in the transformation failing, in the same way as if the error had occurred while evaluating an expression.

5.4 The Static and Dynamic Context

XPath defines the concept of an expression context^XP which contains all the information that can affect the result of evaluating an expression. The expression context has two parts, the static context^XP, and the dynamic context^XP. The components that make up the expression context are defined in the XPath specification (see Section 2.1 Expression Context^XP). This section describes the way in which these components are initialized when an XPath expression is contained within an XSLT stylesheet.

As well as providing values for the static and dynamic context components defined in the XPath specification, XSLT defines additional context components of its own. These context components are used by XSLT instructions (for example, xsl:next-match and xsl:apply-imports), and also by the functions in the extended function library described in this specification.

The following four sections describe:

5.4.1 Initializing the Static Context
5.4.2 Additional Static Context Components used by XSLT
5.4.3 Initializing the Dynamic Context
5.4.4 Additional Dynamic Context Components used by XSLT

5.5 Patterns

A template rule identifies the nodes to which it applies by means of a pattern. As well as being used in template rules, patterns are used for numbering (see 12 Numbering), for grouping (see 14 Grouping), and for declaring keys (see 16.3 Keys).

[Definition: A pattern specifies a set of conditions on a node. A node that satisfies the conditions matches the pattern; a node that does not satisfy the conditions does not match the pattern. The syntax for patterns is a subset of the syntax for expressions.] As explained in detail below, a node matches a pattern if the node can be selected by deriving an equivalent expression, and evaluating this expression with respect to some possible context.

5.6 Attribute Value Templates

[Definition: In an attribute that is designated as an attribute value template, such as an attribute of a literal result element, an expression can be used by surrounding the expression with curly brackets ({})].

An attribute value template consists of an alternating sequence of fixed parts and variable parts. A variable part consists of an XPath expression enclosed in curly brackets ({}). A fixed part may contain any characters, except that a left curly bracket must be written as {{ and a right curly bracket must be written as }}.

[ERR XTSE0350] It is a static error if an unescaped left curly bracket appears in a fixed part of an attribute value template without a matching right curly bracket.

It is a static error if the string contained between matching curly brackets in an attribute value template does not match the XPath production Expr ^XP, or if it contains other XPath static errors. The error is signaled using the appropriate XPath error code.

[ERR XTSE0370] It is a static error if an unescaped right curly bracket occurs in a fixed part of an attribute value template.

[Definition: The result of evaluating an attribute value template is referred to as the effective value of the attribute.] The effective value is the string obtained by concatenating the expansions of the fixed and variable parts:

• The expansion of a fixed part is obtained by replacing any double curly brackets ({{ or }}) by the corresponding single curly bracket.

• The expansion of a variable part is obtained by evaluating the enclosed XPath expression and converting the resulting value to a string. This conversion is done using the rules given in 5.7.2 Constructing Simple Content.

If backwards compatible behavior is enabled for the attribute, the rules for converting the value of the expression to a string are modified as follows. After atomizing the result of the expression, all items other than the first item in the resulting sequence are discarded, and the effective value is obtained by converting the first item in the sequence to a string. If the atomized sequence is empty, the result is a zero-length string.

Curly brackets are not treated specially in an attribute value in an XSLT stylesheet unless the attribute is specifically designated as one that permits an attribute value template; in an element syntax summary, the value of such attributes is surrounded by curly brackets.

<xsl:variable name="image-dir" select="'/images'"/>

<xsl:template match="photograph">
  <img src="{$image-dir}/{href}" width="{size/@width}"/>
</xsl:template>

<photograph>
  <href>headquarters.jpg</href>
  <size width="300"/>
</photograph>

<img src="/images/headquarters.jpg" width="300"/>

<temperature readings="{10.32, 5.50, 8.31}"/>

<temperature readings="10.32 5.5 8.31"/>

Curly brackets are not recognized recursively inside expressions.

<a href="#{id({@ref})/title}">

<a href="#{id(@ref)/title}">

5.7 Sequence Constructors

[Definition: A sequence constructor is a sequence of zero or more sibling nodes in the stylesheet that can be evaluated to return a sequence of nodes and atomic values. The way that the resulting sequence is used depends on the containing instruction.]

Many XSLT elements, and also literal result elements, are defined to take a sequence constructor as their content.

Four kinds of nodes may be encountered in a sequence constructor:

• Text nodes appearing in the stylesheet (if they have not been removed in the process of whitespace stripping: see 4.2 Stripping Whitespace from the Stylesheet) are copied to create a new parentless text node in the result sequence.

• Literal result elements are evaluated to create a new parentless element node, having the same expanded-QName as the literal result element, which is added to the result sequence: see 11.1 Literal Result Elements

• XSLT instructions produce a sequence of zero, one, or more items as their result. These items are added to the result sequence. For most XSLT instructions, these items are nodes, but some instructions (xsl:sequence and xsl:copy-of) can also produce atomic values. Several instructions, such as xsl:element, return a newly constructed parentless node (which may have its own attributes, namespaces, children, and other descendants). Other instructions, such as xsl:if, pass on the items produced by their own nested sequence constructors. The xsl:sequence instruction may return atomic values, or existing nodes.

• Extension instructions (see 18.2 Extension Instructions) also produce a sequence of items as their result. The items in this sequence are added to the result sequence.

There are several ways the result of a sequence constructor may be used.

• The sequence may be bound to a variable or returned from a stylesheet function, in which case it becomes available as a value to be manipulated in arbitrary ways by XPath expressions. The sequence is bound to a variable when the sequence constructor appears within one of the elements xsl:variable, xsl:param, or xsl:with-param, when this instruction has an as attribute. The sequence is returned from a stylesheet function when the sequence constructor appears within the xsl:function element.

  Note:

  This will typically expose to the stylesheet elements, attributes, and other nodes that have not yet been attached to a parent node in a result tree. The semantics of XPath expressions when applied to parentless nodes are well-defined; however, such expressions should be used with care. For example, the expression / causes a type error if the root of the tree containing the context node is not a document node..

