XSL Transformations (XSLT) Version 3.0

1.1 What is XSLT?

This specification defines the syntax and semantics of the XSLT 3.0 language.

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

A stylesheet generally includes elements that are defined by XSLT as well as elements that are not defined by XSLT. XSLT-defined elements are distinguished by use of the namespace http://www.w3.org/1999/XSL/Transform (see 3.1 XSLT Namespace), which is referred to in this specification as the XSLT namespace. Thus this specification is a definition of the syntax and semantics of the XSLT namespace.

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

A transformation expressed in XSLT describes rules for transforming 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.

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

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

[Definition: For a given transformation, one package functions as the top-level package. The complete stylesheet is assembled by finding the packages referenced directly or indirectly from the top-level package using xsl:use-package declarations: see 3.6.2 Dependencies between Packages.]

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

[Definition: Within a package, one stylesheet module functions as the principal stylesheet module. The complete package is assembled by finding the stylesheet modules referenced directly or indirectly from the principal stylesheet module using xsl:include and xsl:import elements: see 3.12.2 Stylesheet Inclusion and 3.12.3 Stylesheet Import.]

1.2 What's New in XSLT 3.0?

A major focus for enhancements in XSLT 3.0 is the requirement to enable streaming of source documents. This is needed when source documents become too large to hold in main memory, and also for applications where it is important to start delivering results before the entire source document is available.

While implementations of XSLT that use streaming have always been theoretically possible, the nature of the language has made it very difficult to achieve this in practice. The approach adopted in this specification is twofold: it identifies a set of restrictions which, if followed by stylesheet authors, will enable implementations to adopt a streaming mode of operation without placing excessive demands on the optimization capabilities of the processor; and it provides new constructs to indicate that streaming is required, or to express transformations in a way that makes it easier for the processor to adopt a streaming execution plan.

Capabilities provided in this category include:

• A new xsl:stream instruction, which reads and processes a source document in streaming mode;

• The ability to declare that a mode is a streaming mode, in which case all the template rules using that mode must be streamable;

• A new xsl:iterate instruction, which iterates over the items in a sequence, allowing parameters for the processing of one item to be set during the processing of the previous item;

• A new xsl:merge instruction, allowing multiple input streams to be merged into a single output stream;

• A new xsl:fork instruction, allowing multiple computations to be performed in parallel during a single pass through an input document.

• Accumulators, which allow a value to be computed progressively during streamed processing of a document, and accessed as a function of a node in the document, without compromise to the functional nature of the XSLT language.

A second focus for enhancements in XSLT 3.0 is the introduction of a new mechanism for stylesheet modularity, called the package. Unlike the stylesheet modules of XSLT 1.0 and 2.0 (which remain available), a package defines an interface that regulates which functions, variables, templates and other components are visible outside the package, and which can be overridden. There are two main goals for this facility: it is designed to deliver software engineering benefits by improving the reusability and maintainability of code, and it is intended to streamline stylesheet deployment by allowing packages to be compiled independently of each other, and compiled instances of packages to be shared between multiple applications.

Other significant features in XSLT 3.0 include:

• An xsl:evaluate instruction allowing evaluation of XPath expressions that are dynamically constructed as strings, or that are read from a source document;

• Enhancements to the syntax of patterns, in particular enabling the matching of atomic values as well as nodes;

• An xsl:try instruction to allow recovery from dynamic errors;

• The element xsl:global-context-item, used to declare the stylesheet's expectations of the global context item (notably, its type).

• A new instruction xsl:assert to assist developers in producing correct and robust code.

XSLT 3.0 also delivers enhancements made to the XPath language and to the standard function library, including the following:

• Variables can now be bound in XPath using the let expression.

• Functions are now first class values, and can be passed as arguments to other (higher-order) functions, making XSLT a fully-fledged functional programming language.

• A number of new functions are available, for example trigonometric functions, and the functions parse-xml^FO30 and serialize^FO30 to convert between lexical and tree representations of XML.

This Working Draft includes support for maps (a data structure consisting of key/value pairs, sometimes referred to in other programming languages as dictionaries, hashes, or associative arrays). This feature extends the data model, provides new syntax in XPath, and adds a number of new functions and operators. The XSL Working Group intends that these changes should eventually become part of XPath: however, this has not yet been agreed with all interested parties.

A full list of changes is at J Changes since XSLT 2.0.

2.1 Terminology

For a full glossary of terms, see C Glossary.

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

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

[Definition: The term 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 post-processing the raw result of a stylesheet evaluation. The conditions under which this happens are described in 2.4 Executing a Transformation. Any final result tree may be serialized as described in 25 Serialization.

