XSL Transformations (XSLT)
Version 2.0

1.1 Stylesheets and Transformation

This specification defines the syntax and semantics of the XSLT language. A transformation in the XSLT language is expressed in the form of a stylesheet, whose syntax is well-formed XML [XML] conforming to the Namespaces in XML Recommendation [XML Names]. A stylesheet generally includes both elements that are defined by XSLT and elements that are not defined by XSLT. XSLT-defined elements are distinguished by belonging to a specific XML namespace (http://www.w3.org/1999/XSL/Transform: see [2.1 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.

NOTE: 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, or into another presentation-oriented format such as HTML, XHTML, or SVG.

The software responsible for transforming a source document into a result document 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. A specific product that performs the functions of an XSLT processor is referred to as an implementation.

A transformation expressed in XSLT describes rules for transforming a source tree into a result tree. 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 content constructor, which can be evaluated to produce part of the result tree. The structure of the result tree can be completely different from the structure of the source tree. In constructing the result tree, nodes from the source tree 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.

NOTE: More generally, a transformation can process several source trees and produce several result trees.

A stylesheet may consist of several stylesheet modules, contained in different XML documents. One of these functions as the principal stylesheet module. The complete stylesheet is assembled by finding the stylesheet modules referenced directly or indirectly from the principal stylesheet module using xsl:include and xsl:import elements: see [2.8.1 Stylesheet Inclusion] and [2.8.2 Stylesheet Import].

1.2 Initiating a Transformation

A stylesheet can process further source documents in addition to the principal source document. These additional documents can be loaded using the functions document or collection described in [Functions and Operators], or they can be supplied as stylesheet parameters (see [6.2 Global Variables and Parameters]), or as the result of an extension function (see [16.1 Extension Functions]

NOTE: Sometimes it is useful to write an XSLT stylesheet that does not require input from a principal source document. However, the semantics of the language require that a principal source document is always present. Implementors may provide a mechanism that supplies a default document, containing just a document node with no children, as the principal source document to be used in the absence of any other source document.

1.4 Executing a Transformation

A stylesheet contains a set of template rules. A template rule has two parts: a pattern which is matched against nodes in the source tree and a content constructor which is evaluated to produce a sequence of nodes: these nodes are typically used to form part of the result tree. This allows a stylesheet to be applicable to a wide class of documents that have similar source tree structures.

A content constructor is evaluated for a particular node in the source tree, to create part of the result tree. A content constructor can contain elements (called literal result elements) and text nodes that specify part of the result structure directly. A content constructor can also contain elements from the XSLT namespace that are instructions for creating parts of the result tree.

When a content constructor is evaluated, each instruction is evaluated to produce a sequence of zero or more nodes; the result of the content constructor as a whole is a sequence of nodes formed by concatenating the results of each of the instructions and literal results nodes that it contains, in the order that they appear in the content constructor. The resulting nodes are typically attached as children to an element or document node constructed by the instruction that contains the content constructor, thus forming a tree. During this process, adjacent text nodes will be merged into a single text node. When a content constructor is evaluated to create new nodes, the tree to which these nodes are added is referred to as the current result tree. When the transformation is initiated, a result tree is created, and becomes the current result tree. This tree is referred to as the principal result tree. Various XSLT instructions, (including xsl:variable and xsl:result-document) establish a new current result tree for the nodes created by the content constructor that they contain.

The elements occurring within a content constructor are classified as being either literal result elements or instructions. If the element is in the XSLT namespace, or in a namespace designated as an extension namespace, then it is an instruction. Otherwise, it is a literal result element.

The element syntax summary notation used to describe the syntax of XSLT-defined elements is described in [20 Notation], and a full list of these elements is provided in [C Element Syntax Summary]

Instructions can select and process other nodes in a source tree. The typical way of processing a source node is to create a sequence of result nodes by finding the applicable template rule and evaluating its content constructor. Note that source nodes are processed only if they are selected by such an instruction.

Instructions that select nodes from the source document, or that derive information from these nodes for inclusion in the result document, always access the source tree by means of an Expression in the XPath language, described in [XPath 2.0]. A stylesheet written to use XSLT 2.0 will contain expressions whose syntax and semantics are defined by XPath 2.0 (but see also [2.6 Backwards-Compatible Processing] and [2.7 Forwards-Compatible Processing]).

Execution of a stylesheet against the principal source document proceeds by creating a document node for the principal result tree, finding the template rule that matches the document node of the source tree, and evaluating the content constructor of this template rule to create the children of the new document node. By the time evaluation of this content constructor is complete, these children will typically each act as the parent of further result nodes, so a complete tree is constructed.

It is also possible for the execution of a stylesheet to start at a node in the source document other than the document node, determined by the implementation-specific mechanism for invoking a stylesheet. In this situation, the complete tree remains available for processing by the stylesheet; the only difference is the choice of the node used when applying the first template rule.

In the process of finding the applicable template rule, more than one template rule may have a pattern that matches a given node. However, only one template rule will be applied. The method for deciding which template rule to apply is described in [5.4 Conflict Resolution for Template Rules].

A single content constructor by itself has considerable power. It can create structures of arbitrary complexity; it can pull string values out of arbitrary locations in the source tree; and it can generate structures that are repeated according to the occurrence of nodes in the source tree.

For simple transformations where the structure of the result tree is independent of the structure of the source tree, a stylesheet can often consist of only a single literal result element, containing a content constructor which functions as a template for building the complete result tree. Transformations on XML documents that represent data with a regular and predictable structure (for example, data extracted from a relational database) are often of this kind. XSLT allows a simplified syntax for such stylesheets (see [2.5 Simplified Stylesheet Modules]).

1.5 Maintaining Position: the Focus

When a content constructor is evaluated, the processor keeps track of which nodes are being processed by means of a set of implicit variables referred to collectively as the focus. More specifically, the focus consists of the following five values:

• The context item is the item currently being processed. An item (see [Data Model]) is either a simple value (such as an integer, date, or string), or a node. If the context item is a node, then it will always be a node in the context document. The initial context node is the same as the context document. It changes whenever instructions such as xsl:apply-templates and xsl:for-each are used to process a sequence of items; each item in such a sequence becomes the context item while that item is being processed. The context item is returned by the XPath expression . (dot).

• The context position is the position of the context item within the sequence of items currently being processed. It changes whenever the context item changes. When an instruction such as xsl:apply-templates or xsl:for-each is used to process a sequence of items, the first item in the sequence is processed with a context position of 1, the second item with a context position of 2, and so on. The context position is returned by the XPath expression position().

• The context size is the number of items in the sequence of items currently being processed. It changes whenever instructions such as xsl:apply-templates and xsl:for-each are used to process a sequence of items; during the processing of each one of those items, the context size is set to the count of the number of items in the sequence (or equivalently, the position of the last item in the sequence). The context size is returned by the XPath expression last().

• If the context item is a node (as distinct from a simple value such as an integer), then it is also referred to as the context node. The context node is not an independent variable, it changes whenever the context item changes. When the context item is a simple value, there is no context node: its value is an empty sequence. The context node is returned by the XPath expression self::node(), and it is used as the starting node for all relative path expressions.

• The context document is the source document currently being processed. This is initially set to the document node of the principal source document. It changes when instructions such as xsl:apply-templates and xsl:for-each are used to process nodes in a document other than the principal source document. When such an instruction is processing a node, the context document is the document containing that node. When such an instruction is processing a simple value (an item that is not a node), the context document is the same as the context document for the content constructor containing the xsl:apply-templates or xsl:for-each instruction. The context document is returned by the XPath expression / (slash), and it is used as the starting node for all absolute path expressions.

On completion of an instruction which changes the focus (such as xsl:apply-templates or xsl:for-each), the focus reverts to its previous value.

The description above gives an outline of the way the focus works. Detailed rules for the effect of each instruction are given separately with the description of that instruction. In the absence of specific rules, an instruction uses the same focus as its parent instruction.

Sometimes the focus is based on a single node rather than a sequence. A singleton focus based on a node N has the context item (and therefore the context node) set to N, the context document set to the document containing N, and the context position and context size both set to 1 (one).

1.6 Parsing and Serialization

As explained in the previous section, an XSLT stylesheet describes a process that constructs a result tree from a source tree.

The stylesheet does not describe how the source tree is constructed. Frequently an implementation will operate in conjunction with an XML parser (or more strictly, in the terminology of [XML], an XML processor), to build the 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 [DOM2]). 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 data model 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.

A frequent requirement is to output the 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 should be able to perform serialization. However, for pragmatic reasons, this specification describes a declaration (the xsl:output element, see [18 Serialization]) which allows a stylesheet to specify the desired properties of a serialized output file. Implementations that do not serialize the result tree are allowed to ignore this declaration.

Because it is a common requirement to perform a transformation on a document while retaining lexical characteristics such as CDATA section boundaries, entity references, and the like, an appendix to this specification (see [G Representation of Lexical XML Constructs]) describes a way in which these constructs can be represented within the data model by means of elements in a special namespace. If such a representation is chosen, the tree is transformed in the same way as any other tree. The process of constructing such a tree is something that happens before XSLT transformation starts, and the process of interpreting such a tree and reconstituting the lexical representation is part of the serialization process. Neither of these processes is properly within the scope of XSLT transformation, and therefore, this specification places no requirement on an XSLT processor to support this representation of lexical properties.

1.7 Extensibility

XSLT provides two "hooks" for extending the language, one hook for extending the set of instruction elements used in content constructors and one hook for extending the set of functions used in XPath expressions. These hooks are both based on XML namespaces: see [16 Extensibility and Fallback] for further details. Extension instructions and extension functions defined according to these rules may be provided by the implementor of the XSLT processor, and the implementor 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.

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 should be described in the vendor's documentation.

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.

1.8 Error Handling

An error that is detected by examining a stylesheet before execution starts (that is, before the source document and values of stylesheet parameters are available) is referred to as a static error. Errors classified in this specification as static errors must be signaled by all implementations: that is, the processor must indicate that the error is present, and it must not use the stylesheet to produce a result tree. A static error must be signaled even if it occurs in a part of the stylesheet that is never evaluated.

There is an exception to this rule when the stylesheet specifies forwards-compatible behavior (see [2.7 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.

An error that is not detected until a source document is being transformed is referred to as a dynamic error. In many cases, this specification allows dynamic errors to be signaled (by reporting the error condition and terminating execution), but also defines a recovery action that can be taken. It is implementation-defined whether the error is signaled or the recovery action is taken. If the implementation does choose to take recovery action, it must take the recovery action defined in this specification.

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

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 result tree 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 can 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 report 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 [6.3 Circular Definitions].

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 reported as a dynamic error. An implementation may also, optionally, report a type error 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. For example, the following construct contains a type error, because 42 is not allowed as an operand of the xsl:apply-templates instruction. An implementation may optionally report this as a static error, even though the offending instruction will never be evaluated, and the type error would therefore never be reported as a dynamic error.

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

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

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 produce a result tree.

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.

If a transformation has successfully produced a 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. As with other aspects of serialization, the handling of serialization errors is implementation-defined: see [18 Serialization].

2.1 XSLT Namespace

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.

NOTE: The 1999 in the URI indicates the year in which the URI was allocated by the W3C. It does not indicate the version of XSLT being used, which is specified by attributes (see [2.4 Stylesheet Element] and [2.5 Simplified Stylesheet Modules]).

XSLT processors must use the XML namespaces mechanism [XML Names] 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 [C 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 [16.2 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.

An element from the XSLT namespace may have any attribute not from the XSLT namespace, provided that the expanded-QName (see [XPath 2.0]) of the attribute has a non-null namespace URI. The presence of such attributes must not change the behavior of XSLT elements and functions defined in this document or in the XPath specification, though they may be used to modify the behavior of extension functions and extension instructions. Thus, an implementation is always free to ignore such attributes, and must ignore such attributes without giving an error if it does not recognize the namespace URI. Such attributes can provide, for example, unique identifiers, optimization hints, or documentation. The set of namespaces that are recognized for such attributes is implementation-defined.

For example, the following code might be used to provide a hint to a particular implementation that a call to an extension function has side effects:

<xsl:value-of select="abc:set-property('someprop', 3)" 
    abc:side-effects="yes"
    xmlns:abc="http://some.vendor.com/xslt/extensions"/>

[ERR002] It is a static error for an element from the XSLT namespace to have an attribute with an expanded-QName that has a null namespace URI (i.e. an attribute with an unprefixed name) other than attributes defined for the element in this document.

NOTE: The conventions used for the names of XSLT elements, attributes and functions are that names are all lower-case, use hyphens to separate words, and use abbreviations only if they already appear in the syntax of a related language such as XML or HTML.

2.2 XSLT Media Type

The MIME media types text/xml and application/xml [RFC2376] should be used for XSLT stylesheets. It is possible that a media type will be registered specifically for XSLT stylesheets; if and when it is, that media type may also be used.

2.3 Standard Attributes

There are a number of standard attributes that may appear on any XSLT element: specifically version, exclude-result-prefixes, extension-element-prefixes, and default-xpath-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:version, xsl:exclude-result-prefixes, xsl:extension-element-prefixes, or xsl:default-xpath-namespace.

It is recommended that these attributes should also be permitted on extension instructions, but this is at the discretion of the implementor 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, and any descendant elements of the element they appear on, together with attributes of those descendant elements. The two forms with and without the XSLT namespace have the same effect; the XSLT namespace is used for the attribute if and only if its parent element is not in the XSLT namespace.

In the case of [xsl:]version and [xsl:]default-xpath-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 containing element on which the attribute appears.

In the case of [xsl:]exclude-result-prefixes and [xsl:]extension-element-prefixes the values are cumulative. For these attributes, the value is a whitespace-separated list of namespace prefixes, and the effective value for an element is the combined set of 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.

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 description of the xsl:stylesheet and xsl:transform elements only. This reflects the fact that these attributes are often used on the xsl:stylesheet element, in which case they apply to the entire stylesheet module.

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

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

• [xsl:]version: see [2.6 Backwards-Compatible Processing] and [2.7 Forwards-Compatible Processing]
• [xsl:]default-xpath-namespace: see [4.4 Unprefixed Names in Expressions and Patterns].
• [xsl:]exclude-result-prefixes: see [8.1.2 Namespace Nodes for Literal Result Elements].
• [xsl:]extension-element-prefixes: see [16.2 Extension Instructions].