  Parentless attribute nodes require particular care because they have no namespace nodes associated with them. A parentless attribute node is not permitted to contain namespace-sensitive content (for example, a QName or an XPath expression) because there is no information enabling the prefix to be resolved to a namespace URI. Parentless attributes can be useful in an application (for example, they provide an alternative to the use of attribute sets: see 10.2 Named Attribute Sets) but they need to be handled with care.

• The sequence may be returned as the result of the containing element. This happens when the instruction containing the sequence constructor is xsl:analyze-string, xsl:apply-imports, xsl:apply-templates, xsl:call-template, xsl:choose, xsl:fallback, xsl:for-each, xsl:for-each-group, xsl:if, xsl:matching-substring, xsl:next-match, xsl:non-matching-substring, xsl:otherwise, xsl:perform-sort, xsl:sequence, or xsl:when

• The sequence may be used to construct the content of a new element or document node. This happens when the sequence constructor appears as the content of a literal result element, or of one of the instructions xsl:copy, xsl:element, xsl:document, xsl:result-document, or xsl:message. It also happens when the sequence constructor is contained in one of the elements xsl:variable, xsl:param, or xsl:with-param, when this instruction has no as attribute. For details, see 5.7.1 Constructing Complex Content.

• The sequence may be used to construct the string value of an attribute node, text node, namespace node, comment node, or processing instruction node. This happens when the sequence constructor is contained in one of the elements xsl:attribute, xsl:value-of, xsl:namespace, xsl:comment, or xsl:processing-instruction. For details, see 5.7.2 Constructing Simple Content.

<td>
  <xsl:attribute name="valign">top</xsl:attribute>
  <xsl:value-of select="@description"/>
</td>

<doc>
  <e><xsl:sequence select="1 to 5"/></e>
  <f>
    <xsl:for-each select="1 to 5">
      <xsl:value-of select="."/>
    </xsl:for-each>
  </f>
</doc>

<doc>
  <e>1 2 3 4 5</e>
  <f>12345</f>
</doc>

<doc>
  <xsl:attribute name="e" select="1 to 5"/>
  <xsl:attribute name="f">
    <xsl:for-each select="1 to 5">
      <xsl:value-of select="."/>
    </xsl:for-each>
  </xsl:attribute>
</doc>

<doc e="1 2 3 4 5" f="12345"/>

5.8 URI References

[Definition: Within this specification, the term URI Reference, unless otherwise stated, refers to a string in the lexical space of the xs:anyURI data type as defined in [XML Schema Part 2].] Note that this is a wider definition than that in [RFC3986]: in particular, it is designed to accommodate Internationalized Resource Identifiers (IRIs) as described in [RFC3987], and thus allows the use of non-ASCII characters without escaping.

URI References are used in XSLT with three main roles:

As namespace URIs
As collation URIs
As identifiers for resources such as stylesheet modules; these resources are typically accessible using a protocol such as HTTP. Examples of such identifiers are the URIs used in the href attributes of xsl:import, xsl:include, and xsl:result-document.

The rules for namespace URIs are given in [Namespaces in XML 1.0] and [Namespaces in XML 1.1]. Those specifications deprecate the use of relative URIs as namespace URIs.

The rules for collation URIs are given in [Functions and Operators].

URI references used to identify external resources must conform to the same rules as the locator attribute (href) defined in section 5.4 of [XLink]. If the URI reference is relative, then it is resolved (unless otherwise specified) against the base URI of the containing element node, according to the rules of [RFC3986], after first escaping all characters that need to be escaped to make it a valid RFC3986 URI reference. (But a relative URI in the href attribute of xsl:result-document is resolved against the Base Output URI.)

Other URI references appearing in an XSLT stylesheet document, for example the system identifiers of external entities or the value of the xml:base attribute, must follow the rules in their respective specifications.

6.1 Defining Templates

<!-- Category: declaration -->
<xsl:template
  match? = pattern
  name? = qname
  priority? = number
  mode? = tokens
  as? = sequence-type>
  <!-- Content: (xsl:param*, sequence-constructor) -->
</xsl:template>

[Definition: An xsl:template declaration defines a template, which contains a sequence constructor for creating nodes and/or atomic values. A template can serve either as a template rule, invoked by matching nodes against a pattern, or as a named template, invoked explicitly by name. It is also possible for the same template to serve in both capacities.]

[ERR XTSE0500] An xsl:template element must have either a match attribute or a name attribute, or both. An xsl:template element that has no match attribute must have no mode attribute and no priority attribute.

If an xsl:template element has a match attribute, then it is a template rule. If it has a name attribute, then it is a named template.

A template may be invoked in a number of ways, depending on whether it is a template rule, a named template, or both. The result of invoking the template is the result of evaluating the sequence constructor contained in the xsl:template element (see 5.7 Sequence Constructors).

If an as attribute is present, the as attribute defines the required type of the result. The result of evaluating the sequence constructor is then converted to the required type using the function conversion rules. If no as attribute is specified, the default value is item()*, which permits any value. No conversion then takes place.

[ERR XTTE0505] It is a type error if the result of evaluating the sequence constructor cannot be converted to the required type.

6.2 Defining Template Rules

This section describes template rules. Named templates are described in 10.1 Named Templates.

A template rule is specified using the xsl:template element with a match attribute. The match attribute is a Pattern that identifies the node or nodes to which the rule applies. The result of applying the template rule is the result of evaluating the sequence constructor contained in the xsl:template element, with the matching node used as the context node.

This is an <emph>important</emph> point.

<xsl:template match="emph">
  <fo:wrapper font-weight="bold" xmlns:fo="http://www.w3.org/1999/XSL/Format">
    <xsl:apply-templates/>
  </fo:wrapper>
</xsl:template>

A template rule is evaluated when an xsl:apply-templates instruction selects a node that matches the pattern specified in the match attribute. The xsl:apply-templates instruction is described in the next section. If several template rules match a selected node, only one of them is evaluated, as described in 6.4 Conflict Resolution for Template Rules.

6.3 Applying Template Rules

<!-- Category: instruction -->
<xsl:apply-templates
  select? = expression
  mode? = token>
  <!-- Content: (xsl:sort | xsl:with-param)* -->
</xsl:apply-templates>

The xsl:apply-templates instruction takes as input a sequence of nodes (typically nodes in a source tree), and produces as output a sequence of items; these will often be nodes to be added to a result tree.