[Definition: The term source tree means any tree provided as input to the transformation. This includes the document containing the global context item if any, documents containing nodes present in the initial match selection, documents containing nodes supplied as the values of stylesheet parameters, documents obtained from the results of functions such as document, doc^FO30, and collection^FO30, documents read using the xsl:stream instruction, and documents returned by extension functions or extension instructions. In the context of a particular XSLT instruction, the term source tree means any tree provided as input to that instruction; this may be a source tree of the transformation as a whole, or it may be a temporary tree produced during the course of the transformation.]

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

The use of the term "tree" in phrases such as source tree, result tree, and temporary tree is not confined to documents that the processor materializes in memory in their entirety. The processor may, and in some cases must, use streaming techniques to limit the amount of memory used to hold source and result documents. When streaming is used, the nodes of the tree may never all be in memory at the same time, but at an abstract level the information is still modeled as a tree of nodes, and the document is therefore still described as a tree. Unless otherwise stated, the term "tree" refers to a tree rooted at a parentless node: that is, the term does not include subtrees of larger trees. Every node therefore belongs to exactly one tree.

In this specification the phrases must, must not, should, should not, may, required, and recommended, when used in normative text and rendered in capitals, are to be interpreted as described in [RFC2119].

Where the phrase must, must not, or required relates to the behavior of the XSLT processor, then an implementation is not conformant unless it behaves as specified, subject to the more detailed rules in 26 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 3.0] 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 ^XP30. It is a process that takes as input a sequence of items, and returns a sequence of atomic values, in which the nodes are replaced by their typed values as defined in [Data Model].] For some items (for example, elements with element-only content, and function items), atomization generates a dynamic error.

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

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

• [Definition: The term XPath 1.0 compatibility mode is defined in Section 2.1.1 Static Context ^XP30. 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 3.0 and XPath 1.0.]

[Definition: The core functions are: functions specified in [Functions and Operators] in either the standard function namespace or the namespace http://www.w3.org/2005/xpath-functions/math; plus functions defined in this specification in namespace http://www.w3.org/2005/xpath-functions/map.]

2.2 Notation

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

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

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

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

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

  The types used are as follows:

  Type name	Meaning
  boolean	One of the strings "yes", "true", or "1" to indicate the value true, or one of the strings "no", "false", or "0" to indicate the value false. Note: the values are synonyms; where this specification uses a phrase such as "If required='yes' is specified ..." this is to be interpreted as meaning "If the attribute named required is present, and has the value yes, true, or 1 (after stripping leading and trailing whitespace) ...".
  string	Any string
  expression	An XPath expression
  pattern	A pattern as described in 5.6 Patterns.
  sequence-type	A SequenceTypeXP30 as defined in the XPath specification
  uri; uris	A URI, for example a namespace URI or a collation URI; a whitespace-separated list of URIs
  qname	A lexical QName as defined in 5.1 Qualified Names
  eqname; eqnames	An EQName as defined in 5.1 Qualified Names; a whitespace-separated list of EQNames
  token; tokens	A string containing no significant whitespace; a whitespace-separated list of such strings
  nmtoken; nmtokens	A string conforming to the XML schema rules for the type xs:NMTOKEN; a whitespace-separated list of such strings.
  char	A string comprising a single Unicode character
  integer	An integer, that is a string in the lexical space of the schema type xs:integer
  decimal	A decimal value, that is a string in the lexical space of the schema type xs:decimal
  ncname	An unprefixed name: a string in the value space of the schema type xs:NCName
  prefix	An xs:NCName representing a namespace prefix, which must be in scope for the element on which it appears
  id	An xs:NCName used as a unique identifier for an element in the containing XML document

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

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

The rules in the element syntax summary (both for the element structure and for its attributes) apply to the stylesheet content after preprocessing as described in 3.14 Stylesheet Preprocessing.

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

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

• [ERR XTDE0030] It is a dynamic error if the effective value of an attribute written using curly brackets, in a position where an attribute value template is permitted, is a value that is not one of the permitted values for that attribute. If the processor is able to detect the error statically (for example, when any XPath expressions within the curly brackets can be evaluated statically), then the processor may optionally 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 3.11 Forwards Compatible Processing.

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

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

2.3 Initiating a Transformation

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

The execution of a stylesheet necessarily involves two activities: static analysis and dynamic evaluation. Static analysis consists of those tasks that can be performed by inspection of the stylesheet alone, including the binding of static variables, the evaluation of [xsl:]use-when expressions (see 3.14.1 Conditional Element Inclusion), and shadow attributes (see 3.14.2 Shadow Attributes) and detection of static errors. Dynamic evaluation consists of tasks which in general cannot be carried out until a source document is available.

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

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

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

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

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

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

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

2.4 Executing a Transformation

The classic method of executing an XSLT transformation is to apply template rules to an input document (see 2.3.3 Apply-Templates Invocation), and then delivering the result as a result tree which is then optionally serialized (see 2.3.6 Post-processing the Raw Result). This processing model is described in this section. Other ways of invoking a transformation or processing the result are outlined in the previous section.