2.4 Stylesheet Element

<xsl:stylesheet
  id = id
  extension-element-prefixes = tokens
  exclude-result-prefixes = tokens
  version = number
  default-xpath-namespace = uri>
  <!-- Content: (xsl:import*, top-level-elements) -->
</xsl:stylesheet>

<xsl:transform
  id = id
  extension-element-prefixes = tokens
  exclude-result-prefixes = tokens
  version = number
  default-xpath-namespace = uri>
  <!-- Content: (xsl:import*, top-level-elements) -->
</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.

[ERR003] An xsl:stylesheet element must have a version attribute, indicating the version of XSLT that the stylesheet requires. [ERR004] The value of the version attribute must be a number (specifically, it must be a DecimalLiteral as defined in [XPath 2.0].) For this version of XSLT, the value should normally be 2.0. When the value is less than 2.0, backwards-compatible processing behavior is enabled (see [2.6 Backwards-Compatible Processing]). When the value is greater than 2.0, forwards-compatible behavior is enabled (see [2.7 Forwards-Compatible Processing]).

[ERR005] An xsl:stylesheet element must have no text node children, other than text nodes consisting entirely of whitespace.

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

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 [2.4.1 User-defined Data Elements])

The xsl:stylesheet element may contain the following types of declaration:

• xsl:import
• xsl:include
• xsl:attribute-set
• xsl:decimal-format
• xsl:function
• xsl:import-schema
• xsl:key
• xsl:namespace-alias
• xsl:output
• xsl:param
• xsl:preserve-space
• xsl:principal-result-document
• xsl:sort-key
• xsl:strip-space
• xsl:template
• xsl:variable

The order in which the children of the xsl:stylesheet element occur is not significant except for xsl:import elements and for error recovery. Users are free to order the elements as they prefer, and stylesheet creation tools need not provide control over the order in which the elements occur.

2.5 Simplified Stylesheet Modules

A simplified syntax is allowed for a stylesheet module that consists of only a single template rule for the document node. The stylesheet module may consist of just a literal result element (see [8.1 Literal Result Elements]). Such a stylesheet is equivalent to a standard stylesheet module whose xsl:stylesheet element contains a template rule containing the literal result element; the template rule has a match pattern of /. For example:

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

has the same meaning as

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

More formally, a simplified stylesheet module is equivalent to the standard stylesheet module that would be generated by applying the following transformation to the simplified stylesheet module:

<xsl:stylesheet version="2.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:template match="/*">
  <xsl:element name="xsl:stylesheet">
    <xsl:attribute name="version">
      <xsl:value-of select="@xsl:version"/>
    </xsl:attribute>
    <xsl:element name="xsl:template">
      <xsl:attribute name="match">/</xsl:attribute>
      <xsl:copy-of select="."/>
    </xsl:element>
  </xsl:element>
</xsl:template>  

</xsl:stylesheet>

If the outermost element of the simplified stylesheet is not the document element, then in the above transformation the attribute match="/*" must be replaced by a pattern than matches this outermost element. Note that if the element has an ID attribute to enable it to be selected (for example in the fragment identifier of a URI reference), then this ID attribute will be copied to the result document in the same way as other attributes of a literal result element.

[ERR008] 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 should normally be 2.0; the value must be a DecimalLiteral as defined in [XPath 2.0].

Other literal result elements may also have an xsl:version attribute. When the xsl:version attribute is numerically less than 2.0, backwards-compatible processing behavior is enabled (see [2.6 Backwards-Compatible Processing]). When the xsl:version attribute is numerically greater than 2.0, forwards-compatible behavior is enabled (see [2.7 Forwards-Compatible Processing]).

The allowed content of a literal result element when used as a simplified stylesheet is the same as when it occurs within a content constructor. Thus, a literal result element used as the document element of a simplified stylesheet cannot contain declarations.

2.6 Backwards-Compatible Processing

An element enables backwards-compatible behavior for itself, its attributes, its descendants and their attributes if either it has an [xsl:]version attribute (see [2.3 Standard Attributes]) whose value is less than 2.0. An element that has an [xsl:]version attribute whose value is greater than or equal to 2.0 disables backwards-compatible behavior for itself, its attributes, its descendants and their attributes. The compatibility behavior established by an element overrides any compatibility behavior established by an ancestor element.

If an attribute containing an XPath expression is processed with backwards-compatible behavior, then:

• It is constrained to use syntax permitted by XPath 1.0

• It is guaranteed to return the same result as would be returned by XPath 1.0, after conversion of any variables that it references to the equivalent XPath 1.0 data type. This conversion is done as follows. Any numeric value is converted to the nearest XPath 1.0 number. Boolean values remain as booleans; any other simple value is converted to a string. [ERR009] If the value is an empty sequence or a sequence that consists entirely of nodes, then it is converted to a node-set; it is a dynamic error if the value is any other sequence of two or more items. The processor must signal the error. The result of the expression is converted to an XPath 2.0 value by representing any node-set as a sequence of nodes in document order.

It is implementation-defined whether a particular XSLT 2.0 implementation supports backwards-compatible behavior. [ERR010] If an implementation does not support backwards-compatible behavior, then it is a dynamic error if any element is evaluated that enables backwards-compatible behavior. The processor must signal the error.

2.7 Forwards-Compatible Processing

An element enables forwards-compatible behavior for itself, its attributes, its descendants and their attributes if it has an [xsl:]version attribute (see [2.3 Standard Attributes]) whose value is greater than 2.0. An element that has an [xsl:]version attribute whose value is less than or equal to 2.0 disables forwards-compatible behavior for itself, its attributes, its descendants and their attributes. The compatibility behavior established by an element overrides any compatibility behavior established by an ancestor element.

Within a section of a stylesheet where forwards-compatible behavior is enabled, errors that would normally be static errors are treated instead as dynamic errors. This means that no error is reported unless the construct containing the error is actually evaluated.

This means, for example, that when an element is processed with forwards-compatible behavior:

• if it is a top-level element and XSLT 2.0 does not allow such elements as top-level elements, then the element must be ignored along with its content;

• if it is an element in a content constructor and XSLT 2.0 does not allow such elements to occur in content constructors, then if the element is not evaluated, no error must be signaled, and if the element is evaluated, the processor must perform fallback for the element as specified in [16.2.3 Fallback];

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

• if an attribute of the element contains an XPath expression that does not match the allowed syntax of an XPath 2.0 expression, or one that calls a function with an unprefixed name that is not defined in XPath 2.0 or XSLT 2.0, or that calls such a function with the wrong number or type of arguments, the error must not be signaled unless the expression is actually evaluated.

Thus, any XSLT 2.0 processor must be able to process the following stylesheet without error, although the stylesheet includes elements from the XSLT namespace that are not defined in this specification:

<xsl:stylesheet version="17.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="/">
    <xsl:choose>
      <xsl:when test="system-property('xsl:version') >= 17.0">
        <xsl:exciting-new-17.0-feature/>
      </xsl:when>
      <xsl:otherwise>
        <html>
        <head>
          <title>XSLT 17.0 required</title>
        </head>
        <body>
          <p>Sorry, this stylesheet requires XSLT 17.0.</p>
        </body>
        </html>
      </xsl:otherwise>
    </xsl:choose>
  </xsl:template>
</xsl:stylesheet>

NOTE: If a stylesheet depends crucially on a declaration introduced by a version of XSLT after 2.0, then the stylesheet can use an xsl:message element with terminate="yes" (see [15 Messages]) to ensure that implementations that conform to an earlier version of XSLT will not silently ignore the declaration. For example,

<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="system-property('xsl:version') < 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>

2.8 Combining Stylesheet Modules

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

• an inclusion mechanism that allows stylesheet modules to be combined without changing the semantics of the modules being combined, and
• an import mechanism that allows stylesheet modules to override each other.

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

2.9 Embedded Stylesheet Modules

A standard stylesheet module is a complete XML document with the xsl:stylesheet element as its document element. However, a stylesheet module may also be embedded in another resource. Two forms of embedding are possible:

• the XSLT stylesheet may be textually embedded in a non-XML resource, or
• the xsl:stylesheet element may occur in an XML document other than as the document element.

To facilitate the second form of embedding, the xsl:stylesheet element is allowed to have an ID attribute that specifies a unique identifier.

NOTE: In order for such an attribute to be used with the XPath id function, it must actually be declared in the DTD as being of type ID. The same requirement typically applies if the identifier is to be used as a fragment identifier in a URI reference.

The following example shows how the xml-stylesheet processing instruction (see [XML Stylesheet]) can be used to allow a source document to contain its own stylesheet. The URI reference uses a relative URI with a fragment identifier to locate the xsl:stylesheet element:

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

NOTE: A stylesheet module that is embedded in the document to which it is to be applied or that may be included or imported into a stylesheet that is so embedded typically needs to contain a template rule that specifies that xsl:stylesheet elements are to be ignored.

NOTE: The above example uses the pseudo-attribute type="text/xml" in the xml-stylesheet processing instruction to denote an XSLT stylesheet. This usage was defined provisionally in XSLT 1.0, and is subject to change. In the absence of a registered media type for XSLT stylesheets, some vendors' products have adopted different conventions, notably type="text/xsl".

NOTE: Support for the xml-stylesheet processing instruction is not a requirement for conformance with this Recommendation.

Issue (schema-explanation): We need to say something here about schemas and DTDs.

3.1 Parentless Nodes

The data model defined in [Data Model] allows a node to be part of a tree whose root is a node other than a document node.

XSLT does not allow nodes with no parent (other than document nodes) to be created, other than transiently: a node created during the course of XSLT processing is not available for further processing until it has been attached to a tree that is rooted at a document node. However, implementations may allow parentless nodes to be supplied as input to the transformation, for example as parameters to the stylesheet.

3.2 Document Node Children

Ed. Note: This section can be removed when it is confirmed that the data model permits "well-balanced" trees. At the time of writing, this is still an open issue in the data model (Issue 0041).

The normal restrictions on the children of the document node are relaxed for the result tree and for temporary trees constructed during the evaluation of the stylesheet. The document node of such a tree may have any sequence of nodes as children that would be possible for an element node. In particular, it may have text node children, and any number of element node children. When written out using the XML output method (see [18 Serialization]), it is possible that a result tree will not be a well-formed XML document; however, it will always be a well-formed external general parsed entity.

For example, a stylesheet might produce the following output. This is a well-formed external general parsed entity, but it is not a well-formed XML document:

<?xml version="1.0" encoding="iso-8859-1"?>A <i>fine</i> mess!

When a source tree is created by parsing a well-formed XML document, the document node of the source tree will automatically satisfy the normal restrictions of having no text node children and exactly one element child. When a source tree is created in some other way, for example by using the DOM, the usual restrictions are relaxed for the source tree as for the result tree.

3.3 Unparsed Entities

Ed. Note: Unparsed entities don't currently appear in the data model, though we have asked for them to be added. This section can be deleted when the Data Model is updated to support unparsed entities.

The document node has a mapping that gives the URI for each unparsed entity declared in the document's DTD. The URI is generated from the system identifier and public identifier specified in the entity declaration. The processor may use the public identifier to generate a URI for the entity instead of the URI specified in the system identifier. If the processor does not use the public identifier to generate the URI, it must use the system identifier; if the system identifier is a relative URI, it must be resolved into an absolute URI using the URI of the resource containing the entity declaration as the base URI [RFC2396].

3.4 Whitespace Stripping

The source document supplied as input to the transformation process may contain whitespace nodes (that is, text nodes consisting solely of whitespace characters) that are of no interest, and that do not need to be retained by the transformation. Conceptually, such whitespace nodes may be removed from the tree before the transformation commences. This process is referred to as whitespace stripping. The source tree itself must not be modified: the processor may implement whitespace stripping either by creating a copy of the tree from which the whitespace nodes have been removed, or by working on a virtual tree in which the whitespace nodes are treated as if they were absent.

The stripping process takes as input a set of element names whose child whitespace nodes must be preserved. The stripping process is applied to both stylesheets and source documents, but the set of whitespace-preserving element names is determined differently for stylesheets and for source documents.

NOTE: Where multiple transformations are to be applied to the same source document, a useful optimization is to do the whitespace stripping only once. Implementations may therefore allow whitespace stripping to be controlled as a separate operation from the rest of the transformation process.

A text node is preserved if any of the following apply:

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

• The text node contains at least one non-whitespace character. As in XML, a whitespace character is #x20, #x9, #xD or #xA.

  Issue (NEL-char): Should we make provision for XML 1.1 and its introduction of NEL as a whitespace character?

• 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 text node is stripped.

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

NOTE: This implies that if an xml:space attribute is specified on a literal result element, it will be included in the result.

For stylesheets, the set of whitespace-preserving element names consists of just xsl:text.

Processing instructions and comments in a stylesheet module are ignored: the stylesheet module is treated as if the processing instructions and comments were not there. This also means that sibling text nodes that are separated by a processing instruction or comment in a stylesheet module are concatenated into a single text node; and a text node is classified as a whitespace text node for the purpose of whitespace stripping only after this concatenation has taken place.

The content model for some XSLT elements (for example xsl:stylesheet and xsl:choose) does not permit text nodes as children of these elements. If the xml:space="preserve" attribute is used to suppress the stripping of whitespace text nodes within such elements, then any whitespace used for the layout of such elements will be retained in the stylesheet tree in the form of whitespace text nodes. Such text nodes must not be reported as an error. [ERR018] Within an XSLT element that is required to be empty, any content other than comments or processing instructions, including any whitespace-only text node preserved using the xml:space="preserve" attribute, is a static error.

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

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

For source documents, 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 amongst 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; an element name matches an xsl:strip-space or xsl:preserve-space element if it matches one of the NameTests. An element matches a NameTest if and only if the NameTest would be true for the element as an XPath node test. When more than one xsl:strip-space and xsl:preserve-space element matches, the best matching element is determined by the best matching NameTest. This is determined in the same way as with template rules:

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

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

[ERR019] It is an dynamic error if this leaves more than one match. The processor must either signal the error, of must recover by choosing, from amongst the matches that are left, the one that occurs last in declaration order.

NOTE: A source document is supplied as input to the XSLT processor in the form of a tree. Nothing in this specification states that this tree must be built by parsing an XML document; nor does it state that the application that constructs the tree is required to treat whitespace in any particular way. The provisions in this section relate only to whitespace text nodes that are present in the tree supplied as input to the processor. In particular, the processor cannot preserve whitespace text nodes unless they were actually present in the supplied tree.