If the instruction has one or more xsl:sort children, then the input sequence is sorted as described in 13 Sorting. The result of this sort is referred to below as the sorted sequence; if there are no xsl:sort elements, then the sorted sequence is the same as the input sequence.

Each node in the input sequence is processed by finding a template rule whose pattern matches that node. If there is more than one, the best among them is chosen, using rules described in 6.4 Conflict Resolution for Template Rules. If there is no template rule whose pattern matches the node, a built-in template rule is used (see 6.6 Built-in Template Rules). The chosen template rule is evaluated. The rule that matches the Nth node in the sorted sequence is evaluated with that node as the context item, with N as the context position, and with the length of the sorted sequence as the context size. Each template rule that is evaluated produces a sequence of items as its result. The resulting sequences (one for each node in the sorted sequence) are then concatenated, to form a single sequence. They are concatenated retaining the order of the nodes in the sorted sequence. The final concatenated sequence forms the result of the xsl:apply-templates instruction.

<message>Proceed <emph>at once</emph> to the exit!</message>

<xsl:template match="message">
  <p>
    <xsl:apply-templates select="child::node()"/>
  </p>
</xsl:template>

<xsl:template match="emph">
  <b>
    <xsl:apply-templates select="child::node()"/>
  </b>
</xsl:template>

<p>Proceed <b>at once</b> to the exit!</p>

The default value of the select attribute is child::node(), which causes all the children of context node to be processed.

[ERR XTTE0510] It is a type error if an xsl:apply-templates instruction with no select attribute is evaluated when the context item is not a node.

A select attribute can be used to process nodes selected by an expression instead of processing all children. The value of the select attribute is an expression. The expression must evaluate to a sequence of nodes (it can contain zero, one, or more nodes).

[ERR XTTE0520] It is a type error if the sequence returned by the select expression contains an item that is not a node.

<xsl:template match="author-group">
  <fo:wrapper>
    <xsl:apply-templates select="author/given-name"/>
  </fo:wrapper>
</xsl:template>

<xsl:template match="employee">
  <fo:block>
    Employee <xsl:apply-templates select="name"/> belongs to group
    <xsl:apply-templates select="ancestor::department/group"/>
  </fo:block>
</xsl:template>

<xsl:template match="product">
  <table>
    <xsl:apply-templates select="*"/>
  </table>
</xsl:template>

<xsl:template match="product/*" priority="3">
  <tr>
    <td><xsl:value-of select="name()"/></td>
    <td><xsl:next-match/></td>
  </tr>
</xsl:template>

<xsl:template match="product/element(*, xs:decimal) | 
                     product/element(*, xs:double)" priority="2">  
  <xsl:value-of select="format-number(xs:double(.), '#,###0.00')"/>
</xsl:template>

<xsl:template match="product/element(*, xs:date)" priority="2">
  <xsl:value-of select="format-date(., '[Mn] [D], [Y]')"/>
</xsl:template>

<xsl:template match="product/*" priority="1.5">
  <xsl:value-of select="."/>
</xsl:template>

<xsl:template match="product">
  <table>
    <xsl:apply-templates select="sales/domestic"/>
  </table>
  <table>
    <xsl:apply-templates select="sales/foreign"/>
  </table>
</xsl:template>

<doc><div><div></div></div></doc>

<xsl:template match="doc">
  <xsl:apply-templates select=".//div"/>
</xsl:template>

<xsl:template match="foo">
  <xsl:apply-templates select="."/>
</xsl:template>

6.4 Conflict Resolution for Template Rules

It is possible for a node in a source document to match more than one template rule. When this happens, only one template rule is evaluated for the node. The template rule to be used is determined as follows:

1. First, only the matching template rule or rules with the highest import precedence are considered. Other matching template rules with lower precedence are eliminated from consideration.

2. Next, of the remaining matching rules, only those with the highest priority are considered. Other matching template rules with lower priority are eliminated from consideration. The priority of a template rule is specified by the priority attribute on the xsl:template declaration.

   [ERR XTSE0530] The value of this attribute must conform to the rules for the xs:decimal type defined in [XML Schema Part 2]. Negative values are permitted..

   [Definition: If no priority attribute is specified on the xsl:template element, a default priority is computed, based on the syntax of the pattern supplied in the match attribute.] The rules are as follows:

   • If the pattern contains multiple alternatives separated by | , then the template rule is treated equivalently to a set of template rules, one for each alternative. However, it is not an error if a node matches more than one of the alternatives.

   • If the pattern has the form /, then the priority is −0.5.

   • If the pattern has the form of a QName optionally preceded by a PatternAxis or has the form processing-instruction( StringLiteral^XP) or processing-instruction(NCName ^Names) optionally preceded by a PatternAxis, then the priority is 0.

   • If the pattern has the form of an ElementTest^XP or AttributeTest^XP, optionally preceded by a PatternAxis, then the priority is as shown in the table below. In this table, the symbols E, A, and T represent an arbitrary element name, attribute name, and type name respectively, while the symbol * represents itself. The presence or absence of the symbol ? following a type name does not affect the priority.

     Format	Priority	Notes
     element()	−0.5	(equivalent to *)
     element(*)	−0.5	(equivalent to *)
     attribute()	−0.5	(equivalent to @*)
     attribute(*)	−0.5	(equivalent to @*)
     element(E)	0	(equivalent to E)
     element(*,T)	0	(matches by type only)
     attribute(A)	0	(equivalent to @A)
     attribute(*,T)	0	(matches by type only)
     element(E,T)	0.25	(matches by name and type)
     schema-element(E)	0.25	(matches by substitution group and type)
     attribute(A,T)	0.25	(matches by name and type)
     schema-attribute(A)	0.25	(matches by name and type)

   • If the pattern has the form of a DocumentTest^XP, then if it includes no ElementTest^XP or SchemaElementTest^XP the priority is −0.5. If it does include an ElementTest^XP or SchemaElementTest^XP, then the priority is the same as the priority of that ElementTest^XP or SchemaElementTest^XP, computed according to the table above.

   • If the pattern has the form NCName ^Names:* or *:NCName ^Names, optionally preceded by a PatternAxis, then the priority is −0.25.