[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 source document is typically processed by finding the template rule that best matches its root (document) node, and evaluating the sequence constructor contained in this template rule, as described in 5.8 Sequence Constructors.

The raw result sequence produced by evaluating this initial template is handled as follows:

1. If the template has an as attribute, then the raw result sequence is checked against the required type in the same way as for any other template, and is converted if necessary to the required type by applying the function conversion rules.

2. If the result sequence is non-empty, then it is typically post-processed (as described in 2.3.6 Post-processing the Raw Result) to construct an implicit final result tree, following the rules described in 5.8.1 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>
   

The process of generating a result tree from the raw result of the initial template may be replaced with the very similar sequence normalization process described in 25 Serialization. In the latter case the serialization parameter item-separator may be used to separate items in the raw result with a string other than space (for example, newlines).

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 23.2 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:try, xsl:for-each, xsl:for-each-group, xsl:fork, xsl:iterate and its subordinate instructions xsl:next-iteration and xsl:break;

• 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:assert, xsl:message, xsl:result-document, xsl:stream, xsl:perform-sort, xsl:merge.

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

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

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

The most commonly used component of the dynamic context is the context item. This is an implicit variable whose value is the item currently being processed (it may be a node, an atomic value, or a function item). 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 25 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 Packages and Modules

In previous versions of the XSLT language, it has been possible to structure a stylesheet as a collection of modules, using the xsl:include and xsl:import declarations to express the dependency of on module on others.

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

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

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

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

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

To achieve this, packages (unlike modules):

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

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

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

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

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

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

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

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

2.8 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 23.1 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 23.2 Extension Instructions. A stylesheet that uses extension instructions in a particular namespace must declare that it is doing so by using the [xsl:]extension-element-prefixes attribute.

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

This specification defines how extension instructions and extension functions are invoked, but the facilities for creating new extension instructions and extension functions are implementation-defined. For further details, see 23 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 3.8.3 User-defined Data Elements).

2.9 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^XP30 syntax defined in [XPath 3.0].

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

[Definition: The schema components that may be referenced by name in a 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 3.0, defined in 26 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^FO30 function) and other constructs may produce different results depending on the datatype (for example, given the element <product price="10.00" discount="2.00"/>, the expression @price gt @discount will return true if the attributes have type xs:decimal, but will return false if they are untyped).

There is a standard set of type definitions that are always available as in-scope schema components in every stylesheet. These are defined in 3.15 Built-in Types.

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

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

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

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

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

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

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

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

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

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

2. The processor may maintain additional metadata, beyond that described in [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 2.11 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 24.4 Validation.

2.10 Streaming

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

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

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

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

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

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

Nothing in this specification, nor in its predecessors [XSLT 1.0] and [XSLT 2.0], prevents a processor using streaming whenever it sees an opportunity to do so. However, experience has shown that in order to achieve streaming, it is often necessary to write stylesheet code in such a way as to make this possible. Therefore, XSLT 3.0 provides explicit constructs allowing the stylesheet author to request streaming, and defines explicit static constraints on the structure of the code which are designed to make streaming possible.

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

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

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

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

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

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

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

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

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

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

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

Streaming applications often fall into one of the following categories:

• Aggregation applications, where a single aggregation operation (perhaps count^FO30, sum^FO30, exists^FO30, or distinct-values^FO30) is applied to a set of elements selected from the streamed source document by means of a path expression.

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

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

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

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

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

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

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

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

2.11 Error Handling

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

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 3.11 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.]

When a dynamic error occurs, and is not caught using xsl:catch, 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 9.11 Circular Definitions.

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

A dynamic error may also be signaled during the static analysis phase if the error occurs during evaluation of a static expression.

The XPath specification states (see Section 2.3.1 Kinds of Errors ^XP30) 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 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:number select="42"/>
</xsl:if>

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

If more than one error arises, an implementation is not required to 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 25 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.

Errors defined in related specifications ([XPath 3.0], [Functions and Operators] [XSLT and XQuery Serialization]) use QNames with a similar structure, in the same namespace. When errors occur in processing XPath expressions, an XSLT processor should use the original error code reported by the XPath processor, unless a more specific XSLT error code is available.

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

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

3.1 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] 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 23.2 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 for standard functions defined in this specification.]

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

• The namespace http://www.w3.org/2005/xpath-functions/map is used for functions defined in this specification relating to the manipulation of maps.

• [Definition: The XML namespace, defined in [Namespaces in XML] 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.

• [Definition: The standard error namespace http://www.w3.org/2005/xqt-errors is used for error codes defined in this specification and related specifications. It is also used for the names of certain predefined variables accessible within the scope of an xsl:catch element.]