3.5 Namespace Fixup

Ed. Note: The process of namespace fixup would ideally be described along with the node construction functions defined in the XPath 2.0 data model.

In a tree constructed by parsing an XML document, the following constraints relating to namespace nodes will be satisfied:

• If an element node has an expanded-QName with a non-null namespace URI, then that element node will have at least one namespace node whose string-value is the same as that namespace URI.

• If an attribute node has an expanded-QName with a non-null namespace URI, then the parent element of that attribute will have at least one namespace node whose string-value is the same as that namespace URI and whose expanded-QName has a non-empty local part.

• If an element node has a namespace node with an expanded-QName with a non-empty local part, then every child element of that element will also have a namespace node with that expanded-QName (possibly with a different string-value).

• Every element has a namespace node whose expanded-QName has local-part xml and whose string-value is http://www.w3.org/XML/1998/namespace.

However, when a tree is being constructed as the result of an XSLT transformation, these constraints might not be satisfied unless special action is taken. In particular, since xsl:element and xsl:attribute instructions do not create namespace nodes, they will often cause these constraints not to be satisfied. The process of namespace fixup modifies a tree by adding namespace nodes so that it satisfies all constraints affecting namespace nodes. What namespace nodes are added and where they are added by namespace fixup is implementation-dependent, provided that the resulting tree satisfies the constraints and provided that all namespaces nodes in the resulting tree are allowable, where a namespace node is allowable for an element E if any of the following conditions applies:

• The namespace node was in the tree before namespace fixup.

• The local-part of the expanded-QName of the namespace node is xmland its string-value is http://www.w3.org/XML/1998/namespace.

• The namespace node has a string-value equal to the namespace URI of the expanded-QName of element E.

• The namespace node has a string-value equal to the namespace URI of the expanded-QName of an attribute of element E; this applies only if the local part of the expanded-QName of the namespace node is non-empty.

• Element E has a parent element with a namespace node that is allowable and that has the same expanded-QName and same string-value as the other namespace node; this applies only if the local part of the expanded-QName of the namespace node is non-empty.

Namespace fixup must not result in an element having multiple namespace nodes with the same expanded-QName.

Namespace fixup is performed in two situations:

• It is applied to a result tree, before the result tree is made available to the calling application (whether by serialization or otherwise: see [18 Serialization]).

• It is applied to a temporary tree, before the temporary tree is made available for processing by stylesheet instructions. (see [6.1 Values of Variables and Parameters]).

There is no requirement to perform namespace fixup for the principal source document, nor for any document loaded using the document function, nor for any document supplied as the value of a global parameter, nor for any document returned by an extension function. [ERR020] It is a dynamic error if such a document does not already satisfy the constraints listed above . The processor may signal the error, or may recover by performing namespace fixup, or may produce implementation-dependent results.

3.6 Disable Output Escaping

If an implementation supports the disable-output-escaping attribute of xsl:text and xsl:value-of (see [18.5 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.

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

This property is preserved when a text node is copied using xsl:copy or xsl:copy-of.

NOTE: There are many ways an implementation can avoid the overhead of actually storing a boolean flag with every character.

4.1 Qualified Names

The name of an internal XSLT object, specifically a named template (see [7.1 Named Templates]), a mode (see [5.6 Modes]), an attribute set (see [7.2 Named Attribute Sets]), a key (see [14.3 Keys]), a named sort specification (see [12.4 Using Named Sort Specifications]), a decimal-format (see [14.4 Number Formatting]), a variable or parameter (see [6 Variables and Parameters]), a stylesheet function (see [7.3 Stylesheet Functions]), or a named output definition (see [18 Serialization]), is specified as a QName.

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

An expanded-QName is a pair of values containing a namespace URI and a local name. A QName is expanded by replacing the namespace prefix with the corresponding namespace URI, from the namespace declarations that are in scope at the point where the QName is written. Two expanded-QNames are equal if the namespace URIs are the same and the local names are the same.

QNames always occur either as the value of an attribute node in a stylesheet module, or within an XPath expression contained in such an attribute node, or as the result of evaluating an XPath expression contained in such an attribute node. The element containing this attribute node is referred to as the defining element of the QName.

If the QName has a prefix, then the prefix is expanded into a URI reference using the namespace declarations in effect on its defining element. The expanded-QName consisting of the local part of the name and the possibly null URI reference is used as the name of the object. The default namespace (as defined by a namespace declaration of the form xmlns="some.uri") is not used for unprefixed names.

In the case of an unprefixed QName used as a NameTest within an XPath expression (see [4.2 Expressions]) or within a pattern (see [4.3 Patterns]), or in the elements attribute of the xsl:strip-space and xsl:preserve-space instructions, the namespace to be used in expanding the QName may be specified by means of the [xsl:]default-xpath-namespace attribute, as specified in [4.4 Unprefixed Names in Expressions and Patterns].

[ERR021] In the case of a QName used as the value of an attribute in the stylesheet, or appearing within the text of an XPath expression in the the stylesheet, it is a static error if the defining element has no namespace node whose name matches the prefix of the QName.

[ERR022] In the case of a QName produced by evaluating an XPath expression, it is a dynamic error if the defining element has no namespace node whose name matches the prefix of the QName. The error is a dynamic error even if the value of the expression is known statically, for example if the QName is written as a string literal. The required action depends on the defining element.

4.2 Expressions

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

• selecting nodes for processing;
• specifying conditions for different ways of processing a node;
• generating text to be inserted in the result tree.

An expression must match the XPath production Expr.

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

[ERR023] It is a static error if the value of such an attribute, or the text between curly braces in an attribute value template, does not match the XPath production Expr, 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.

The context within a stylesheet where an XPath expression may appear determines the required type of the expression. The required type indicates the data type of value that the expression is expected to return.

[ERR024] It is a type error if an XPath expression contains a type error, or if the type of the XPath expression is incompatible with the required type. The processor must either signal a type error as a static error, or must attempt to recover by converting the result of the expression to the required type using the standard type conversion rules; if conversion is not possible under these rules, the processor must signal a dynamic error

The context for evaluation of an XPath expression is determined according to the following rules. The context has two parts: the static context, and the dynamic expression evaluation context.

The static context depends on the element in the stylesheet that contains the attribute holding the XPath expression ("the containing element") as follows:

• The type exception policy is, by default, flexible, meaning that the system attempts to convert supplied values to the required type when possible (using the fallback conversion rules defined in [XPath 2.0]). An implementation may provide the alternative policy, strict, as a user-selectable option.

• The in-scope namespaces are the namespace declarations that are in scope for the containing element.

• The default namespace for element names is the namespace defined by the innermost [xsl:]default-xpath-namespace attribute, as described in [4.4 Unprefixed Names in Expressions and Patterns]. The value of this attribute is a namespace URI.

• The default namespace for function names is the namespace http://www.w3.org/2001/12/xquery-operators, defined in [Functions and Operators]. This means that it is not necessary to declare this namespace in the stylesheet, nor to use the prefix xf used in the specification of the core functions.

  Ed. Note: The current draft of the Functions & Operators document includes an issue suggesting this namespace may change.

• The in-scope type definitions includes the built-in types of XML Schema (see [XML Schema]), plus any types imported using the xsl:import-schema declaration.

• The in-scope variables are the variable-bindings that are in scope for the containing element (see [6 Variables and Parameters]).

• The in-scope functions are the core functions defined by XPath, the additional functions defined in this specification, the stylesheet functions defined in the stylesheet, plus any extension functions bound using implementation-defined mechanisms (see [16 Extensibility and Fallback]). [ERR025] It is a dynamic error for an expression to call any function that is not included in the in-scope functions. The processor must signal the error, but only if the function call is actually evaluated.

• The in-scope collations are implementation-defined.

• The default collation is implementation-defined.

  Issue (default-collation): Should there be facilities in XSLT to select the default collation? If so, how should this be scoped?

• The base URI is the base URI of the containing element.

The evaluation context, which includes the focus, is determined as follows:

• The variable values are the current values of the in-scope variables.
• The set of accessible documents is determined by the implementation.

Where the containing element is an instruction or a literal result element, the focus is established as follows. In other cases, the rules are given for the specific containing element.

• The initial context document, context item, context position, and context size for the XPath expression are the same as the context document, context item, context position, and context size for the evaluation of the containing instruction or literal result element. (Note that these values may change for evaluation of sub-expressions within the XPath expression, according to the XPath rules.)

4.3 Patterns

A template rule identifies the nodes to which it applies by means of a pattern. As well as being used in template rules, patterns are used for numbering (see [9 Numbering]), for grouping (see [13 Grouping]), and for declaring keys (see [14.3 Keys]).

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

Here are some examples of patterns:

• para matches any para element

• * matches any element

• chapter|appendix matches any chapter element and any appendix element

• olist/item matches any item element with an olist parent

• appendix//para matches any para element with an appendix ancestor element

• / matches the document node of any source document

• text() matches any text node

• node() matches any node other than an attribute node, namespace node, or document node

• id("W11") matches the element with unique ID W11

• para[1] matches any para element that is the first para child element of its parent

• item[position() mod 2 = 1] matches any item element that is an odd-numbered item child of its parent.

• div[@class="appendix"]//p matches any p element with a div ancestor element that has a class attribute with value appendix

• @class matches any class attribute (not any element that has a class attribute)

• @* matches any attribute node

[ERR026] Where an attribute is defined to contain a pattern, it is a static error if the pattern does not match the production Pattern. Every pattern is a legal XPath expression, but the converse is not true: 2+2 is an example of a legal XPath expression that is not a pattern. The XPath expressions that can be used as patterns are those that match the grammar for Pattern, given below.

Informally, a Pattern is a set of path expressions separated by |, where each step in the path expression is constrained to be an AxisStepExpr that uses only the child or attribute axes. Patterns may also use the // operator, and they may start with an id or key function call provided its arguments are string literals. Predicates in a pattern (the construct enclosed between square brackets) can contain arbitrary XPath expressions in the same way as predicates in a path expression.

If a pattern occurs in part of the stylesheet where backwards compatible behavior is enabled (see [2.6 Backwards-Compatible Processing]), then the pattern is restricted to use the syntax for patterns defined in XSLT 1.0, and will match a node if and only if it would have matched that node under the rules defined in XSLT 1.0.

Patterns

The constructs NodeTest, StringLiteral, and Expr are part of the XPath expression language, and are defined in [XPath 2.0].

The meaning of a pattern is defined formally as follows. To determine whether a node N matches a pattern PAT, evaluate the expression //(PAT) with a singleton focus based on N. If the result is a sequence of nodes that includes N, then node N matches the pattern; otherwise node N does not match the pattern. This expression is constructed by textually inserting the pattern PAT exactly as written in the stylesheet.

For example, p matches any p element, because a p element will always be present in the result of evaluating the expression //(p). Similarly, / matches a document node, and only a document node, because the result of the expression //(/) when applied using a particular document as context document returns only the document node of that document.

NOTE: Although the semantics of patterns are specified formally in terms of expression evaluation, it is possible to understand pattern matching using a different model. In a pattern, | indicates alternatives; a pattern with one or more | separated alternatives matches if any one of the alternatives matches. A pattern such as book/chapter/section can be examined from right to left. A node will only match this pattern if it is a section element; and then, only if its parent is a chapter; and then, only if the parent of that chapter is a book. When the pattern uses the // operator, one can still read it from right to left, but this time testing the ancestors of a node rather than its parent. For example appendix//section matches every section element that has an ancestor appendix element. The formal definition, however, is useful for understanding the meaning of a pattern such as item[1]. This matches any node selected by the expression //(item[1]): that is, any item element that is the first item child of its parent.

The pattern node() matches all nodes selected by the expression //(node()), that is, all element, text, comment, and processing instruction nodes. It does not match attribute or namespace nodes because the expression does not select nodes using the attribute or namespace axes.

NOTE: An implementation, of course, may use any algorithm it wishes for evaluating patterns, so long as the result corresponds with the formal definition above. An implementation that followed the formal semantics by evaluating the equivalent expression and then testing the membership of a specific node in the result would probably be very inefficient.

4.4 Unprefixed Names in Expressions and Patterns

The attribute [xsl:]default-xpath-namespace (see [2.3 Standard Attributes]) may be used on an element in the stylesheet to define the namespace URI that will be used for an unprefixed name used as a NameTest within a step of an XPath PathExpression or an XSLT Pattern or in the elements attribute of the xsl:strip-space or xsl:preserve-space instructions, where the NameTest occurs in an attribute of that stylesheet element or an attribute of a descendant of that stylesheet element. The value of the attribute is the namespace URI to be used.

This default namespace URI applies only to a NameTest applied to an axis whose principal node type is elements: it does not apply when the step is using the attribute or namespace axis. The default namespace URI for such a name is the value of the [xsl:]default-xpath-namespace attribute on the innermost ancestor element that has such an attribute, considering all ancestor elements of the attribute containing the XPath expression or XSLT pattern. The [xsl:]default-xpath-namespace attribute must be in the XSLT namespace only if its parent element is not in the XSLT namespace.

In the absence of this attribute, an unqualified NameTest (that is, a NameTest that is an NCName) matches an expanded-QName whose namespace URI is null: the default namespace (as defined by an xmlns="some-uri" declaration) is not used.

The default-xpath-namespace only affects unqualified names (names containing no colon) used in a NameTest. It does not affect other names, for example function names, variable names, or names used as arguments to the key or system-property functions.

Ed. Note: Do we need to add this attribute to all element proformas and to the DTD?

4.5 Attribute Value Templates

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

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

NOTE: An expression within a variable part may contain an unescaped curly brace within a StringLiteral.

[ERR027] It is a static error if a left curly brace appears in an attribute value template without a matching right curly brace.

[ERR028] It is a static error if the string contained between matching curly braces in an attribute value template does not match the XPath production Expr.

[ERR029] It is a static error if a right curly brace occurs in an attribute value template outside an expression without being followed by a second right curly brace. A right curly brace inside a StringLiteral in an expression is not recognized as terminating the expression.

The required type of each expression within an attribute value template is xsd:string.