   • If the pattern is any other NodeTest ^XP, optionally preceded by a PatternAxis, then the priority is −0.5.

   • Otherwise, the priority is 0.5.

   Note:

   In many cases this means that highly selective patterns have higher priority than less selective patterns. The most common kind of pattern (a pattern that tests for a node of a particular kind, with a particular expanded-QName or a particular type) has priority 0. The next less specific kind of pattern (a pattern that tests for a node of a particular kind and an expanded-QName with a particular namespace URI) has priority −0.25. Patterns less specific than this (patterns that just test for nodes of a given kind) have priority −0.5. Patterns that specify both the name and the required type have a priority of +0.25, putting them above patterns that only specify the name or the type. Patterns more specific than this, for example patterns that include predicates or that specify the ancestry of the required node, have priority 0.5.

   However, it is not invariably true that a more selective pattern has higher priority than a less selective pattern. For example, the priority of the pattern node()[self::*] is higher than that of the pattern salary. Similarly, the patterns attribute(*, xs:decimal) and attribute(*, xs:short) have the same priority, despite the fact that the latter pattern matches a subset of the nodes matched by the former. Therefore, to achieve clarity in a stylesheet it is good practice to allocate explicit priorities.

[ERR XTRE0540] It is a recoverable dynamic error if the conflict resolution algorithm for template rules leaves more than one matching template rule. The optional recovery action is to select, from the matching template rules that are left, the one that occurs last in declaration order.

6.5 Modes

[Definition: Modes allow a node in a source tree to be processed multiple times, each time producing a different result. They also allow different sets of template rules to be active when processing different trees, for example when processing documents loaded using the document function (see 16.1 Multiple Source Documents) or when processing temporary trees.]

[Definition: There is always a default mode available. The default mode is an unnamed mode, and it is used when no mode attribute is specified on an xsl:apply-templates instruction.]

Every mode other than the default mode is identified by a QName.

A template rule is applicable to one or more modes. The modes to which it is applicable are defined by the mode attribute of the xsl:template element. If the attribute is omitted, then the template rule is applicable to the default mode. If the attribute is present, then its value must be a non-empty whitespace-separated list of tokens, each of which defines a mode to which the template rule is applicable. Each token must be one of the following:

• a QName, which is expanded as described in 5.1 Qualified Names to define the name of the mode

• the token #default, to indicate that the template rule is applicable to the default mode

• the token #all, to indicate that the template rule is applicable to all modes (that is, to the default mode and to every mode that is named in an xsl:apply-templates instruction or xsl:template declaration anywhere in the stylesheet).

[ERR XTSE0550] It is a static error if the list is empty, if the same token is included more than once in the list, if the list contains an invalid token, or if the token #all appears together with any other value.

The xsl:apply-templates element also has an optional mode attribute. The value of this attribute must either be a QName, which is expanded as described in 5.1 Qualified Names to define the name of a mode, or the token #default, to indicate that the default mode is to be used, or the token #current, to indicate that the current mode is to be used. If the attribute is omitted, the default mode is used.

When searching for a template rule to process each node selected by the xsl:apply-templates instruction, only those template rules that are applicable to the selected mode are considered.

[Definition: At any point in the processing of a stylesheet, there is a current mode. When the transformation is initiated, the current mode is the default mode, unless a different initial mode has been supplied, as described in 2.3 Initiating a Transformation. Whenever an xsl:apply-templates instruction is evaluated, the current mode becomes the mode selected by this instruction.] When a stylesheet function is called, the current mode becomes the default mode. No other instruction changes the current mode. On completion of the xsl:apply-templates instruction, or on return from a stylesheet function call, the current mode reverts to its previous value. The current mode is used when an xsl:apply-templates instruction uses the syntax mode="#current"; it is also used by the xsl:apply-imports and xsl:next-match instructions (see 6.7 Overriding Template Rules).

6.6 Built-in Template Rules

When a node is selected by xsl:apply-templates and there is no template rule in the stylesheet that can be used to process that node, a built-in template rule is evaluated instead.

The built-in template rules apply to all modes.

The built-in rule for document nodes and element nodes is equivalent to calling xsl:apply-templates with no select attribute, and with the mode attribute set to #current. If the built-in rule was invoked with parameters, those parameters are passed on in the implicit xsl:apply-templates instruction.

<xsl:apply-templates select="title" mode="mm">
  <xsl:with-param name="init" select="10"/>
</xsl:apply-templates>

<xsl:template match="title" mode="#all">
  <xsl:param name="init"/>
  <xsl:apply-templates mode="#current">
    <xsl:with-param name="init" select="$init"/>
  </xsl:apply-templates>
</xsl:template>

The built-in template rule for text and attribute nodes returns a text node containing the string value of the context node. It is effectively:

<xsl:template match="text()|@*" mode="#all">
  <xsl:value-of select="string(.)"/>
</xsl:template>

The built-in template rule for processing instructions and comments does nothing (it returns the empty sequence).

<xsl:template match="processing-instruction()|comment()" mode="#all"/>

The built-in template rule for namespace nodes is also to do nothing. There is no pattern that can match a namespace node, so the built-in template rule is always used when xsl:apply-templates selects a namespace node.

The built-in template rules have lower import precedence than all other template rules. Thus, the stylesheet author can override a built-in template rule by including an explicit template rule.

6.7 Overriding Template Rules

<!-- Category: instruction -->
<xsl:apply-imports>
  <!-- Content: xsl:with-param* -->
</xsl:apply-imports>

<!-- Category: instruction -->
<xsl:next-match>
  <!-- Content: (xsl:with-param | xsl:fallback)* -->
</xsl:next-match>

A template rule that is being used to override another template rule (see 6.4 Conflict Resolution for Template Rules) can use the xsl:apply-imports or xsl:next-match instruction to invoke the overridden template rule. The xsl:apply-imports instruction only considers template rules in imported stylesheet modules; the xsl:next-match instruction considers all other template rules of lower import precedence and/or priority. Both instructions will invoke the built-in template rule for the node (see 6.6 Built-in Template Rules) if no other template rule is found.