• The namespace http://www.w3.org/2000/xmlns/ is reserved for use as described in [Namespaces in XML]. No element or attribute node can have a name in this namespace, and although the prefix xmlns is implicitly bound to this namespace, no namespace node will ever define this binding.

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, an accumulator, or a character map; except that the name xsl:initial-template is permitted as a template name.

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 3.0]) of the attribute has a non-null namespace URI. These attributes are referred to as extension attributes.] The presence of an extension attribute must not cause the final result trees produced by the transformation to be different from the result trees that a conformant XSLT 3.0 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 [XSLT and XQuery Serialization], the presence of an extension attribute must not cause the serializer to behave in a way that is inconsistent with the mandatory provisions of that specification.

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

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

3.4 XSLT Media Type

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

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

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

3.5 Standard Attributes

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

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

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

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

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

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

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

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

The effect of the [xsl:]use-when attribute is described in 3.14.1 Conditional Element Inclusion.

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

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

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

[xsl:]default-collation

see 3.8.1 The default-collation Attribute

[xsl:]default-mode

see 3.8.2 The [xsl:]default-mode Attribute

[xsl:]default-validation

see 24.4 Validation

[xsl:]exclude-result-prefixes

see 11.1.3 Namespace Nodes for Literal Result Elements

[xsl:]expand-text

see 5.7.2 Text Value Templates

[xsl:]extension-element-prefixes

see 23.2 Extension Instructions

[xsl:]use-when

see 3.14.1 Conditional Element Inclusion

[xsl:]version

see 3.10 Backwards Compatible Processing and 3.11 Forwards Compatible Processing

[xsl:]xpath-default-namespace

see 5.2 Unprefixed Lexical QNames in Expressions and Patterns

3.6 Packages

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

<xsl:package
  name? = uri
  package-version? = string
  version = decimal
  input-type-annotations? = "preserve" | "strip" | "unspecified"
  declared-modes? = boolean
  default-mode? = eqname | "#unnamed"
  default-validation? = "preserve" | "strip"
  default-collation? = uris
  extension-element-prefixes? = prefixes
  exclude-result-prefixes? = prefixes
  expand-text? = boolean
  use-when? = expression
  xpath-default-namespace? = uri >
  <!-- Content: ((xsl:use-package | xsl:expose | xsl:global-context-item | declarations)*) -->
</xsl:package>

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

The version attribute indicates the version of the XSLT language specification to which the package manifest conforms. The value should normally be 3.0. If the value is numerically less than 3.0, the content of the xsl:package element is processed using the rules for backwards compatible behavior (see 3.10 Backwards Compatible Processing). If the value is numerically greater than 3.0, it is processed using the rules for forwards compatible behavior (see 3.11 Forwards Compatible Processing).

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

A package may have a version identifier, given in its package-version attribute. This is used to distinguish different versions of a package. The value of the version attribute, after trimming leading and trailing whitespace, must conform to the syntax given in 3.6.1 Versions of a Package. If no version number is specified for a package, version 1 is assumed.

The attributes default-collation, default-mode, default-validation, exclude-result-prefixes, expand-text, extension-element-prefixes, use-when, version, and xpath-default-namespace are standard attributes that can appear on any XSLT element, and potentially affect all descendant elements. Their meaning is described in 3.5 Standard Attributes.

The package manifest contains the following elements, arbitrarily ordered:

1. Zero or more xsl:use-package elements which identify the packages used by this package, including subsidiary elements that constrain the way in which the components contained in those packages are used.

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

3. Zero or more xsl:expose declarations that define the interface offered by this package to the outside world.

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

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

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

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

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

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

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

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

3.7 Stylesheet Modules

[Definition: A package consists of one or more stylesheet modules, each one forming all or part of an XML document.]

The principal stylesheet module of a package may take one of three forms:

• A package manifest, as described in 3.6 Packages, which is a subtree rooted at an xsl:package element

• An implicit package, which is a subtree rooted at an xsl:stylesheet or xsl:transform element. This is transformed automatically to a package as described in 3.6 Packages.

• A simplified stylesheet, which is a subtree rooted at a literal result element, as described in 3.9 Simplified Stylesheet Modules. This is first converted to an implicit package by wrapping it in an xsl:stylesheet element using the transformation described in 3.9 Simplified Stylesheet Modules, and then to an explicit package (rooted at an xsl:package element) using the transformation described in 3.6 Packages.

A stylesheet module other than the principal stylesheet module of a package may take either of two forms:

• [Definition: A standard stylesheet module, which is a subtree rooted at an xsl:stylesheet or xsl:transform element.]

• [Definition: A simplified stylesheet, which is a subtree rooted at a literal result element, as described in 3.9 Simplified Stylesheet Modules. This is first converted to a standard stylesheet module by wrapping it in an xsl:stylesheet element using the transformation described in 3.9 Simplified Stylesheet Modules.]