The result of evaluating an attribute value template is referred to as the effective value of the attribute. The effective value is the string obtained by concatenating the expansions of the fixed and variable parts. The expansion of a fixed part is obtained by replacing any double curly braces ({{ or }}) by the corresponding single curly brace. The expansion of a variable part is obtained by evaluating the enclosed XPath expression and converting the resulting value to a string as if by a call to the string function.

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

NOTE: Not all attributes are interpreted as attribute value templates. Attributes whose value is an expression or pattern, attributes of top-level elements and attributes that refer to named XSLT objects are not interpreted as attribute value templates. In addition, xmlns attributes are not interpreted as attribute value templates; it would not be conformant with the XML Namespaces Recommendation to do this. Two exceptions are the xsl:output and xsl:principal-result-document declarations: although these are top-level elements, some of their attributes are interpreted as attribute value templates.

The following example creates an img result element from a photograph element in the source; the value of the src attribute of the img element is computed from the value of the image-dir variable and the string-value of the href child of the photograph element; the value of the width attribute of the img element is computed from the value of the width attribute of the size child of the photograph element:

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

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

With this source

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

the result would be

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

Curly braces are not recognized recursively inside expressions. For example:

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

is not allowed. Instead, use simply:

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

4.6 Content Constructors

NOTE: The term content constructor replaces template as used in XSLT 1.0. The change is made partly for clarity (to avoid confusion with template rules and named templates), but also to reflect a more formal definition of the semantics. Whereas XSLT 1.0 described a template as a sequence of instructions that write to the result tree, XSLT 2.0 describes a content constructor as something that can be evaluated to return a sequence of nodes; what happens to these nodes depends on the containing instruction.

Many XSLT elements (including literal result elements) are defined to take as their content a content constructor.

A content constructor is a sequence of nodes in the stylesheet that, when evaluated, constructs and returns a sequence of new nodes suitable for adding to the result tree. This sequence is referred to below as the result sequence.

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

• Text nodes appearing in the stylesheet (if they have not been removed in the process of whitespace stripping: see [3.4 Whitespace Stripping]) are copied to create a new text node in the result sequence.

• Literal result elements are evaluated to create a new element node, having the same name as the literal result element, which is added to the result sequence: see [8.1 Literal Result Elements]

• XSLT instructions produce a sequence of zero, one, or more nodes as their result. These nodes are added to the result sequence. Some instructions, such as xsl:element, return a newly-constructed node (which may have its own attributes, namespaces, children, and other descendants); others, such as xsl:if, return nodes produced by their own nested content constructors.

• Extension instructions also produce a sequence of nodes as their result, which is added to the result sequence.

The node sequences produced by each node in the content constructor are concatenated to form a single result sequence. This concatenation retains the order of the nodes within the content constructor: if while evaluating a content constructor, node M is constructed by instruction I, and node N is constructed by a different instruction J, then N will appear after M in the result sequence if and only if J follows I in document order. This does not mean that the nodes in a content constructor must be evaluated sequentially: on the contrary, they may be evaluated in any order, or in parallel, provided that their results are assembled in the correct sequence on completion.

If the result sequence contains two or more adjacent text nodes, these adjacent text nodes are concatenated to form a single text node.

The result sequence will never contain a document node. [ERR030] It is an dynamic error if an extension instruction attempts to return a sequence containing a document node. The processor must signal the error.

[ERR031] It is a dynamic error if the result sequence (after concatenating the results of individual instructions) contains a namespace node that is preceded in the sequence by a node that is not a namespace node. The processor must either signal the error, or must recover by ignoring the offending namespace node.

[ERR032] It is a dynamic error if the result sequence (after concatenating the results of individual instructions) contains an attribute node that is preceded in the sequence by a node that is neither a namespace node nor an attribute node. The processor must either signal the error, or must recover by ignoring the offending attribute node.

What actually happens to the nodes in the result sequence depends on the element containing the content constructor. Some elements, such as xsl:if, simply return the sequence of nodes, which are added to the result of the content constructor containing the xsl:if instruction. Other elements, such as xsl:element, xsl:comment, and xsl:variable, construct a new node, and use the value returned by the content constructor to create the children or the text content of the new node.

Specifically:

• [ERR033] Elements such as xsl:variable, xsl:param, xsl:message, and xsl:result-document construct a new document node, and use the result sequence returned by the content constructor to form the children of this document node. In this case it is an dynamic error if the result sequence contains namespace or attribute nodes. The processor must either signal the error, or must recover by ignoring the offending nodes. The elements, comments, processing instructions, and text nodes in the node sequence form the children of the newly constructed document node.

• Elements such as xsl:element, xsl:copy, and literal result elements construct a new element node, and use the result sequence returned by the content constructor to form the children of this element node. The elements, comments, processing instructions, and text nodes in the result sequence form the children of the newly constructed document node.

  • Any namespace nodes in the result sequence are merged with any existing namespace nodes in the same document, for the same namespace prefix and URI, and are then added to the in-scope namespaces of the target element. [ERR034] It is a dynamic error to add a namespace node to an element if the element already has a namespace node with the same name, unless both namespace nodes have the same string-value, in which case the duplicate is ignored. It is also a dynamic error to add a namespace node to an element if the namespace node has a null name and the element has a null namespace URI. In both cases the processor must either signal the error, or must recover by ignoring the offending namespace node.

  • Any attribute nodes in the result sequence are added to the target element. If the target element already has an attribute with the same expanded-QName, the new attribute overwrites the existing attribute. If two or more attributes in the result sequence have the same expanded-QName, the one that appears latest in the result sequence is the only one that is used.

• The xsl:comment, xsl:attribute, xsl:processing-instruction, xsl:text, and xsl:namespace elements create nodes that cannot have children. In these cases the result sequence produced by the content constructor is used to establish the string-value of the new node. [ERR035] It is a dynamic error if the result sequence contains nodes other than text nodes. The processor must either signal the error, or must recover by ignoring the non-text nodes together with their content.

5.1 Processing Model

The xsl:apply-templates instruction takes as input a sequence of nodes in the source tree, and produces as output a sequence of nodes which are typically added to the result tree. Each node in the input sequence is processed by finding a template rule whose pattern matches that node. If there is more than one, the best among them is chosen; if there is none, a built-in template rule is used. The content constructor that forms the body of the chosen template rule is evaluated to produce a sequence of nodes. The resulting sequences of nodes (one for each node in the input sequence) are then concatenated, to form a single sequence. They are concatenated retaining the order of the nodes in the original input sequence, unless a different order is requested using xsl:sort. The final concatenated sequence of nodes forms the result of the xsl:apply-templates instruction: what happens to it next depends on the context in which the xsl:apply-templates instruction appeared.

Ed. Note: Given that the above paragraph explains an important concept, it could be more clearly written.

The stylesheet as a whole is evaluated by evaluating an implicit xsl:apply-templates instruction with the document node (that is, the root) of the principal source document as the (singleton) input node sequence.

NOTE: An implementation may also allow processing to start with some different node sequence, using implementation-defined mechanisms.

A content constructor will often contain an xsl:apply-templates instruction selecting the children of the node as the input sequence for processing. This will cause a template rule to be invoked for each of these children. The nodes constructed by the content constructor for a child node can be added as children to the nodes constructed by the content constructor for their parent. Thus it is possible to construct a result tree whose structure corresponds to the structure of the source tree.

Implementations are free to process the source document in any way that produces the same result as if it were processed using this processing model.

5.2 Defining Template Rules

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

A template rule is specified with the xsl:template element. The match attribute is a Pattern that identifies the source node or nodes to which the rule applies. The match attribute is required unless the xsl:template element has a name attribute (see [7.1 Named Templates]). It is an static error for the value of the match attribute to contain a Variable. The result of applying the template rule is the result of evaluating the content constructor contained in the xsl:template element, with the matching node used as the context node

For example, an XML document might contain:

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

The following template rule matches emph elements and produces a fo:wrapper formatting object with a font-weight property of bold.

<xsl:template match="emph">
  <fo:wrapper font-weight="bold">
    <xsl:apply-templates/>
  </fo:wrapper>
</xsl:template>

NOTE: Examples in this document use the fo: prefix for the namespace http://www.w3.org/1999/XSL/Format, which is the namespace of the formatting objects defined in [XSL Formatting Objects].

As described next, the xsl:apply-templates element recursively processes the children of the source element.

5.3 Applying Template Rules

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

This example creates a block for a chapter element and then processes its immediate children.

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

In the absence of a select attribute, the xsl:apply-templates instruction processes all of the children of the context node, including text nodes. [ERR036] It is a dynamic error if the context item is not a node. The processor must either signal the error, or must recover by returning an empty sequence.

Whitespace text nodes that have been stripped as specified in [3.4 Whitespace Stripping] will not be processed. If stripping of whitespace nodes has not been enabled for an element, then all whitespace in the content of the element will be processed as text, and thus whitespace between child elements will count in determining the position of a child element as returned by the position function.

A select attribute can be used to process nodes selected by an expression instead of processing all children. The value of the select attribute is an expression. The expression must evaluate to a sequence (which can contain zero, one, or more nodes). The selected nodes are processed in the order of this sequence, unless a sorting specification is present (see [12 Sorting]).

[ERR037] It is a dynamic error if the sequence returned by the select expression contains an item that is not a node. The processor must either signal the error, or must recover by ignoring the offending items.

NOTE: In XSLT 1.0, the select attribute selected a set of nodes, which by default were processed in document order. In XSLT 2.0, it selects a sequence of nodes. In cases that would have been valid in XSLT 1.0, the expression will return a sequence of nodes in document order, so the effect is the same.

The following example processes all of the author children of the author-group:

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

The following example processes all of the given-names of the authors that are children of author-group:

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

This example processes all of the heading descendant elements of the book element.

<xsl:template match="book">
  <fo:block>
    <xsl:apply-templates select=".//heading"/>
  </fo:block>
</xsl:template>

It is also possible to process elements that are not descendants of the context node. This example assumes that a department element has group children and employee descendants. It finds an employee's department and then processes the group children of the department.

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

Multiple xsl:apply-templates elements can be used within a single template to do simple reordering. The following example creates two HTML tables. The first table is filled with domestic sales while the second table is filled with foreign sales.

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

NOTE: It is possible for there to be two matching descendants where one is a descendant of the other. This case is not treated specially: both descendants will be processed as usual. For example, given a source document

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

the rule

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

will process both the outer div and inner div elements.

NOTE: Typically, xsl:apply-templates is used to process only nodes that are descendants of the context node. Such use of xsl:apply-templates cannot result in non-terminating processing loops. However, when xsl:apply-templates is used to process elements that are not descendants of the context node, the possibility arises of non-terminating loops. For example,

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

Implementations may be able to detect such loops in some cases, but the possibility exists that a stylesheet may enter a non-terminating loop that an implementation is unable to detect. This may present a denial of service security risk.

5.4 Conflict Resolution for Template Rules

It is possible for a node in a source document to match more than one template rule. The template rule to be used is determined as follows:

1. First, all matching template rules that have lower import precedence than the matching template rule or rules with the highest import precedence are eliminated from consideration.

2. Next, all matching template rules that have lower priority than the matching template rule or rules with the highest priority are eliminated from consideration. The priority of a template rule is specified by the priority attribute on the template rule. [ERR038] The value of this must be a real number (positive or negative), matching the production NumericLiteral with an optional leading minus sign (-). [ERR039] If an xsl:template element does not have a match attribute, then it must not have a priority attribute.

   If no priority attribute is specified on the xsl:template element, the default priority is computed as follows:

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

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

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

   • Otherwise, if the pattern consists of just a NodeTest optionally preceded by a PatternAxis, then the priority is -0.5.

   • Otherwise, the priority is 0.5.

   NOTE: In many cases this means that highly-selective patterns have higher priority than less-selective patterns. The most common kind of pattern (a pattern that tests for a node with a particular type and a particular expanded-QName) has priority 0. The next less specific kind of pattern (a pattern that tests for a node with a particular type and an expanded-QName with a particular namespace URI) has priority -0.25. Patterns less specific than this (patterns that just tests for nodes with particular types) have priority -0.5. Patterns more specific than the most common kind of pattern have priority 0.5. However, it is not invariably true that a more selective pattern has higher priority than a less selective pattern. For example, the priority of the pattern node()[self::*] is higher than that of the pattern item. Therefore, to achieve clarity in a stylesheet it is good practice to allocate explicit priorities.

[ERR040] It is a dynamic error if this leaves more than one matching template rule. The processor must either signal the error, or must recover by choosing, from amongst the matching template rules that are left, the one that occurs last in declaration order.

5.5 Overriding Template Rules

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

A template rule that is being used to override a template rule in an imported stylesheet (see [5.4 Conflict Resolution for Template Rules]) can use the xsl:apply-imports element to invoke the overridden template rule.

At any point in the processing of a stylesheet, there may be a current template rule. Whenever a template rule is chosen by matching a pattern, the template rule becomes the current template rule for the evaluation of the rule's content constructor. When an xsl:for-each or xsl:for-each-group instruction is evaluated, the current template rule becomes null for the evaluation of the content of the xsl:for-each or xsl:for-each-group instruction.

The current template rule is not affected by invoking named templates (see [7.1 Named Templates]), named attribute sets (see [7.2 Named Attribute Sets]), nor stylesheet functions (see [7.3 Stylesheet Functions]). While evaluating a global variable or parameter (see [6.2 Global Variables and Parameters]) the current template rule is null.

xsl:apply-imports searches for a template rule that matches the context node, and whose mode is the same as the current template rule's mode (see [5.6 Modes]). In choosing a template rule, it uses the usual criteria such as the priority and import precedence of the template rules, but it considers as candidates only those template rules contained in stylesheet levels that are descendants in the import tree of the stylesheet level that contains the current template rule.

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

If no matching template rule is found that satisfies these criteria, the built-in template rule for the node type is used (see [5.7 Built-in Template Rules]).

[ERR041] It is an error if the xsl:apply-imports instruction is evaluated when the context item is not a node. The processor must either signal the error, or must recover by returning an empty sequence.

An xsl:apply-imports element may use xsl:with-param child elements to pass parameters to the chosen template rule (see [7.1.1 Passing Parameters to Templates]).

[ERR042] It is a dynamic error if xsl:apply-imports is evaluated when the current template rule is null. The processor must signal the error.