[Definition: At any point in the processing of a stylesheet, there may be a current template rule. Whenever a template rule is chosen as a result of evaluating xsl:apply-templates, xsl:apply-imports, or xsl:next-match, the template rule becomes the current template rule for the evaluation of the rule's sequence constructor. When an xsl:for-each, xsl:for-each-group, or xsl:analyze-string instruction is evaluated, or when evaluating a sequence constructor contained in an xsl:sort or xsl:key element, or when a stylesheet function is called (see 10.3 Stylesheet Functions), the current template rule becomes null for the evaluation of that instruction or function.]

The current template rule is not affected by invoking named templates (see 10.1 Named Templates) or named attribute sets (see 10.2 Named Attribute Sets). While evaluating a global variable or the default value of a stylesheet parameter (see 9.5 Global Variables and Parameters) the current template rule is null.

Both xsl:apply-imports and xsl:next-match search for a template rule that matches the context node, and that is applicable to the current mode (see 6.5 Modes). In choosing a template rule, they use the usual criteria such as the priority and import precedence of the template rules, but they consider as candidates only a subset of the template rules in the stylesheet. This subset differs between the two instructions:

• The xsl:apply-imports instruction considers as candidates only those template rules contained in stylesheet levels that are descendants in the import tree of the stylesheet level that contains the current template rule.

  Note:

  This is not the same as saying that the search considers all template rules whose import precedence is lower than that of the current template rule.

• The xsl:next-match instruction considers as candidates all those template rules that come after the current template rule in the ordering of template rules implied by the conflict resolution rules given in 6.4 Conflict Resolution for Template Rules. That is, it considers all template rules with lower import precedence than the current template rule, plus the template rules that are at the same import precedence that have lower priority than the current template rule. If the processor has recovered from the error that occurs when two matching template rules have the same import precedence and priority, then it also considers all matching template rules with the same import precedence and priority that occur before the current template rule in declaration order.

  Note:

  As explained in 6.4 Conflict Resolution for Template Rules, a template rule whose match pattern contains multiple alternatives separated by | is treated equivalently to a set of template rules, one for each alternative. This means that where the same node matches more than one alternative, and the alternatives have different priority, it is possible for an xsl:next-match instruction to cause the current template rule to be invoked recursively. This situation does not occur when the alternatives have the same priority.

If no matching template rule is found that satisfies these criteria, the built-in template rule for the node kind is used (see 6.6 Built-in Template Rules).

An xsl:apply-imports or xsl:next-match instruction may use xsl:with-param child elements to pass parameters to the chosen template rule (see 10.1.1 Passing Parameters to Templates). It also passes on any tunnel parameters as described in 10.1.2 Tunnel Parameters.

[ERR XTDE0560] It is a non-recoverable dynamic error if xsl:apply-imports or xsl:next-match is evaluated when the current template rule is null.

<xsl:template match="example">
  <pre><xsl:apply-templates/></pre>
</xsl:template>

<xsl:import href="doc.xsl"/>

<xsl:template match="example">
  <div style="border: solid red">
     <xsl:apply-imports/>
  </div>
</xsl:template>

<div style="border: solid red"><pre>...</pre></div>

An xsl:fallback instruction appearing as a child of an xsl:next-match instruction is ignored by an XSLT 2.0 processor, but can be used to define fallback behavior when the stylesheet is processed by an XSLT 1.0 processor in forwards-compatible mode.

8.1 Conditional Processing with xsl:if

<!-- Category: instruction -->
<xsl:if
  test = expression>
  <!-- Content: sequence-constructor -->
</xsl:if>

The xsl:if element has a mandatory test attribute, which specifies an expression. The content is a sequence constructor.

The result of the xsl:if instruction depends on the effective boolean value^XP of the expression in the test attribute. The rules for determining the effective boolean value of an expression are given in [XPath 2.0]: they are the same as the rules used for XPath conditional expressions.

If the effective boolean value of the expression is true, then the sequence constructor is evaluated (see 5.7 Sequence Constructors), and the resulting node sequence is returned as the result of the xsl:if instruction; otherwise, the sequence constructor is not evaluated, and the empty sequence is returned.

<xsl:template match="namelist/name">
  <xsl:apply-templates/>
  <xsl:if test="not(position()=last())">, </xsl:if>
</xsl:template>

<xsl:template match="item">
  <tr>
    <xsl:if test="position() mod 2 = 0">
       <xsl:attribute name="bgcolor">yellow</xsl:attribute>
    </xsl:if>
    <xsl:apply-templates/>
  </tr>
</xsl:template>

8.2 Conditional Processing with xsl:choose

<!-- Category: instruction -->
<xsl:choose>
  <!-- Content: (xsl:when+, xsl:otherwise?) -->
</xsl:choose>

<xsl:when
  test = expression>
  <!-- Content: sequence-constructor -->
</xsl:when>

<xsl:otherwise>
  <!-- Content: sequence-constructor -->
</xsl:otherwise>

The xsl:choose element selects one among a number of possible alternatives. It consists of a sequence of one or more xsl:when elements followed by an optional xsl:otherwise element. Each xsl:when element has a single attribute, test, which specifies an expression. The content of the xsl:when and xsl:otherwise elements is a sequence constructor.

When an xsl:choose element is processed, each of the xsl:when elements is tested in turn (that is, in the order that the elements appear in the stylesheet), until one of the xsl:when elements is satisfied. If none of the xsl:when elements is satisfied, then the xsl:otherwise element is considered, as described below.

An xsl:when element is satisfied if the effective boolean value^XP of the expression in its test attribute is true. The rules for determining the effective boolean value of an expression are given in [XPath 2.0]: they are the same as the rules used for XPath conditional expressions.

The content of the first, and only the first, xsl:when element that is satisfied is evaluated, and the resulting sequence is returned as the result of the xsl:choose instruction. If no xsl:when element is satisfied, the content of the xsl:otherwise element is evaluated, and the resulting sequence is returned as the result of the xsl:choose instruction. If no xsl:when element is satisfied, and no xsl:otherwise element is present, the result of the xsl:choose instruction is an empty sequence.

Only the sequence constructor of the selected xsl:when or xsl:otherwise instruction is evaluated. The test expressions for xsl:when instructions after the selected one are not evaluated.