Whichever of the above forms a module takes, the outermost element (xsl:package, xsl:stylesheet, or a literal result element) may either be the outermost element of an XML document, or it may be a child of some (non-XSLT) element in a host document.

[Definition: A stylesheet module whose outermost element is the child of a non-XSLT element in a host document is referred to as an embedded stylesheet module. See 3.13 Embedded Stylesheet Modules.]

3.8 Stylesheet Element

<xsl:stylesheet
  id? = id
  version = decimal
  default-mode? = eqname | "#unnamed"
  default-validation? = "preserve" | "strip"
  input-type-annotations? = "preserve" | "strip" | "unspecified"
  default-collation? = uris
  extension-element-prefixes? = prefixes
  exclude-result-prefixes? = prefixes
  expand-text? = boolean
  use-when? = expression
  xpath-default-namespace? = uri >
  <!-- Content: (declarations) -->
</xsl:stylesheet>

<xsl:transform
  id? = id
  version = decimal
  default-mode? = eqname | "#unnamed"
  default-validation? = "preserve" | "strip"
  input-type-annotations? = "preserve" | "strip" | "unspecified"
  default-collation? = uris
  extension-element-prefixes? = prefixes
  exclude-result-prefixes? = prefixes
  expand-text? = boolean
  use-when? = expression
  xpath-default-namespace? = uri >
  <!-- Content: (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.

The version attribute indicates the version of XSLT that the stylesheet module requires. The attribute is required.

[ERR XTSE0110] The value of the version attribute must be a number: specifically, it must be a valid instance of the type xs:decimal as defined in [XML Schema Part 2].

The version attribute is intended to indicate the version of the XSLT specification against which the stylesheet is written. In a stylesheet written to use XSLT 3.0, the value should normally be set to 3.0. If the value is numerically less than 3.0, the stylesheet is processed using the rules for backwards compatible behavior (see 3.10 Backwards Compatible Processing). If the value is numerically greater than 3.0, the stylesheet is processed using the rules for forwards compatible behavior (see 3.11 Forwards Compatible Processing).

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

The [xsl:]default-validation attribute defines the default value of the validation attribute of all relevant instructions appearing within its scope. For details of the effect of this attribute, see 24.4 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, xsl:transform, or xsl:override 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 3.8.3 User-defined Data Elements)].

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

xsl:import
xsl:include
xsl:accumulator
xsl:attribute-set
xsl:character-map
xsl:decimal-format
xsl:function
xsl:import-schema
xsl:key
xsl:mode
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.

The child elements of the xsl:stylesheet element may appear in any order. In most cases, the ordering of these elements does not affect the results of the transformation; however:

• As described in 6.4 Conflict Resolution for Template Rules, when two template rules with the same priority match the same nodes, there are situations where the order of the template rules will affect which is chosen.

• Forwards references to static variables are not allowed in static expressions.

3.9 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="3.0"
      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="3.0"
                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="3.0"
                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" select="'/'"/>
      <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 3.0 ; the value must be a valid instance of the type xs:decimal as defined in [XML Schema Part 2].

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 template rules, 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 match selection, to be matched by the implicit match="/" template rule using the unnamed mode.

3.10 Backwards Compatible Processing

[Definition: The effective version of an element in a stylesheet module or package manifest is the decimal value of the [xsl:]version attribute (see 3.5 Standard Attributes) on that element or on the innermost ancestor element that has such an attribute, excluding the version attribute on an xsl:output element.]

[Definition: An element is processed with backwards compatible behavior if its effective version is less than 3.0.]

Specifically:

• If the effective version is equal to 1.0, then the element is processed with XSLT 1.0 behavior as described in 3.10.1 XSLT 1.0 Compatibility Mode.

• If the effective version is equal to 2.0, then the element is processed with XSLT 2.0 behavior as described in 3.10.2 XSLT 2.0 Compatibility Mode.

• If the effective version is any other value less than 3.0, the recommended action is to report a static error; however, processors may recognize such values and process the element in an implementation-defined way.

  Note:

  XSLT 1.0 allowed the version attribute to take any decimal value, and invoked forwards compatible processing for any value other than 1.0. XSLT 2.0 allowed the attribute to take any decimal value, and invoked backwards compatible (i.e. 1.0-compatible) processing for any value less than 2.0. Some stylesheets may therefore be encountered that use values other than 1.0 or 2.0. In particular, the value 1.1 is sometimes encountered, as it was used at one stage in a draft language proposal.

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 3.0 implementation supports backwards compatible behavior for any XSLT version earlier than XSLT 3.0.

[ERR XTDE0160] It is a dynamic error if an element has an effective version of V (with V < 3.0) when the implementation does not support backwards compatible behavior for XSLT version V.