For example, suppose the stylesheet doc.xsl contains a template rule for example elements:

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

Another stylesheet could import doc.xsl and modify the treatment of example elements as follows:

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

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

The combined effect would be to transform an example into an element of the form:

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

5.6 Modes

Modes allow a node in the source tree to be processed multiple times, each time producing a different result. They also allow different sets of template rules to be active when processing different trees, for example when processing documents loaded using the document function (see [14.1 Multiple Source Documents]) or when processing temporary trees (see [6 Variables and Parameters])

Both xsl:template and xsl:apply-templates have an optional mode attribute. The value of the mode attribute is a QName, which is expanded as described in [4.1 Qualified Names]. If an xsl:apply-templates element has a mode attribute, then it applies only to those template rules from xsl:template elements that have a mode attribute with the same value; if an xsl:apply-templates element does not have a mode attribute, then it applies only to those template rules from xsl:template elements that do not have a mode attribute.

[ERR043] If an xsl:template element does not have a match attribute, then it is a static error if it has a mode attribute.

5.7 Built-in Template Rules

There is a built-in template rule to allow recursive processing to continue in the absence of a successful pattern match by an explicit template rule in the stylesheet. This template rule applies to both element nodes and the document node. The following shows the equivalent of the built-in template rule:

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

There is also a built-in template rule for each mode, which allows recursive processing to continue in the same mode in the absence of a successful pattern match by an explicit template rule in the stylesheet. This template rule applies to both element nodes and the document node. The following shows the equivalent of the built-in template rule for mode m.

<xsl:template match="*|/" mode="m">
  <xsl:apply-templates mode="m"/>
</xsl:template>

The built-in rule for document nodes and element nodes is equivalent to calling xsl:apply-templates with no select attribute. If the built-in rule was invoked in a non-default mode, that mode is used in the implicit xsl:apply-templates instruction. If the built-in rule was invoked with parameters, those parameters are again used in the implicit xsl:apply-templates instruction.

For example, suppose the stylesheet contains the following instruction:

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

If there is no explicit template rule that matches the title element, then the following implicit rule is used:

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

There is also a built-in template rule for text and attribute nodes that copies text through:

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

The built-in template rule for processing instructions and comments is to do nothing.

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

The built-in template rule for namespace nodes is also to do nothing. There is no pattern that can match a namespace node; so, the built-in template rule is the only template rule that is applied for namespace nodes.

The built-in template rules for text, attribute, comment, processing-instruction, and namespace nodes for a specific mode are the same as the rules shown above that apply where no mode is specified.

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

Issue (variable-type-semantics): We need to say more about the permitted values of the type attribute, and their meaning, once the XPath rules are clearer. For example, are all the permitted names of types known statically, and if so, where do these names come from?

Issue (variable-type-conversion): Should the type attribute on xsl:variable cause the supplied value to be converted to the required type, or should it cause a dynamic error to be signaled if the supplied value does not conform to the required type: perhaps with conversion as a recovery action?

Issue (variable-validation): Is "validate" invoked automatically when xsl:variable is used to create a temporary tree, and a type is specified? How does this work, given that the type will be a document type, not an element type? What happens if validation fails (and what does it mean for validation to fail)? Should it be possible to request lax validation?

6.1 Values of Variables and Parameters

A variable-binding element can specify the supplied value of the variable in three alternative ways.

• [ERR045] If the variable-binding element has a select attribute, then the value of the attribute must be an expression and the supplied value of the variable is the value that results from evaluating the expression. In this case, the content of the variable-binding element must be empty.

• If a variable-binding element has no select attribute and has non-empty content (i.e. the variable-binding element has one or more child nodes), then the content of the variable-binding element specifies the supplied value. The content of the variable-binding element is a content constructor; a new document (referred to as a temporary tree) is constructed with a document node having as its children the sequence of nodes that results from evaluating the content constructor. Namespace fixup is performed on the temporary tree (see [3.5 Namespace Fixup]). The value of the variable is a single node, the document node of the temporary tree.

  The base URI of a node in the temporary tree is determined as if all the nodes in the temporary tree came from a single entity whose URI was the base URI of the variable-binding element (see [Data Model]). Thus, the base URI of the document node will be equal to the base URI of the variable-binding element; an xml:base attribute within the temporary tree will change the base URI for its parent element and that element's descendants, just as it would within a document constructed by parsing.

  A temporary tree is available for processing in exactly the same way as any source document. For example, its nodes are accessible using path expressions, and they can be processed using instructions such as xsl:apply-templates and xsl:for-each. For example, the following stylesheet uses a temporary tree as the intermediate result of a two-phase transformation, using different modes for the two phases (see [5.6 Modes]):

  Ed. Note: We need to add examples to clarify how the type attribute works, especially with temporary trees.

  <xsl:stylesheet
    version="2.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  
  <xsl:import href="phase1.xsl"/>
  <xsl:import href="phase2.xsl"/>
  
  <xsl:variable name="intermediate">
    <xsl:apply-templates select="/" mode="phase1"/>
  </xsl:variable>
  
  <xsl:template match="/">
    <xsl:apply-templates select="$intermediate" mode="phase2"/>
  </xsl:template>
  
  </xsl:stylesheet>

  NOTE: The algorithm for matching nodes against template rules is exactly the same regardless which tree the nodes come from; if nodes from different trees cannot be distinguished by their expanded-QName, it is therefore a good idea to use modes to ensure that each tree is processed using the appropriate set of template rules.

• If the variable-binding element has empty content and does not have a select attribute, then the supplied value of the variable is an empty string. Thus

  <xsl:variable name="x"/>

  is equivalent to

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

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

NOTE: When a variable is used to select nodes by position, be careful not to do:

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

This will output the value of the first item element, because the variable n will be bound to a node, not a number. Instead, do one of the following:

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

or

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

or

<xsl:variable name="n" type="xsd:int">2</xsl:variable>
...
<xsl:value-of select="item[$n]"/>

6.2 Global Variables and Parameters

Both xsl:variable and xsl:param are allowed as top-level elements. A top-level variable-binding element declares a global variable that is visible everywhere (except where it is shadowed by another binding). A top-level xsl:param element declares a global parameter to the stylesheet; XSLT does not define the mechanism by which parameters are passed to the stylesheet.

If a stylesheet contains more than one binding for a global variable or parameter of a particular name, then the binding with the highest import precedence is used. [ERR046] It is a static error if a stylesheet contains more than one binding of a global variable with the same name and same import precedence.

For a global variable or parameter, the expression or content constructor specifying the variable value is evaluated with a singleton focus based on the document node of the principal source document.

The following example declares a global variable para-font-size, which it references in an attribute value template.

<xsl:variable name="para-font-size" type="xsd:string">12pt</xsl:variable>

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

6.3 Circular Definitions

If the expression or content constructor specifying the value of a global variable X references a global variable Y, then the value for Y must be computed before the value of X. If it is impossible to do this for all global variable definitions, then a circularity is said to exist.

For example the following two declarations create a circularity:

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

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

The definition of a global variable can be circular even if no other variable is involved. For example the following two declarations (see [7.3 Stylesheet Functions] for an explanation of the xsl:function element) also create a circularity:

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

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

The definition of a variable is also circular if the evaluation of the variable invokes an xsl:apply-templates instruction and the variable is referenced in the pattern used in the match attribute of any template rule in the stylesheet. For example the following definition is circular:

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

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

Similarly, a variable definition is circular if it causes a call on the key function, and the definition of that key refers to that variable in its match or use attributes. So the following definition is circular:

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

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

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

[ERR047] In general, a circularity in a stylesheet is a dynamic error. The processor must signal the error. However, as with all other dynamic errors, an implementation will signal the error only if it actually executes the instructions and expressions that participate in the circularity. Because different implementations may optimize the execution of a stylesheet in different ways, it is implementation-dependent whether a particular circularity will actually be signaled.

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

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

<xsl:function name="f">
  <xsl:param name="a" select="$x"/>
  <xsl:result select="$a"/>
</xsl:function>

Circularities usually involve global variables or parameters, but they can also exist between key definitions (see [14.3 Keys]), between named attribute sets (see [7.2 Named Attribute Sets]), or between any combination of these constructs. For example, a circularity exists if a key definition invokes a function that references a global variable that uses an attribute set that calls the key function, supplying the name of the original key definition as an argument.

Circularity is not the same as recursion. Stylesheet functions (see [7.3 Stylesheet Functions]) and named templates (see [7.1 Named Templates]) may call other functions and named templates without restriction. With careless coding, recursion may be non-terminating. Implementations are required to signal circularity as a dynamic error, but they are not required to detect non-terminating recursion.

6.4 Local Variables and Parameters

As well as being allowed as top-level elements, both xsl:variable and xsl:param are also allowed in content constructors and within the xsl:function element (see [7.3 Stylesheet Functions]. Such a variable or parameter is known as a local variable or parameter.

• A local xsl:variable is allowed anywhere within a content constructor that an instruction is allowed. Within an xsl:function element, it is allowed after any xsl:param elements and before the xsl:result element. In these cases, the binding is visible for all following siblings and their descendants. Note that the binding is not visible for the xsl:variable element itself.

• A local xsl:param is allowed only as a child at the beginning of an xsl:template or xsl:function element. In this context, the binding is visible for all following siblings and their descendants. Note that the binding is not visible for the xsl:param element itself.

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

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

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

However, the following is allowed:

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

NOTE: Once a variable has been given a value, the value cannot subsequently be changed. XSLT does not provide an equivalent to the assignment operator available in many procedural programming languages. This is because an assignment operator would make it harder to create an implementation that processes a document other than in a batch-like way, starting at the beginning and continuing through to the end.

7.1 Named Templates

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

Templates can be invoked by name. An xsl:template element with a name attribute specifies a named template. The value of the name attribute is a QName, which is expanded as described in [4.1 Qualified Names]. If an xsl:template element has a name attribute, it may, but need not, also have a match attribute. An xsl:call-template element invokes a template by name; it has a required name attribute that identifies the template to be invoked. Unlike xsl:apply-templates, the xsl:call-template instruction does not change the focus.

The match, mode and priority attributes on an xsl:template element do not affect whether the template is invoked by an xsl:call-template element. Similarly, the name attribute on an xsl:template element does not affect whether the template is invoked by an xsl:apply-templates element.

[ERR048] It is a static error if a stylesheet contains more than one template with the same name and same import precedence.

The result of evaluating an xsl:call-template instruction is the node sequence produced by evaluating the content constructor contained in the associated xsl:template element.

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

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

7.2 Named Attribute Sets

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

The xsl:attribute-set element defines a named set of attributes. The name attribute specifies the name of the attribute set. The value of the name attribute is a QName, which is expanded as described in [4.1 Qualified Names]. The content of the xsl:attribute-set element consists of zero or more xsl:attribute elements that specify the attributes in the set.

Attribute sets are used by specifying a use-attribute-sets attribute on the xsl:element, xsl:copy (see [8.8 Copying Nodes from the Source Tree to the Result Tree]) or xsl:attribute-set elements. The value of the use-attribute-sets attribute is a whitespace-separated list of names of attribute sets. Each name is specified as a QName, which is expanded as described in [4.1 Qualified Names]. Specifying a use-attribute-sets attribute is equivalent to adding xsl:attribute elements for each of the attributes in each of the named attribute sets to the beginning of the content of the element with the use-attribute-sets attribute, in the same order in which the names of the attribute sets are specified in the use-attribute-sets attribute. [ERR049] It is a dynamic error if use of use-attribute-sets attributes on xsl:attribute-set elements causes an attribute set to use itself, directly or indirectly. The processor must signal the error

Attribute sets can also be used by specifying an xsl:use-attribute-sets attribute on a literal result element. The value of the xsl:use-attribute-sets attribute is a whitespace-separated list of names of attribute sets. The xsl:use-attribute-sets attribute has the same effect as the use-attribute-sets attribute on xsl:element with the additional rule that attributes specified on the literal result element itself are treated as if they were specified by xsl:attribute elements before any actual xsl:attribute elements but after any xsl:attribute elements implied by the xsl:use-attribute-sets attribute. Thus, in the node sequence produced by evaluating the content constructor for a literal result element, attributes from attribute sets named in an xsl:use-attribute-sets attribute will appear first, in the order listed in the attribute; these will be followed by attributes specified on the literal result element will be added; finally, any attributes specified by xsl:attribute elements will appear. Since in a sequence of attribute nodes produced by a content constructor, only the last attribute with a given expanded-QName has any effect (see [4.6 Content Constructors]), this means that attributes specified in attribute sets can be overridden by attributes specified on the literal result element itself.

The content constructor within each xsl:attribute element in an xsl:attribute-set element is evaluated each time the attribute set is used; it is evaluated using the same focus as is used for evaluating the element bearing the use-attribute-sets or xsl:use-attribute-sets attribute. However, it is the position in the stylesheet of the xsl:attribute element rather than of the element bearing the use-attribute-sets or xsl:use-attribute-sets attribute that determines which variable bindings are visible (see [6 Variables and Parameters]); thus, only global variables and parameters are visible.

The following example creates a named attribute set title-style and uses it in a template rule.

<xsl:template match="chapter/heading">
  <fo:block font-stretch="condensed" xsl:use-attribute-sets="title-style">
    <xsl:apply-templates/>
  </fo:block>
</xsl:template>

<xsl:attribute-set name="title-style">
  <xsl:attribute name="font-size">12pt</xsl:attribute>
  <xsl:attribute name="font-weight">bold</xsl:attribute>
</xsl:attribute-set>

Multiple definitions of an attribute set with the same expanded-QName are merged. An attribute from a definition that has higher import precedence takes precedence over an attribute from a definition that has lower import precedence. [ERR050] It is a dynamic error if there are two attribute sets that have the same expanded-QName and equal import precedence and that both contain the same attribute, unless there is a definition of the attribute set with higher import precedence that also contains the attribute. The processor must either signal the error, or must recover by choosing from amongst the definitions that specify the attribute that have the highest import precedence the one that was specified last in declaration order.

Where the attributes in an attribute set were specified is relevant only in merging the attributes into the attribute set; it makes no difference when the attribute set is used. For each attribute set name occurring in a use-attribute-sets attribute on an xsl:attribute-set element, all definitions of an attribute set with that name must be merged before the use-attribute-sets attribute is replaced by the equivalent sequence of xsl:attribute child elements. Any use-attribute-sets attribute on an xsl:attribute-set element must be replaced by the equivalent sequence of xsl:attribute child elements before that xsl:attribute-set element is merged with other xsl:attribute-set elements with the same expanded-QName. When xsl:attribute-set elements with the same expanded-QName are merged, any xsl:attribute child elements added to replace a use-attribute-sets attribute are treated exactly as if they had originally been specified in the stylesheet as child elements.