<xsl:template match="orderedlist/listitem">
  <fo:list-item indent-start='2pi'>
    <fo:list-item-label>
      <xsl:variable name="level"
                    select="count(ancestor::orderedlist) mod 3"/>
      <xsl:choose>
        <xsl:when test='$level=1'>
          <xsl:number format="i"/>
        </xsl:when>
        <xsl:when test='$level=2'>
          <xsl:number format="a"/>
        </xsl:when>
        <xsl:otherwise>
          <xsl:number format="1"/>
        </xsl:otherwise>
      </xsl:choose>
      <xsl:text>. </xsl:text>
    </fo:list-item-label>
    <fo:list-item-body>
      <xsl:apply-templates/>
    </fo:list-item-body>
  </fo:list-item>
</xsl:template>

9.1 Variables

<!-- Category: declaration -->
<!-- Category: instruction -->
<xsl:variable
  name = qname
  select? = expression
  as? = sequence-type>
  <!-- Content: sequence-constructor -->
</xsl:variable>

The xsl:variable element has a required name attribute, which specifies the name of the variable. The value of the name attribute is a QName, which is expanded as described in 5.1 Qualified Names.

The xsl:variable element has an optional as attribute, which specifies the required type of the variable. The value of the as attribute is a SequenceType ^XP, as defined in [XPath 2.0].

[Definition: The value of the variable is computed using the expression given in the select attribute or the contained sequence constructor, as described in 9.3 Values of Variables and Parameters. This value is referred to as the supplied value of the variable.] If the xsl:variable element has a select attribute, then the sequence constructor must be empty.

If the as attribute is specified, then the supplied value of the variable is converted to the required type, using the function conversion rules.

[ERR XTTE0570] It is a type error if the supplied value of a variable cannot be converted to the required type.

If the as attribute is omitted, the supplied value of the variable is used directly, and no conversion takes place.

9.2 Parameters

<!-- Category: declaration -->
<xsl:param
  name = qname
  select? = expression
  as? = sequence-type
  required? = "yes" | "no"
  tunnel? = "yes" | "no">
  <!-- Content: sequence-constructor -->
</xsl:param>

The xsl:param element may be used as a child of xsl:stylesheet, to define a parameter to the transformation; or as a child of xsl:template to define a parameter to a template, which may be supplied when the template is invoked using xsl:call-template, xsl:apply-templates, xsl:apply-imports or xsl:next-match; or as a child of xsl:function to define a parameter to a stylesheet function, which may be supplied when the function is called from an XPath expression.

The xsl:param element has a required name attribute, which specifies the name of the parameter. The value of the name attribute is a QName, which is expanded as described in 5.1 Qualified Names.

[ERR XTSE0580] It is a static error if two parameters of a template or of a stylesheet function have the same name.

The supplied value of the parameter is the value supplied by the caller. If no value was supplied by the caller, and if the parameter is not mandatory, then the supplied value is computed using the expression given in the select attribute or the contained sequence constructor, as described in 9.3 Values of Variables and Parameters. If the xsl:param element has a select attribute, then the sequence constructor must be empty.

The xsl:param element has an optional as attribute, which specifies the required type of the parameter. The value of the as attribute is a SequenceType ^XP, as defined in [XPath 2.0].

If the as attribute is specified, then the supplied value of the parameter is converted to the required type, using the function conversion rules.

[ERR XTTE0590] It is a type error if the conversion of the supplied value of a parameter to its required type fails.

If the as attribute is omitted, the supplied value of the parameter is used directly, and no conversion takes place.

The optional required attribute may be used to indicate that a parameter is mandatory. This attribute may be specified for stylesheet parameters and for template parameters; it must not be specified for function parameters, which are always mandatory. A parameter is mandatory if it is a function parameter or if the required attribute is present and has the value yes. Otherwise, the parameter is optional. If the parameter is mandatory, then the xsl:param element must be empty and must not have a select attribute.

[ERR XTTE0600] If a default value is given explicitly, that is, if there is either a select attribute or a non-empty sequence constructor, then it is a type error if the default value cannot be converted to the required type, using the function conversion rules.

If an optional parameter has no select attribute and has an empty sequence constructor, and if there is no as attribute, then the default value of the parameter is a zero length string.

[ERR XTDE0610] If an optional parameter has no select attribute and has an empty sequence constructor, and if there is an as attribute, then the default value of the parameter is an empty sequence. If the empty sequence is not a valid instance of the required type defined in the as attribute, then the parameter is treated as a required parameter, which means that it is a non-recoverable dynamic error if the caller supplies no value for the parameter.

The optional tunnel attribute may be used to indicate that a parameter is a tunnel parameter. The default is no; the value yes may be specified only for template parameters. Tunnel parameters are described in 10.1.2 Tunnel Parameters

9.3 Values of Variables and Parameters

A variable-binding element may specify the supplied value of the variable or parameter in four different ways.

• If the variable-binding element has a select attribute, then the value of the attribute must be an expression and the supplied value of the variable is the value that results from evaluating the expression. In this case, the content of the variable-binding element must be empty.

• If the variable-binding element has empty content and has neither a select attribute nor an as attribute, then the supplied value of the variable is a zero-length string. Thus

  <xsl:variable name="x"/>
  

  is equivalent to

  <xsl:variable name="x" select="''"/>
  

• If a variable-binding element has no select attribute and has non-empty content (that is, the variable-binding element has one or more child nodes), and has no as attribute, then the content of the variable-binding element specifies the supplied value. The content of the variable-binding element is a sequence constructor; a new document is constructed with a document node having as its children the sequence of nodes that results from evaluating the sequence constructor and then applying the rules given in 5.7.1 Constructing Complex Content. The value of the variable is then a singleton sequence containing this document node. For further information, see 9.4 Creating implicit document nodes.

• If a variable-binding element has an as attribute but no select attribute, then the supplied value is the sequence that results from evaluating the (possibly empty) sequence constructor contained within the variable-binding element (see 5.7 Sequence Constructors).

These combinations are summarized in the table below.

[ERR XTSE0620] It is a static error if a variable-binding element has a select attribute and has non-empty content.