3.11 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 3.0, while retaining the ability to execute the stylesheet with an XSLT 3.0 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 3.14.1 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 3.0 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 is processed with forwards compatible behavior if its effective version is greater than 3.0.]

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.

When an element is processed with forwards compatible behavior:

• If the element is in the XSLT namespace and appears as a child of the xsl:stylesheet element, and XSLT 3.0 does not allow the element to appear as a child of the xsl:stylesheet element, then the element and its content must be ignored.

• If the element has an attribute that XSLT 3.0 does not allow the element to have, then the attribute must be ignored.

• If the element is in the XSLT namespace and appears as a child of an element whose content model requires a sequence constructor, and XSLT 3.0 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 3.0 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>

3.12 Combining Stylesheet Modules

XSLT provides two mechanisms to construct a package 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="3.0"
                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>

3.13 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="3.0"
                    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>

3.14 Stylesheet Preprocessing

This specification provides two features that cause the raw stylesheet to be preprocessed as the first stage of static processing: elements may be conditionally included or excluded by means of an [xsl:]use-when attribute as described in 3.14.1 Conditional Element Inclusion, and attributes may be conditionally computed as described in 3.14.2 Shadow Attributes.

Note that many of the rules affecting the validity of stylesheet documents apply to a stylesheet after this preprocessing phase has been carried out.

<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'"/>

<xsl:include _href="common{$VERSION}.xsl"/>

<xsl:param name="streamable" as="xs:boolean" required="yes" static="yes"/>
<xsl:mode _streamable="{$streamable}" on-no-match="shallow-skip"/>

<xsl:param name="NS" as="xs:string" static="yes" select="'http://example.com/ns/one'"/>

_xpath-default-namespace="{$NS}"

          <xsl:param name="filter" static="yes"
                     as="xs:string" select="'true()'"/>
          <xsl:function name="local:filter" as="xs:boolean">
            <xsl:param name="e" as="element(employee)"/>
            <xsl:sequence _select="$e/({$filter})"/>
          </xsl:function>
          <xsl:template match="/">
            <report>
              <xsl:apply-templates mode="report" select="//employee[local:filter(.)]"/>
            </report>
          </xsl:template>
                     

3.15 Built-in Types

Every XSLT 3.0 processor includes the following named type definitions in the in-scope schema components:

• All built-in types defined in [XML Schema Part 2], including xs:anyType and xs:anySimpleType.

• The following types defined in [XPath 3.0]: xs:yearMonthDuration, xs:dayTimeDuration, xs:anyAtomicType, xs:untyped, and xs:untypedAtomic.

XSLT 3.0 processors may optionally include types defined in XSD 1.1 (see [XML Schema 1.1 Part 1]). XSD 1.1 adopts the types xs:yearMonthDuration, xs:dayTimeDuration, and xs:anyAtomicType previously defined in XPath 2.0, and adds one new type: xs:dateTimeStamp. XSD 1.1 also allows implementers to define additional primitive types, and XSLT 3.0 permits such types to be supported by an XSLT processor.

A schema-aware XSLT processor additionally supports:

• User-defined types, and element and attribute declarations, that are imported using an xsl:import-schema declaration as described in 3.16 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 23.1 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.

3.16 Importing Schema Components

<!-- Category: declaration -->
<xsl:import-schema
  namespace? = uri
  schema-location? = uri >
  <!-- 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 3.15 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 package, 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 within the declaring package; importing components in one stylesheet module makes them available throughout the package.

The schema components imported into different packages within a stylesheet must be consistent. Specifically, it is not permitted to use the same name in the same XSD symbol space to refer to different schema components within different packages; and the union of the schema components imported into the packages of a stylesheet must constitute a valid schema (as well as the set of schema components imported into each package forming a valid schema in its own right).

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 one of the following must be true:

• the namespace attribute of the xsl:import-schema element and the targetNamespace attribute of the xs:schema element are both absent (indicating a no-namespace schema), or

• the namespace attribute of the xsl:import-schema element and the targetNamespace attribute of the xs:schema element are both present and both have the same value, or

• the namespace attribute of the xsl:import-schema element is absent and the targetNamespace attribute of the xs:schema element is present, in which case the target namespace is as given on the xs:schema element.

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

<xsl:variable name="condition" select="local:yes-no('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]) 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 3.0 can be used with either of these XML versions.

Construction of the XDM tree is outside the scope of this specification, so XSLT 3.0 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 25 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 3.0 and XPath 3.0, 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 (and, indeed, between different editions of XML 1.0). Implementations may support any version; it is recommended that an XSLT 3.0 processor that implements the 1.1 versions should also provide a mode that supports the 1.0 versions. It is thus implementation-defined which versions and editions of XML and XML Namespaces are supported by the implementation..