7.3 Stylesheet Functions

An xsl:function declaration declares the name, parameters, and implementation of a stylesheet function that can be called from any XPath expression within the stylesheet. [ERR051] A stylesheet function must have a prefixed name, to remove any risk of a clash with a system-defined function. It is a static error if the name has no prefix.

NOTE: To prevent the namespace declaration used for the function name appearing in the result document, use the exclude-result-prefixes attribute on the xsl:stylesheet element: see [8.1.2 Namespace Nodes for Literal Result Elements].

<xsl:transform 
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:str="http://user.com/namespace"
  version="2.0"
  exclude-result-prefixes="str">

<xsl:function name="str:reverse">
  <xsl:param name="sentence"/>
  <xsl:result 
     select="if (contains($sentence, ' '))
             then concat(str:reverse(substring-after($sentence, ' ')),
                         ' ',
                         substring-before($sentence, ' '))
             else $sentence"/>             
</xsl:function>

<xsl:template match="/">
<output>
  <xsl:value-of select="str:reverse('DOG BITES MAN')"/>
</output>
</xsl:template>

</xsl:transform>

<xsl:function name="num:roman">
  <xsl:param name="value" type="xsd:integer"/>
  <xsl:variable name="roman-number" type="xsd:string">
    <xsl:number value="$value" format="i"/>
  </xsl:variable>
  <xsl:result select="string($roman-number)" type="xsd:string"/>             
</xsl:function>

8.1 Literal Result Elements

In a content constructor, an element in the stylesheet that does not belong to the XSLT namespace and that is not an extension instruction (see [16.2 Extension Instructions]) is classified as a literal result element. A literal result element is evaluated to construct a new element node with the same expanded-QName. The result of evaluating a literal result element is a node sequence containing one element, the newly-constructed element node.

The content of the element is a content constructor, which is evaluated to return a sequence of nodes that define the attributes and children of the created element node. The children of the newly-constructed element node will be the sequence of elements, text nodes, comment nodes, and processing instructions that results from evaluating this content constructor.

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

<xsl:namespace-alias stylesheet-prefix="axsl" result-prefix="xsl"/>

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

<xsl:template match="block">
  <axsl:template match="{.}">
     <fo:block><axsl:apply-templates/></fo:block>
  </axsl:template>
</xsl:template>

</xsl:stylesheet>

<elements>
<block>p</block>
<block>h1</block>
<block>h2</block>
<block>h3</block>
<block>h4</block>
</elements>

8.2 Creating Element Nodes using xsl:element

<!-- Category: instruction -->
<xsl:element
  name = { qname }
  namespace = { uri-reference }
  use-attribute-sets = qnames>
  <!-- Content: content-constructor -->
</xsl:element>

The xsl:element instruction allows an element to be created with a computed name. The expanded-QName of the element to be created is specified by a required name attribute and an optional namespace attribute. The content of the xsl:element instruction is a content constructor for the attributes and children of the created element.

The result of evaluating the xsl:element instruction, except in error cases, is the newly-constructed element node.

The name attribute is interpreted as an attribute value template. [ERR056] It is an dynamic error if the effective value is not a QName. The processor must either signal the error, or must recover by making the result of evaluating the xsl:element element be the sequence of nodes created by evaluating the content of the xsl:element element, excluding any initial attribute nodes.

If the namespace attribute is not present then the QName is expanded into an expanded-QName using the namespace declarations in effect for the xsl:element element, including any default namespace declaration.

If the namespace attribute is present, then it too is interpreted as an attribute value template. The effective value should be a URI reference. It is not an error if the string is not a syntactically legal URI reference. If the string is empty, then the expanded-QName of the element has a null namespace URI. Otherwise, the string is used as the namespace URI of the expanded-QName of the element to be created. The local part of the QName specified by the name attribute is used as the local part of the expanded-QName of the element to be created.

Implementations may make use of the prefix of the QName specified in the name attribute when selecting the prefix used for outputting the created element as XML; however, they are not required to do so.

For the effect of the use-attribute-sets attribute, see [7.2 Named Attribute Sets]

8.3 Creating Attribute Nodes using xsl:attribute

<!-- Category: instruction -->
<xsl:attribute
  name = { qname }
  namespace = { uri-reference }>
  <!-- Content: content-constructor -->
</xsl:attribute>

The xsl:attribute element can be used to add attributes to result elements whether created by literal result elements in the stylesheet or by instructions such as xsl:element. The expanded-QName of the attribute to be created is specified by a required name attribute and an optional namespace attribute. The result of evaluating an xsl:attribute instruction is the newly-constructed attribute node.

The content of the xsl:attribute element is a content constructor for the value of the created attribute.

The name attribute is interpreted as an attribute value template. [ERR057] It is a dynamic error if the effective value is not a QName or is the string xmlns. The processor must either signal the error, or must recover by not adding the attribute to the result tree.

If the namespace attribute is not present, then the QName is expanded into an expanded-QName using the namespace declarations in effect for the xsl:attribute element, not including any default namespace declaration.

If the namespace attribute is present, then it too is interpreted as an attribute value template. The effective value should be a URI reference. It is not an error if the string is not a syntactically legal URI reference. If the string is empty, then the expanded-QName of the attribute has a null namespace URI. Otherwise, the string is used as the namespace URI of the expanded-QName of the attribute to be created. The local part of the QName specified by the name attribute is used as the local part of the expanded-QName of the attribute to be created.

Implementations may make use of the prefix of the QName specified in the name attribute when selecting the prefix used for outputting the created attribute as XML; however, they are not required to do so and, if the prefix is xmlns, they must not do so. Thus, although it is not an error to do:

<xsl:attribute name="xmlns:xsl" namespace="whatever">http://www.w3.org/1999/XSL/Transform</xsl:attribute>

it will not result in a namespace declaration being output.

As described in [4.6 Content Constructors], any attribute nodes must appear in the result of evaluating a content constructor before any element, text, comment, or processing instruction nodes, but after any namespace nodes. Where the result of the content constructor contains two or more attribute nodes with the same expanded-QName, the one that comes last is the only one that takes effect. When evaluating the content of a node other than an element, returning a node sequence that includes attribute nodes is a dynamic error; the recovery action (if any) depends on the instruction that makes use of the constructed node sequence.

NOTE: When an xsl:attribute element contains a text node with a newline, then the XML output must contain a character reference. For example,

<xsl:attribute name="a">x
y</xsl:attribute>

will result in the output

a="x
y"

(or with any equivalent character reference). The XML output cannot be

a="x
y"

This is because XML 1.0 requires newline characters in attribute values to be normalized into spaces but requires character references to newline characters not to be normalized. The attribute values in the data model represent the attribute value after normalization. If a newline occurring in an attribute value in the tree were output as a newline character rather than as character reference, then the attribute value in the tree created by reparsing the XML would contain a space not a newline, which would mean that the tree had not been output correctly.

8.4 Creating Text Nodes

8.5 Creating Processing Instructions

<!-- Category: instruction -->
<xsl:processing-instruction
  name = { ncname }>
  <!-- Content: content-constructor -->
</xsl:processing-instruction>

The xsl:processing-instruction element is evaluated to create a processing instruction node. The content of the xsl:processing-instruction element is a content constructor for the string-value of the processing instruction node. The xsl:processing-instruction element has a required name attribute that specifies the name of the processing instruction node. The value of the name attribute is interpreted as an attribute value template.

Except in error situations, the result of evaluating the xsl:processing-instruction instruction is a single node, the newly-constructed processing instruction.

For example, this

<xsl:processing-instruction name="xml-stylesheet">href="book.css" type="text/css"</xsl:processing-instruction>

would create the processing instruction

<?xml-stylesheet href="book.css" type="text/css"?>

[ERR058] It is a dynamic error if the effective value of the name attribute is not both an NCName and a PITarget. The processor must either signal the error, or must recover by returning an empty sequence.

NOTE: This means that xsl:processing-instruction cannot be used to output an XML declaration. The xsl:output declaration should be used to control this instead (see [18 Serialization]).

[ERR059] It is a dynamic error if the result of evaluating the content of the xsl:processing-instruction contains the string ?>. The processor must either signal the error, or must recover by inserting a space after any occurrence of ? that is followed by a >

8.6 Creating Namespace Nodes

<!-- Category: instruction -->
<xsl:namespace
  name = { ncname }>
  <!-- Content: content-constructor -->
</xsl:namespace>

The xsl:namespace element is evaluated to create a namespace node. The content of the xsl:namespace element is a content constructor for the string-value of the namespace node (that is, the namespace URI). The xsl:namespace element has a required name attribute that specifies the name of the namespace node (that is, the namespace prefix). The value of the name attribute is interpreted as an attribute value template.

Except in error situations, the result of evaluating the xsl:namespace instruction is a single node, the newly-constructed namespace node. Note the restrictions described in [4.6 Content Constructors] for the position of a namespace node relative to other nodes in the node sequence returned by a content constructor.

For example, this

<xsl:namespace name="xsd">http://www.w3.org/2001/XMLSchema</xsl:namespace>

would typically cause the output document to contain the namespace declaration:

xmlns:xsd="http://www.w3.org/2001/XMLSchema"

[ERR060] It is a dynamic error if the effective value of the name attribute is neither an empty string nor an NCName. The processor must either signal the error, or must recover by returning an empty sequence.

[ERR061] It is an dynamic error if evaluating the content of xsl:namespace results in an empty string. The processor must either signal the error, or must recover by returning an empty sequence.

NOTE: It is rarely necessary to use xsl:namespace to create a namespace node in the result tree; in most circumstances, the required namespace nodes will be created automatically, as a side-effect of writing elements or attributes that use the namespace. An example where xsl:namespace is needed is a situation where the required namespace is used only within attribute values in the result document, not in element or attribute names; especially where the required namespace prefix or namespace URI is computed at run-time and is not present in either the source document or the stylesheet.

Ed. Note: We need to add text explaining how the new namespace node is effectively merged with other matching namespace nodes in the same document, once the data model is finalized.

8.7 Creating Comments

<!-- Category: instruction -->
<xsl:comment>
  <!-- Content: content-constructor -->
</xsl:comment>

The xsl:comment element is evaluated to contruct a new comment node. The content of the xsl:comment element is a content constructor for the string-value of the comment node.

The result of evaluating the xsl:comment instruction is a single node, the newly-constructed comment node.

For example, this

<xsl:comment>This file is automatically generated. Do not edit!</xsl:comment>

would create the comment

<!--This file is automatically generated. Do not edit!-->

[ERR062] It is a dynamic error if the result of evaluating the content of the xsl:comment contains the string -- or ends with -. The processor must either signal the error, or must recover by inserting a space after any occurrence of - that is followed by another - or that ends the comment.

8.8 Copying Nodes from the Source Tree to the Result Tree

<xsl:template match="@*|node()">
  <xsl:copy>
    <xsl:apply-templates select="@*|node()"/>
  </xsl:copy>
</xsl:template>

<xsl:template name="apply-templates-copy-lang">
 <xsl:for-each select="@xml:lang">
   <xsl:copy/>
 </xsl:for-each>
 <xsl:apply-templates/>
</xsl:template>

<xsl:call-template name="apply-templates-copy-lang"/>

<xsl:apply-templates/>

Ed. Note: This is a poor example because it would be easier to write <xsl:copy-of select="@xml:lang"/>

<x><xsl:copy-of select="(1,2,3,4)" separator="|"/></x>

<x>1|2|3|4</x>

8.9 Generating Text with xsl:value-of

Within a content constructor, the xsl:value-of instruction can be used to compute generated text, for example by extracting text from the source tree or by inserting the value of a variable. The xsl:value-of instruction computes this text using an expression that is specified as the value of the select attribute.

NOTE: An alternative way of computing a string value, applicable only when the resulting text is to be used within a generated attribute node, is to use an attribute value template, enclosing the expression in curly braces ({}) within an attribute of a literal result element.

<!-- Category: instruction -->
<xsl:value-of
  select = expression
  separator = { string }
  disable-output-escaping = "yes" | "no" />

The xsl:value-of instruction is evaluated to construct a new text node; the result of the instruction is the newly-constructed text node. But if the rules below produce a text node whose string value is the empty string, the result of the instruction is an empty sequence. The required select attribute is an expression. The required type of this expression is sequence, which means that the result of the expression may be any sequence. If the sequence is empty, no text node will be created. If the sequence contains a single item, the resulting text node will have a string-value that is the same as the string-value of this item. If the sequence contains more than one item, the effect depends on whether the separator attribute is present.

If the separator attribute is present, then the string-value of the newly constructed text node will be the concatenation of the string-values of the items in the sequence that results from evaluating the select attribute, with each of these string-values except the last being followed by the string that is the effective value of the separator attribute. If the separator attribute is absent, then (for backwards compatibility with XSLT 1.0) all items in the input sequence other than the first are ignored. If the effective value of the separator attribute is an empty string, then all items in the input sequence are processed and the results are concatenated with no separator.

For example, the instruction:

<x><xsl:value-of select="(1,2,3,4)" separator="|"/></x>

produces the output:

<x>1|2|3|4</x>

NOTE: The separator attribute is used only when the select expression produces a sequence of items. It is not used when the select expression returns a single node whose typed-value is a sequence, for example a node of type IDREFS. The string-value of such a node is obtained in the normal way, as if by calling the string function.

NOTE: The xsl:copy-of element can be used to copy a sequence of nodes to the result tree without converting to a string. See [8.8.2 Deep Copy].

For example, the following creates an HTML paragraph from a person element with given-name and family-name attributes. The paragraph will contain the value of the given-name attribute of the context node followed by a space and the value of the family-name attribute of the context node.

<xsl:template match="person">
  <p>
   <xsl:value-of select="@given-name"/>
   <xsl:text> </xsl:text>
   <xsl:value-of select="@family-name"/>
  </p>
</xsl:template>

For another example, the following creates an HTML paragraph from a person element with given-name and family-name children elements. The paragraph will contain the string-value of the first given-name child element of the context node followed by a space and the string-value of the first family-name child element of the context node.