<xsl:variable name="i" as="xs:integer*" select="1 to 3"/>

<xsl:variable name="i" as="xs:integer" select="@size"/>

<xsl:variable name="z"/>

<xsl:variable name="doc"><c/></xsl:variable>

<xsl:variable name="seq" as="xs:integer*">
  <xsl:for-each select="1 to 3">
    <xsl:sequence select=".*2"/>
  </xsl:for-each>
</xsl:variable>

<xsl:variable name="attset" as="attribute()+">
  <xsl:attribute name="x">2</xsl:attribute>
  <xsl:attribute name="y">3</xsl:attribute>
  <xsl:attribute name="z">4</xsl:attribute>    
</xsl:variable>

<xsl:variable name="empty" as="empty-sequence()"/>

The actual value of the variable depends on the supplied value, as described above, and the required type, which is determined by the value of the as attribute.

<xsl:variable name="n">2</xsl:variable>
...
<xsl:value-of select="td[$n]"/>

<xsl:variable name="n" select="2"/>
...
<xsl:value-of select="td[$n]"/>

<xsl:variable name="n">2</xsl:variable>
...
<xsl:value-of select="td[position()=$n]"/>

<xsl:variable name="n" as="xs:integer">2</xsl:variable>
...
<xsl:value-of select="td[$n]"/>

9.4 Creating implicit document nodes

A document node is created implicitly when evaluating an xsl:variable, xsl:param, or xsl:with-param element that has non-empty content and that has no as attribute. This element is referred to as the variable-binding element. The value of the variable is a single node, the document node of the temporary tree. The content of the document node is formed from the result of evaluating the sequence constructor contained within the variable-binding element, as described in 5.7.1 Constructing Complex Content.

<xsl:variable name="tree">
  <a/>
</xsl:variable>

<xsl:variable name="tree" as="document-node()">
  <xsl:document validation="preserve">
    <a/>
  </xsl:document>  
</xsl:variable>

The base URI of the document node is taken from the base URI of the variable binding element in the stylesheet. (See Section 5.2 base-uri Accessor^DM in [Data Model])

No document-level validation takes place (which means, for example, that there is no checking that ID values are unique). However, type annotations on nodes within the new tree are copied unchanged.

The document-uri and unparsed-entities properties of the new document node are set to empty.

A temporary tree is available for processing in exactly the same way as any source document. For example, its nodes are accessible using path expressions, and they can be processed using instructions such as xsl:apply-templates and xsl:for-each. Also, the key and id ^FO functions can be used to find nodes within a temporary tree rooted at a document node, provided that at the time the function is called, the context item is a node within the temporary tree.

<xsl:stylesheet
  version="2.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

<xsl:import href="phase1.xsl"/>
<xsl:import href="phase2.xsl"/>

<xsl:variable name="intermediate">
  <xsl:apply-templates select="/" mode="phase1"/>
</xsl:variable>

<xsl:template match="/">
  <xsl:apply-templates select="$intermediate" mode="phase2"/>
</xsl:template>

</xsl:stylesheet>

9.5 Global Variables and Parameters

Both xsl:variable and xsl:param are allowed as declaration elements: that is, they may appear as children of the xsl:stylesheet element.

[Definition: A top-level variable-binding element declares a global variable that is visible everywhere (except where it is shadowed by another binding).]

[Definition: A top-level xsl:param element declares a stylesheet parameter. A stylesheet parameter is a global variable with the additional property that its value can be supplied by the caller when a transformation is initiated.] As described in 9.2 Parameters, a stylesheet parameter may be declared as being mandatory, or may have a default value specified for use when no value is supplied by the caller. The mechanism by which the caller supplies a value for a stylesheet parameter is implementation-defined. An XSLT processor must provide such a mechanism.

It is an error if no value is supplied for a mandatory stylesheet parameter [see ERR XTDE0050].

If a stylesheet contains more than one binding for a global variable of a particular name, then the binding with the highest import precedence is used.

[ERR XTSE0630] It is a static error if a stylesheet contains more than one binding of a global variable with the same name and same import precedence, unless it also contains another binding with the same name and higher import precedence.

For a global variable or the default value of a stylesheet parameter, the expression or sequence constructor specifying the variable value is evaluated with a singleton focus based on the root node of the tree containing the initial context node. An XPath error will be reported if the evaluation of a global variable or parameter references the context item, context position, or context size when no initial context node is supplied. The values of other components of the dynamic context are the initial values as defined in 5.4.3 Initializing the Dynamic Context and 5.4.4 Additional Dynamic Context Components used by XSLT.

<xsl:param name="para-font-size" as="xs:string">12pt</xsl:param>

<xsl:template match="para">
 <fo:block font-size="{$para-font-size}">
   <xsl:apply-templates/>
 </fo:block>
</xsl:template>

9.6 Local Variables and Parameters

[Definition: As well as being allowed as declaration elements, the xsl:variable element is also allowed in sequence constructors. Such a variable is known as a local variable.]

[Definition:  An xsl:param element may appear as a child of an xsl:template element, before any non-xsl:param children of that element. Such a parameter is known as a template parameter. A template parameter is a local variable with the additional property that its value can be set when the template is called, using any of the instructions xsl:call-template, xsl:apply-templates, xsl:apply-imports, or xsl:next-match. ]

[Definition:  An xsl:param element may appear as a child of an xsl:function element, before any non-xsl:param children of that element. Such a parameter is known as a function parameter. A function parameter is a local variable with the additional property that its value can be set when the function is called, using a function call in an XPath expression.]

The result of evaluating a local xsl:variable or xsl:param element (that is, the contribution it makes to the result of the sequence constructor it is part of) is an empty sequence.

9.7 Scope of Variables

For any variable-binding element, there is a region (more specifically, a set of element nodes) of the stylesheet within which the binding is visible. The set of variable bindings in scope for an XPath expression consists of those bindings that are visible at the point in the stylesheet where the expression occurs.

A global variable binding element is visible everywhere in the stylesheet (including other stylesheet modules) except within the xsl:variable or xsl:param element itself and any region where it is shadowed by another variable binding.

A local variable binding element is visible for all following siblings and their descendants, with two exceptions: it is not visible in any region where it is shadowed by another variable binding, and it is not visible within the subtree rooted at an xsl:fallback instruction that is a sibling of the variable binding element. The binding is not visible for the xsl:variable or xsl:param element itself.