The current version of [XML Schema 1.1 Part 2] references the XML 1.1 specifications, but the previous version ([XML Schema Part 2]) (that is, XSD 1.0) remains in widespread use, and only references XML 1.0. With processors lacking support for XSD 1.1, therefore, datatypes such as xs:NCName and xs:ID may be constrained by the XML 1.0 rules, and not allow the full range of values permitted by XML 1.1. It is recommended that implementers wishing to support XML 1.1 should consult [XML Schema 1.0 and XML 1.1] for guidance.

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:accumulator
xsl:analyze-string
xsl:apply-imports
xsl:apply-templates
xsl:attribute-set
xsl:call-template
xsl:character-map
xsl:choose
xsl:evaluate
xsl:fork
xsl:merge
xsl:merge-source
xsl:mode
xsl:next-iteration
xsl:next-match
xsl:override
xsl:package
xsl:stylesheet
xsl:transform
xsl:use-package

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

6. Any whitespace text node whose immediate preceding-sibling node is an xsl:catch 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 ^DM30.]

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:package element. This attribute has three permitted values: strip, preserve, and unspecified. The default value is unspecified.

The input-type-annotations attribute may also be specified on the xsl:stylesheet element; if it is specified at this level then it must be consistent for all stylesheet modules within the same package.

[ERR XTSE0265] It is a static error if there is a stylesheet module in a package that specifies input-type-annotations="strip" and another stylesheet module that specifies input-type-annotations="preserve", or if a stylesheet module specifies the value strip or preserve and the same value is not specified on the xsl:package element of the containing package.

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. As with whitespace stripping, the rules for stripping of type annotations may vary from one package to another, and have the effect of modifying the mapping from URIs to document nodes defined in the XPath dynamic context; this means that two calls to the doc^FO30 function (for example) supplying the same URI may produce different document nodes if the calls appear in different packages.

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 global context item if it is a node, any documents containing nodes present in the initial match selection, any document returned by the functions document, doc^FO30, or collection^FO30, and any document read using xsl:stream. It does not include documents passed as the values of stylesheet parameters or parameters of the initial template or function, nor values returned from extension functions.

Each source tree is associated with a package: the relevant package for the global context item is the top-level package; the relevant package for a call on document, doc^FO30, or collection^FO30; is the package in which that call appears; and the relevant package for evaluation of xsl:stream is the package in which that instruction appears.

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.

Formally, the stripping process modifies the mapping from URIs to document nodes defined in the XPath dynamic context. This mapping can therefore vary from one package to another. The mapping that applies to a particular call on document, doc^FO30, or collection^FO30, or a particular evaluation of xsl:stream, is affected by the xsl:strip-space and xsl:preserve-space declarations within the package in which that construct appears. This means that two calls on the doc^FO30 function (for example) may return different nodes if the calls appear in different packages.

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^XP30; an element name matches an xsl:strip-space or xsl:preserve-space element if it matches one of the NameTests^XP30. An element matches a NameTest^XP30 if and only if the NameTest^XP30 would be true for the element as an XPath node test.

The effect of xsl:strip-space and xsl:preserve-space is local to the package in which they appear. Declarations within a library package only affect the handling of documents loaded using a call on the document, doc^FO30, or collection^FO30 functions or an evaluation of an xsl:stream instruction appearing lexically within the same package. Declarations within the top-level package also affect the processing of the main input document.

[ERR XTSE0270] It is a static error if within any package the same NameTest^XP30 appears in both an xsl:strip-space and an xsl:preserve-space declaration if both have the same import precedence. Two NameTests are considered the same if they match the same set of names (which can be determined by comparing them after expanding namespace prefixes to URIs).

Otherwise, when more than one xsl:strip-space and xsl:preserve-space element within the relevant package matches, the best matching element is determined by the best matching NameTest^XP30. The rules are similar to those for 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.

• If several matches have the same default priority (which can only happen if one of the NameTests takes the form *:local and the other takes the form prefix:*), then the declaration that appears last in declaration order is used.

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^FO30 and idref^FO30 described in [Functions and Operators].

4.6 Data Model for Streaming

The data model for nodes in a document that is being streamed is no different from the standard XDM data model, in that it contains the same objects (nodes) with the same properties and relationships. The facilities for streaming do not change the data model; instead they impose rules that limit the ability of stylesheets to navigate the data model.

A useful way to visualize streaming is to suppose that at any point in time, there is a current position in the streamed input document which may be the start or end of the document, the start or end tag of an element, or a text, comment, or processing instruction node. From this position, the stylesheet has access to the following information:

• Properties intrinsic to the node, such as its name, its base URI, its type annotation, and its is-id and is-idref properties.

• The ancestors of the node (but navigation downwards from the ancestors is not permitted).