<xsl:template match="person">
  <p>
   <xsl:value-of select="given-name"/>
   <xsl:text> </xsl:text>
   <xsl:value-of select="family-name"/>
  </p>
</xsl:template>

The following precedes each procedure element with a paragraph containing the security level of the procedure. It assumes that the security level that applies to a procedure is determined by a security attribute on the procedure element or on an ancestor element of the procedure. It also assumes that if more than one such element has a security attribute then the security level is determined by the element that is closest to the procedure.

<xsl:template match="procedure">
  <fo:block>
    <xsl:value-of select="ancestor-or-self::*[@security][1]/@security"/>
  </fo:block>
  <xsl:apply-templates/>
</xsl:template>

For the effect of the disable-output-escaping attribute, see [18.5 Disabling Output Escaping]

9.1 Formatting a Supplied Number

The place marker to be formatted may be specified by an expression. The value attribute contains the expression. The required type of the expression is xsd:integer*, that is, a sequence of integers. The expression is evaluated, to produce a sequence, and each item in this sequence is converted to an integer using the rules of the XPath cast construct.

[ERR064] It is a dynamic error if any item in the sequence cannot be converted to an integer, or if the resulting integer is less than 1 (one). The processor must either signal the error, or must recover by converting that member to a string as if by a call to the string function and inserting the resulting string into the formatted result string in its proper position.

Otherwise, the sequence is formatted as a string using the effective values of the attributes specified in [9.3 Number to String Conversion Attributes]; each of these attributes is interpreted as an attribute value template. After conversion, the xsl:number element constructs a new text node containing the resulting string, and returns this node.

The following example numbers a sorted list:

<xsl:template match="items">
  <xsl:for-each select="item">
    <xsl:sort select="."/>
    <p>
      <xsl:number value="position()" format="1. "/>
      <xsl:value-of select="."/>
    </p>
  </xsl:for-each>
</xsl:template>

9.2 Numbering based on Position in the Source Tree

If no value attribute is specified, then the xsl:number instruction returns a new text node containing a formatted place marker that is based on the position of the context node within its tree. Without loss of generality, the tree containing the context node is referred to in this section as the source tree.

[ERR065] It is a dynamic error if the xsl:number instruction is evaluated, with no value attribute, when the context item is not a node. The processor must either signal the error, or must recover by returning an empty sequence.

The following attributes control how the context node is to be numbered:

• The level attribute specifies what levels of the source tree should be considered; it has the values single, multiple or any. The default is single.

• The count attribute is a pattern that specifies which nodes should be counted at those levels. If count attribute is not specified, then it defaults to the pattern that matches any node with the same node type as the context node and, if the context node has an expanded-QName, with the same expanded-QName as the context node.

• The from attribute is a pattern that specifies where counting starts.

In addition, the attributes specified in [9.3 Number to String Conversion Attributes] are used for number to string conversion, as in the case when the value attribute is specified.

The xsl:number element first constructs a sequence of positive integers using the level, count and from attributes. Where level is single or any, this sequence will either be empty or contain a single number; where level is multiple, the sequence may be of any length. The sequence is constructed as follows:

Let matches-count($node) be a function that returns true if the given node matches the pattern given in the count attribute, or the implied pattern (according to the rules given above) if the count attribute is omitted.

Let matches-from($node) be a function that returns true if the given node matches the pattern given in the from attribute (this function is not used if the from attribute is omitted).

When level="single":

• Let $A be the node sequence selected by the expression

     ancestor-or-self::node()[matches-count(.)][last()]

• If the from pattern is specified, let $F be the node sequence selected by the expression

     ancestor::node()[matches-from(.)][last()]

  otherwise let $F be the root node, /

• Let $AF be the value of

     $A intersect ($F/descendant::node())

• If $AF is empty, return the empty sequence, ()

• Otherwise return the value of

     1 + count($AF/preceding-sibling::node()[matches-count()])

When level="multiple":

• Let $A be the node sequence selected by the expression

     ancestor-or-self::node()[matches-count(.)]

• If the from pattern is specified, let $F be the node sequence selected by the expression

     ancestor::node()[matches-from(.)][last()]

  otherwise let $F be the root node, /

• Let $AF be the value of

     $A intersect ($F/descendant::node())

• Return the result of the expression

     for $af in $AF return 1+count($af/preceding-sibling::node()[matches-count(.)])

When level="any":

• Let $A be the node sequence selected by the expression

     (preceding::node()|ancestor-or-self::node())[matches-count(.)]

• If the from pattern is specified, let $F be the node sequence selected by the expression

     (preceding::node()|ancestor::node())[matches-from(.)][last()]

  and let $AF be the node sequence $A[. >> $F]. Otherwise, let $AF be the sequence $A.

• If $AF is empty, return the empty sequence, ()

• Otherwise return the value of the expression count($AF)

The sequence of numbers (the place marker) is then converted into a string using the effective values of the attributes specified in [9.3 Number to String Conversion Attributes]; each of these attributes is interpreted as an attribute value template. After conversion, the resulting string is inserted in the result tree.

For example, the following will number the items in an ordered list:

<xsl:template match="ol/item">
  <fo:block>
    <xsl:number/><xsl:text>. </xsl:text><xsl:apply-templates/>
  </fo:block>
<xsl:template>

The following two rules will number title elements. This is intended for a document that contains a sequence of chapters followed by a sequence of appendices, where both chapters and appendices contain sections, which in turn contain subsections. Chapters are numbered 1, 2, 3; appendices are numbered A, B, C; sections in chapters are numbered 1.1, 1.2, 1.3; sections in appendices are numbered A.1, A.2, A.3.

<xsl:template match="title">
  <fo:block>
     <xsl:number level="multiple"
                 count="chapter|section|subsection"
                 format="1.1 "/>
     <xsl:apply-templates/>
  </fo:block>
</xsl:template>

<xsl:template match="appendix//title" priority="1">
  <fo:block>
     <xsl:number level="multiple"
                 count="appendix|section|subsection"
                 format="A.1 "/>
     <xsl:apply-templates/>
  </fo:block>
</xsl:template>

The following example numbers notes sequentially within a chapter:

<xsl:template match="note">
  <fo:block>
     <xsl:number level="any" from="chapter" format="(1) "/>
     <xsl:apply-templates/>
  </fo:block>
</xsl:template>

The following example will number H4 elements in HTML with a three-part label:

<xsl:template match="H4">
 <fo:block>
   <xsl:number level="any" from="H1" count="H2"/>
   <xsl:text>.</xsl:text>
   <xsl:number level="any" from="H2" count="H3"/>
   <xsl:text>.</xsl:text>
   <xsl:number level="any" from="H3" count="H4"/>
   <xsl:text> </xsl:text>
   <xsl:apply-templates/>
 </fo:block>
</xsl:template>

9.3 Number to String Conversion Attributes

The following attributes are used to control conversion of a sequence of numbers into a string. The numbers are integers greater than 0. The attributes are all optional.

The main attribute is format. The default value for the format attribute is 1. The format attribute is split into a sequence of tokens where each token is a maximal sequence of alphanumeric characters or a maximal sequence of non-alphanumeric characters. Alphanumeric means any character that has a Unicode category of Nd, Nl, No, Lu, Ll, Lt, Lm or Lo. The alphanumeric tokens (format tokens) specify the format to be used for each number in the sequence. If the first token is a non-alphanumeric token, then the constructed string will start with that token; if the last token is non-alphanumeric token, then the constructed string will end with that token. Non-alphanumeric tokens that occur between two format tokens are separator tokens that are used to join numbers in the sequence. The nth format token will be used to format the nth number in the sequence. If there are more numbers than format tokens, then the last format token will be used to format remaining numbers. If there are no format tokens, then a format token of 1 is used to format all numbers. The format token specifies the string to be used to represent the number 1. Each number after the first will be separated from the preceding number by the separator token preceding the format token used to format that number, or, if there are no separator tokens, then by . (a period character).

Format tokens are a superset of the allowed values for the type attribute for the OL element in HTML 4.0 and are interpreted as follows:

• Any token where the last character has a decimal digit value of 1 (as specified in the Unicode character property database), and the Unicode value of preceding characters is one less than the Unicode value of the last character generates a decimal representation of the number where each number is at least as long as the format token. Thus, a format token 1 generates the sequence 1 2 ... 10 11 12 ..., and a format token 01 generates the sequence 01 02 ... 09 10 11 12 ... 99 100 101.

• A format token A generates the sequence A B C ... Z AA AB AC....

• A format token a generates the sequence a b c ... z aa ab ac....

• A format token i generates the sequence i ii iii iv v vi vii viii ix x ....

• A format token I generates the sequence I II III IV V VI VII VIII IX X ....

• Any other format token indicates a numbering sequence that starts with that token. It is implementation-defined which numbering sequences, additional to those listed above, are supported. If an implementation does not support a numbering sequence that starts with the given token, it must use a format token of 1.

For all format tokens other than the first kind above (one that consists of decimal digits), there may be an implementation-defined upper bound on the range of numbers that can be formatted using this format token. For the numbering sequences described above, this upper bound must not be less than 1000 (one thousand). Numbers that exceed the upper bound must be formatted using the format token 1.

When numbering with an alphabetic sequence, the lang attribute specifies which language's alphabet is to be used; it has the same range of values as xml:lang [XML]; if no lang value is specified, the language should be determined from the system environment. The set of languages for which numbering is supported is implementation-defined.

The letter-value attribute disambiguates between numbering sequences that use letters. In many languages there are two commonly used numbering sequences that use letters. One numbering sequence assigns numeric values to letters in alphabetic sequence, and the other assigns numeric values to each letter in some other manner traditional in that language. In English, these would correspond to the numbering sequences specified by the format tokens a and i. In some languages, the first member of each sequence is the same, and so the format token alone would be ambiguous. A value of alphabetic specifies the alphabetic sequence; a value of traditional specifies the other sequence. If the letter-value attribute is not specified, then it is implementation-dependent how any ambiguity is resolved.

NOTE: It is possible for two conforming implementations not to convert a number to exactly the same string. Some implementations might not support some languages. Furthermore, there may be variations possible in the way conversions are performed for any particular language that are not specifiable by the attributes on xsl:number. Future versions of XSLT may provide additional attributes to provide control over these variations. Implementations may also use implementation-defined namespaced attributes on xsl:number for this.

The grouping-separator attribute gives the separator used as a grouping (e.g. thousands) separator in decimal numbering sequences, and the optional grouping-size specifies the size (normally 3) of the grouping. For example, grouping-separator="," and grouping-size="3" would produce numbers of the form 1,000,000. If only one of the grouping-separator and grouping-size attributes is specified, then it is ignored.

Here are some examples of conversion specifications:

• format="ア" specifies Katakana numbering

• format="イ" specifies Katakana numbering in the "iroha" order

• format="๑" specifies numbering with Thai digits

• format="א" letter-value="traditional" specifies "traditional" Hebrew numbering

• format="ა" letter-value="traditional" specifies Georgian numbering

• format="α" letter-value="traditional" specifies "classical" Greek numbering

• format="а" letter-value="traditional" specifies Old Slavic numbering

11.1 Conditional Processing with xsl:if

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

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

The required type of the expression in the test attribute is xsd:boolean. Type conversion follows the XPath rules for other boolean contexts, which means that a value of () (the empty sequence) is treated as false.

If the result of evaluating the expression is true, then the content constructor is evaluated, and the resulting node sequence is returned as the result of the xsl:if instruction; otherwise, an empty sequence is returned.

In the following example, the names in a group of names are formatted as a comma separated list:

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

The following colors every other table row yellow:

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

11.2 Conditional Processing with xsl:choose

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

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

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

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

The required type of the expression in the test attribute of the xsl:when element is xsd:boolean. Type conversion follows the XPath rules for a choice context, which means that a value of () (the empty sequence) is treated as false.

When an xsl:choose element is processed, each of the xsl:when elements is tested in turn, until one of the expressions evaluates to true. The content of the first, and only the first, xsl:when element whose test is true is evaluated, and the resulting node sequence is returned as the result of the xsl:choose instruction. If no xsl:when condition is true, the content of the xsl:otherwise element is evaluated, and the resulting node sequence is returned as the result of the xsl:choose instruction. If no xsl:when element is true, and no xsl:otherwise element is present, the result of the xsl:choose instruction is an empty sequence.

The content constructors for xsl:when and xsl:otherwise instructions other than the selected one are not evaluated, and the select expressions for xsl:when instructions after the selected one are not evaluated.

The following example enumerates items in an ordered list using arabic numerals, letters, or roman numerals depending on the depth to which the ordered lists are nested.

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

12.2 The xsl:sort Element

<xsl:sort
  select = expression
  lang = { nmtoken }
  data-type = { "text" | "number" | qname-but-not-ncname }
  order = { "ascending" | "descending" }
  collation = { uri }
  case-order = { "upper-first" | "lower-first" } />

Those attributes of the xsl:sort elements whose values are attribute value templates are evaluated using the outer focus. If the element that contains the xsl:sort elements is an xsl:sort-key declaration, then the outer focus is a singleton focus based on the document node of the principal source document. Otherwise, the outer focus is the focus used to evaluate the select attribute of the containing instruction (for example, xsl:for-each or xsl:apply-templates).

The sequence to be sorted is referred to as the initial sequence. The sequence after sorting as defined by the xsl:sort elements is referred to as the sorted sequence.

For each item in the initial sequence, a value is computed for each sort key definition within the sort specification. The value computed for an item by using the Nth sort key definition is referred to as the Nth sort key of that item. Specifically, the Nth sort key is computed by evaluating the expression contained in the select attribute of the Nth xsl:sort element, if there is such an attribute. If there is no select attribute, the sort key is computed by taking the actual item in the initial sequence if it is a simple value, or the typed-value of this item if it is a node.