[Definition: A binding shadows another binding if the binding occurs at a point where the other binding is visible, and the bindings have the same name. ] It is not an error if a binding established by a local xsl:variable or xsl:param shadows a global binding. In this case, the global binding will not be visible in the region of the stylesheet where it is shadowed by the other binding.

<xsl:param name="x" select="1"/>
<xsl:template name="foo">
  <xsl:variable name="x" select="2"/>
</xsl:template>

It is also not an error if a binding established by a local xsl:variable element shadows a binding established by another local xsl:variable or xsl:param.

<xsl:variable name="x" select="1"/>
<xsl:template name="foo">
  <xsl:for-each select="1 to 5">
    <xsl:variable name="x" select="$x+1"/>
  </xsl:for-each>
  <x value="{$x}"/>
</xsl:template>

As well as global variables and local variables, an XPath expression may also declare range variables for use locally within an expression. For details, see [XPath 2.0].

Where a reference to a variable occurs in an XPath expression, it is resolved first by reference to range variables that are in scope, then by reference to local variables and parameters, and finally by reference to global variables and parameters. A range variable may shadow a local variable or a global variable. XPath also allows a range variable to shadow another range variable.

9.8 Circular Definitions

[Definition: A circularity is said to exist if a construct such as a global variable, an attribute set, or a key is defined in terms of itself. For example, if the expression or sequence constructor specifying the value of a global variable X references a global variable Y, then the value for Y must be computed before the value of X. A circularity exists if it is impossible to do this for all global variable definitions.]

<xsl:variable name="x" select="$y+1"/>
<xsl:variable name="y" select="$x+1"/>

<xsl:variable name="x" select="my:f()"/>

<xsl:function name="my:f">
  <xsl:sequence select="$x"/>
</xsl:function>

<xsl:variable name="x">
  <xsl:apply-templates select="//param[1]"/>
</xsl:variable>

<xsl:template match="param[$x]">1</xsl:template>

<xsl:variable name="x" select="my:f(10)"/>

<xsl:function name="my:f">
  <xsl:param name="arg1"/>
  <xsl:sequence select="key('k', $arg1)"/>
</xsl:function>

<xsl:key name="k" match="item[@code=$x]" use="@desc"/>

[ERR XTDE0640] In general, a circularity in a stylesheet is a non-recoverable dynamic error. However, as with all other dynamic errors, an implementation will signal the error only if it actually executes the instructions and expressions that participate in the circularity. Because different implementations may optimize the execution of a stylesheet in different ways, it is implementation-dependent whether a particular circularity will actually be signaled.

For example, in the following declarations, the function declares a local variable $b, but it returns a result that does not require the variable to be evaluated. It is implementation-dependent whether the value is actually evaluated, and it is therefore implementation-dependent whether the circularity is signaled as an error:

<xsl:variable name="x" select="my:f(1)/>

<xsl:function name="my:f">
  <xsl:param name="a"/>
  <xsl:variable name="b" select="$x"/>  
  <xsl:sequence select="$a + 2"/>
</xsl:function>

Circularities usually involve global variables or parameters, but they can also exist between key definitions (see 16.3 Keys), between named attribute sets (see 10.2 Named Attribute Sets), or between any combination of these constructs. For example, a circularity exists if a key definition invokes a function that references an attribute set that calls the key function, supplying the name of the original key definition as an argument.

Circularity is not the same as recursion. Stylesheet functions (see 10.3 Stylesheet Functions) and named templates (see 10.1 Named Templates) may call other functions and named templates without restriction. With careless coding, recursion may be non-terminating. Implementations are required to signal circularity as a dynamic error, but they are not required to detect non-terminating recursion.

10.1 Named Templates

<!-- Category: instruction -->
<xsl:call-template
  name = qname>
  <!-- Content: xsl:with-param* -->
</xsl:call-template>

[Definition: Templates can be invoked by name. An xsl:template element with a name attribute defines a named template.] The value of the name attribute is a QName, which is expanded as described in 5.1 Qualified Names. If an xsl:template element has a name attribute, it may, but need not, also have a match attribute. An xsl:call-template instruction invokes a template by name; it has a required name attribute that identifies the template to be invoked. Unlike xsl:apply-templates, the xsl:call-template instruction does not change the focus.

The match, mode and priority attributes on an xsl:template element have no effect when the template is invoked by an xsl:call-template instruction. Similarly, the name attribute on an xsl:template element has no effect when the template is invoked by an xsl:apply-templates instruction.

[ERR XTSE0650] It is a static error if a stylesheet contains an xsl:call-template instruction whose name attribute does not match the name attribute of any xsl:template in the stylesheet.

[ERR XTSE0660] It is a static error if a stylesheet contains more than one template with the same name and the same import precedence, unless it also contains a template with the same name and higher import precedence.

The target template for an xsl:call-template instruction is the template whose name attribute matches the name attribute of the xsl:call-template instruction and that has higher import precedence than any other template with this name. The result of evaluating an xsl:call-template instruction is the sequence produced by evaluating the sequence constructor contained in its target template (see 5.7 Sequence Constructors).

<xsl:template name="numbered-block">
  <xsl:param name="format">1. </xsl:param>
  <fo:block>
    <xsl:number format="{$format}"/>
    <xsl:apply-templates/>
  </fo:block>
</xsl:template>

<xsl:template match="ol//ol/li">
  <xsl:call-template name="numbered-block">
    <xsl:with-param name="format">a. </xsl:with-param>
  </xsl:call-template>
</xsl:template>

<xsl:template match="equation">
  <xsl:param name="equation-format" select="'(1)'" tunnel="yes"/>
  <xsl:number level="any" format="{$equation-format}"/>
</xsl:template>

<xsl:template match="appendix">
  ...
  <xsl:apply-templates>
    <xsl:with-param name="equation-format" select="'[i]'" tunnel="yes"/>
  </xsl:apply-templates>
  ...
</xsl:template>

10.2 Named Attribute Sets

<!-- Category: declaration -->
<xsl:attribute-set
  name = qname
  use-attribute-sets? = qnames>
  <!-- Content: xsl:attribute* -->
</xsl:attribute-set>

[Definition: The xsl:attribute-set element defines a named attribute set: that is, a collection of attribute definitions that can be used repeatedly on different constructed elements.]