• The attributes of the node, and the attributes of its ancestors. For each such attribute, all the properties of the node including its string value and typed value are available, but there are limitations that restrict navigation from the attribute node to other nodes in the document.

• The in-scope namespace bindings of the node.

• In the case of attributes, text nodes, comments, and processing instructions, the string value and typed value of the node.

The children and other descendants of a node are not accessible except as a by-product of changing the current position in the document. The same applies to properties of an element or document node that require examination of the node's descendants, that is, the string value and typed value. This is enforced by means of a rule that only one expression requiring downward navigation from a node is permitted.

There is an assumption that information about unparsed entities is available at all times during the processing of a document. This has two implications: firstly, the processor may need to read ahead at the start of the document to determine this information so that it is available while processing the document root node; and secondly, the information then needs to be retained for the duration of the processing.

Expressions such as (/) instance of document-node(element(invoice)) also require look-ahead as far as the start-tag of the first element.

A streaming processor is required to read only as much of the source document as is needed to generate correct stylesheet output. It is not required to read the full source document merely in order to satisfy the requirement imposed by the XML Recommendation that an XML Processor must report violations of well-formedness in the input.

More detailed rules are defined in 19 Streamability.

4.7 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 datatypes 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.

4.8 Disable Output Escaping

For backwards compatibility reasons, XSLT 3.0 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 25.1 Character Maps) was 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 25.2 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.

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, an accumulator, a named output definition, or a character map is a qualified name: that is, it consists of a local name and an optional namespace URI.

In most cases where such names are written in a stylesheet, the syntax for expressing the name is given by the production EQName^XP30 in the XPath specification. In practice, this means that three forms are permitted:

• A simple NCName appearing on its own (without any prefix). This represents the local name of the object. The interpretation of unprefixed names is described below.

• A lexical QName written in the form NCName ":" NCName where the first part is a namespace prefix and the second part is the local name. The namespace part of the object's name is then derived from the prefix by examining the in-scope namespace bindings of the element node in the stylesheet where the name appears.

• A URIQualifiedName^XP30 in the form "Q{" URI? "}" NCName where the two parts of the name, that is the namespace part and the local part, both appear explicitly. If the URI part is omitted (for example Q{}local), the resulting expanded QName is a QName whose namespace part is absent.

[Definition: An expanded QName is a value in the value space of the xs:QName datatype as defined in the XDM data model (see [Data Model]): that is, a triple containing namespace prefix (optional), namespace URI (optional), and local name. 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.]

[Definition: An EQName is a string representing an expanded QName where the string, after removing leading and trailing whitespace, is in the form defined by the EQName^XP30 production in the XPath specification.]

[Definition: A lexical QName is a string representing an expanded QName where the string, after removing leading and trailing whitespace, is within the lexical space of the xs:QName datatype as defined in XML Schema (see [XML Schema Part 2]): that is, a local name optionally preceded by a namespace prefix and a colon.]

Note that every lexical QName is an EQName, but the converse is not true.

The following rules are used when interpreting a lexical QName:

1. [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 an attribute or text node within a stylesheet module, or as the result of evaluating an XPath expression contained in such a node. The element containing this attribute or text node is referred to as the defining element of the lexical QName.]

2. If the lexical 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 ^DM30) is not used for unprefixed names.

   [ERR XTSE0280] In the case of a prefixed lexical QName used as the value (or as part of 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 lexical QName.

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

3. If the lexical QName has no prefix, then:

   1. 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 Lexical QNames in Expressions and Patterns.

   2. If the name is in one of the following categories, then the default namespace of the defining element is used:

      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

   3. In all other cases, a lexical QName with no prefix represents an expanded QName in no namespace (that is, an xs:QName value in which both the prefix and the namespace URI are absent).

5.2 Unprefixed Lexical 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 or text node of that element (including XPath expressions in attribute value templates and text value templates). The effect of this is specified in [XPath 3.0]; 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 ^DM30) 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 3.0 [XPath 3.0]. 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^XP30 defined in [XPath 3.0].]

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

Except where forwards compatible behavior is enabled (see 3.11 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 or text value template, does not match the XPath production Expr^XP30, 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 3.0].

The transformation fails with a dynamic error if any XPath expression is evaluated and raises a dynamic error. Error codes are defined in [XPath 3.0].

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 3.0].

[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:  When used in this specification without further qualification, the term function conversion rules means the function conversion rules defined in [XPath 3.0], applied with XPath 1.0 compatibility mode set to false.]

This specification also invokes the XPath 3.0 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^XP30 which contains all the information that can affect the result of evaluating an expression. The expression context has two parts, the static context^XP30, and the dynamic context^XP30. The components that make up the expression context are defined in the XPath specification (see Section 2.1 Expression Context ^XP30). 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