The expression in the select attribute of the xsl:sort element is evaluated with the focus set as follows:

• The context item is the item in the initial sequence whose sort key is being computed.
• The context position is the position of that item in the initial sequence.
• The context size is the size of the initial sequence.
• The context document is the document containing the context item if that is a node, or the context document of the outer focus otherwise

If the xsl:sort element has a data-type attribute, then the sort key is converted to the target data type before comparing it with other items. [ERR067] The target data type for each xsl:sort element is determined by the effective value of its data-type attribute. If this has the value text, the target data type is xsd:string. If it has the value number, the target data type is xsd:double. Otherwise, the target data type must be the name of a primitive data type in XML Schema (see [XML Schema]). It is a dynamic error if any other value is supplied. The processor must either signal the error, or must recover by continuing as if the data-type attribute were not specified. Each sort key is converted to the target data type using the rules for the XPath cast expression. [ERR068] It is a dynamic error if any value obtained by evaluating the select attribute of an xsl:sort element cannot be converted to the target data type. The processor must either signal the error, or must recover by treating the value as being less than any value for which conversion succeeds, but equal to any other value for which conversion fails. This means (if this is the first or only sort key) that values that cannot be converted to the target data type will appear together at the start of the sorted sequence if order is ascending, or at the end if order is descending.

If there is no data-type attribute, then the computed sort keys are not converted before comparison, except in the case where the data type of a computed sort key is a complex type, in which case it is converted to a string as if by the XPath string function.

The items in the initial sequence are ordered into a sorted sequence by comparing their sort keys. The relative position of two items A and B in the sorted sequence is determined as follows. The first sort key of A is compared with the first sort key of B, according to the rules of the first sort key definition. If, under these rules, A is less than B, then A will precede B in the sorted sequence, unless the order attribute of this sort key definition specifies descending, in which case B will precede A in the sorted sequence. If, however, the relevant sort keys compare equal, then the second sort key of A is compared with the second sort key of B, according to the rules of the second sort key definition. This continues until two sort keys are found that compare unequal. If all the sort keys compare equal, then A will precede B in the sorted sequence if A preceded B in the initial sequence, and vice versa.

In general, comparison of two values is performed according to the rules of the XPath lt operator. However, special rules apply to certain data types, as described below. [ERR069] It is a dynamic error if, for any sort key definition, the set of sort keys evaluated for all the items in the initial sequence, after any type conversion requested, contains a pair of values for which the result of the XPath lt operator is an error or an empty sequence. The processor must either signal the error, or must recover by assigning an arbitrary ordering to any such pair of values.

If the target data type is a list data type, then the two sort keys to be compared are considered as sequences. The two sequences are compared pairwise. The ordering of the first pair of values that are unequal determines the ordering of the two sequences. If all items up to the length of the shorter sequence are equal, then the shorter sequence is considered to be less than the longer. If the sequences have the same length, and all their items are equal, then the two sequences are considered equal.

[ERR070] It is a dynamic error if the effective value of the data-type attribute of the xsl:sort element is a data type for which no ordering relation is defined, other than the values xsd:string or text, which are synonymous. The processor must signal the error, or must recover by continuing as if the data-type attribute were omitted.

For comparison of string values, special rules apply. If the xsl:sort element has a collation attribute, then the strings are compared according to the rules for the named collation: that is, they are compared using the XPath function call xf:compare($a, $b, $collation).

The lang and case-order attributes are ignored if a collation attribute is present. But in the absence of a collation attribute, these attributes provide input to an implementation-defined algorithm to identify a suitable collation:

• The lang attribute indicates that a collation suitable for a particular natural language is required. The effective value of the attribute must be a value that would be valid for the xml:lang attribute (see [XML]).
• The case-order attribute indicates whether the desired collation should sort upper-case letters before lower case or vice versa. The effective value of the attribute must be either lower-first (indicating that lower-case letters precede upper-case letters in the collating sequence) or upper-first (indicating that upper-case letters precede lower-case).

In the absence of any of these attributes, the default collation is used.

NOTE: The exact results of sorting are implementation-dependent. Some implementations might not support some languages. Furthermore, there may be variations possible in the sorting of any particular language that are not specified by the attributes on xsl:sort, for example, whether Hiragana or Katakana is sorted first in Japanese. Future versions of XSLT may provide additional attributes to provide control over these variations. Implementations may also use implementation-defined namespaced attributes on xsl:sort for this.

NOTE: It is recommended that implementors consult [UNICODE TR10] for information on internationalized sorting.

There are also special considerations where the target data type is float or double. For sorting purposes, NaN values are considered equal to each other, and less than any other value, including negative infinity. A value that cannot be converted to the target data type is treated in the same way as a NaN value. Negative zero and positive zero are considered equal, as normal.

12.3 Using Unnamed Sort Specifications

When used within xsl:for-each or xsl:apply-templates, a sort specification indicates that the sequence of items selected by that instruction should be processed in sorted order, not in the order of the supplied sequence.

For example, suppose an employee database has the form

<employees>
  <employee>
    <name>
      <given>James</given>
      <family>Clark</family>
    </name>
    ...
  </employee>
</employees>

Then a list of employees sorted by name could be generated using:

<xsl:template match="employees">
  <ul>
    <xsl:apply-templates select="employee">
      <xsl:sort select="name/family"/>
      <xsl:sort select="name/given"/>
    </xsl:apply-templates>
  </ul>
</xsl:template>

<xsl:template match="employee">
  <li>
    <xsl:value-of select="name/given"/>
    <xsl:text> </xsl:text>
    <xsl:value-of select="name/family"/>
  </li>
</xsl:template>

When used within xsl:for-each-group, a sort specification indicates the order in which the groups should be processed. For the effect of xsl:for-each-group, see [13 Grouping]

12.4 Using Named Sort Specifications

<!-- Category: declaration -->
<xsl:sort-key
  name = qname>
  <!-- Content: (xsl:sort+) -->
</xsl:sort-key>

A named sort specification is defined by an xsl:sort-key declaration. This is a top-level element in the stylesheet. The name attribute is mandatory. The value of the name attribute is a QName, which is expanded as described in [4.1 Qualified Names]: it need not have a prefix.

The content of the xsl:sort-key element consists of one or more xsl:sort elements that define the components of the sort specification, in major to minor order.

If a stylesheet contains declarations of two or more named sort specifications with the same expanded-QName, the one with highest import precedence is used. [ERR071] It is a static error for a stylesheet to contain two or more named sort specifications with the same expanded-QName and the same import precedence, unless there is another named sort specification with the same expanded-QName and a higher import precedence.

Function: sequence sort(string, sequence)

In an XPath expression used within an XSLT stylesheet, an additional function sort is available, which sorts a sequence using a named sort specification.

The first argument is evaluated as a string; its value must be a QName whose expanded-QName is the same as the name of a named sort specification defined in the stylesheet. [ERR072] It is a dynamic error if the first argument of the sort function does not match the name of any named sort specification in the stylesheet. The processor must signal the error.

The second argument is evaluated as a sequence. This sequence forms the initial sequence for the sort. The sequence is sorted according to the rules for the named sort specification, and the result of the function call is the resulting sorted sequence.

13.1 The Current Group

Function: sequence current-group()

The evaluation context for XPath expressions includes an additional value called the current group, which is a sequence. The current group is the collection of related items that are processed collectively in one iteration of the xsl:for-each-group element.

While an xsl:for-each-group instruction is being evaluated, the current group will be non-empty. At other times, it will be an empty sequence.

The function current-group returns the current group.

The function takes no arguments.

[ERR073] The current-group function must not be used within a pattern.

13.2 The xsl:for-each-group Element

<!-- Category: instruction -->
<xsl:for-each-group
  select = expression
  group-by = expression
  group-adjacent = expression
  group-starting-with = pattern
  group-ending-with = pattern>
  <!-- Content: (xsl:sort*, content-constructor) -->
</xsl:for-each-group>

This element is an instruction that may be used anywhere within a content constructor.

The xsl:for-each-group instruction partitions a sequence into groups of items (that is, it establishes a set of sequences) based either on common values of a grouping key, or on a pattern that the initial node in a group must match. The content constructor that forms the content of the xsl:for-each-group instruction is evaluated once for each of these groups. A group is never empty.

The sequence of items to be grouped, which is referred to as the population, is determined by evaluating the XPath expression contained in the select attribute. The required type of this expression is sequence. The population is treated as a sequence; the order of items in this sequence is referred to as population order. If the population is empty, the number of groups will be zero. Each item in the population is assigned to exactly one group: the assignment of items to groups depends on the group-by, group-adjacent, and group-starting-with attributes. [ERR074] These four attributes are mutually exclusive: exactly one of the four attributes must be present.

• If the group-by attribute is present, then the group-by expression is evaluated once for each item in the population, with that item as the context item, with its position, in population order, as the context position, and with the size of the population as the context size. The required type of this expression is xsd:string. The result of evaluating the group-by expression is converted to a string; the resulting string is known as the grouping key for that item. All items that have the same value for the grouping key are assigned to the same group, and the number of groups is the same as the number of distinct grouping key values present in the population.

  Issue (grouping-with-collation): The Working Group has decided in principle to add a collation attribute to xsl:for-each-group , to specify the collation under which strings are compared for equality.

  Issue (grouping-with-other-datatypes): Should we allow grouping based on key values whose data type is other than string?

• If the group-adjacent attribute is present, the items in the population are examined, in population order. The group-adjacent expression is evaluated once for each item in the population, with that item as the context item, with its position, in population order, as the context position, and with the size of the population as the context size. The required type of this expression is xsd:string. The result of evaluating the group-adjacent expression is converted to a string; the resulting string is known as the grouping key for that node. If an item has the same value for the grouping key as its preceding node within the population (in population order), then it is assigned to the same group as its preceding node; otherwise a new group is created and the item becomes its first member.

• If the group-starting-with attribute is present, then its value must be a pattern. In this case, the items in the population must all be nodes. [ERR075] It is a dynamic error if the result of evaluating the select expression contains an item that is not a node. The processor must signal the error.

  The nodes in the population are examined in population order. If a node matches the pattern, or is the first node in the population, then a new group is created and the node becomes its first member. Otherwise, the node is assigned to the same group as its preceding node within the population.

• If the group-ending-with attribute is present, then its value must be a pattern. In this case, the items in the population must all be nodes. [ERR076] It is a dynamic error if the result of evaluating the select expression contains an item that is not a node. The processor must signal the error.

  The nodes in the population are examined in population order. If a node is the first node in the population, or if the previous node in the population matches the pattern, then a new group is created and the node becomes its first member. Otherwise, the node is assigned to the same group as its preceding node within the population.

For each group, the item within the group that is first in population order is known as the initial item of the group.

There is an ordering among groups referred to as the order of first appearance. A group G is defined to precede a group H in order of first appearance if the initial item of G precedes the initial item of H in population order.

There is another ordering among groups referred to as processing order.

If there are no xsl:sort elements immediately within the xsl:for-each-group element, the processing order of the groups is the order of first appearance.

Otherwise, the xsl:sort elements immediately within the xsl:for-each-group element define the processing order of the groups (see [12 Sorting]). They do not affect the order of items within each group. Multiple sort keys are allowed, and are evaluated in major-to-minor order. If two groups have the same values for all their sort keys, they are processed in order of first appearance.

The select expression of an xsl:sort element is evaluated once for each group. During this evaluation, the context item is the initial item of the group, the context position is the position of this item within the set of initial items (that is, one item for each group in the population) in population order, the context size is the number of groups, and the current group is the group whose sort key is being determined. This means that if the grouping key is "@category", you can sort the groups in order of their grouping key by writing <xsl:sort select="@category"/>; or you can sort the groups in order of size by writing <xsl:sort select="count(current-group())" data-type="number">

If there are attribute value templates present in the xsl:sort element, the focus for evaluation of the contained expressions is the same as the focus for evaluation of the select attribute of the containing xsl:for-each-group instruction.

The content constructor contained by the xsl:for-each-group element is evaluated once for each of the groups, in processing order. The node sequences that result are concatenated, in processing order, to form the result of the xsl:for-each-group element. Within the content constructor, the context item is the initial item of the relevant group, the context position is the position of this item among the set of initial items (one item for each group) arranged in processing order of the groups, the context size is the number of groups, and the current group is the group being processed. This has the effect that within the the content constructor, a call on position() takes successive values 1, 2, ... last().

On completion of the evaluation of the xsl:for-each-group, the current group reverts to its previous value.

13.3 Examples of Grouping

The following example groups a list of nodes based on common values. The resulting groups are numbered but unsorted, and a total is calculated for each group.

Source XML document:

    <cities>
      <city name="Milano"  country="Italia"      pop="5"/>
      <city name="Paris"   country="France"      pop="7"/>
      <city name="München" country="Deutschland" pop="4"/>
      <city name="Lyon"    country="France"      pop="2"/>
      <city name="Venezia" country="Italia"      pop="1"/>
    </cities>

More specifically, the aim is to produce a four-column table, containing one row for each distinct country. The four columns are to contain first, a sequence number giving the number of the row; second, the name of the country, third, a comma-separated alphabetical list of the city names within that country, and fourth, the sum of the pop attribute for the cities in that country.

Desired output:

    <table>
      <tr>
        <th>Position</th>
        <th>Country</th>
        <th>List of Cities</th>
        <th>Population</th>
      </tr>
      <tr>
        <td>1</td>
        <td>Italia</td>
        <td>Milano, Venezia</td>
        <td>6</td>
      </tr>
      <tr>
        <td>2</td>
        <td>France</td>
        <td>Lyon, Paris</td>
        <td>9</td>
      </tr>  
      <tr>
        <td>3</td>
        <td>Deutschland</td>
        <td>München</td>
        <td>4</td>
      </tr>  
    </table>

Solution:

    <table xsl:version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
      <tr>
        <th>Position</th>
        <th>Country</th>
        <th>City List</th>
        <th>Population</th>
      </tr>
      <xsl:for-each-group select="cities/city" group-by="@country">
        <tr>
          <td><xsl:value-of select="position()"/></td>
          <td><xsl:value-of select="@country"/></td>
          <td>
            <xsl:value-of select="current-group()/@name" separator=","/>
          </td>
          <td><xsl:value-of select="sum(current-group()/@pop)"/></td>
        </tr>
      </xsl:for-each-group>
    </table>

The following example uses the same source document, this time grouping the cities according to the initial letter of the city name. The groups are sorted, and the result includes a count of the nodes within the group. The heading contains a count of the number of groups:

Desired output:

    <html>
      <body>
        <h2>L (1)</h2><p>Lyon</p>
        <h2>M (2)</h2><p>Milano</p><p>München</p>
        <h2>P (1)</h2><p>Paris</p>
        <h2>V (1)</h2><p>Venezia</p>
      </body>
    </html>

Solution:

    <html xsl:version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
      <body>
        <xsl:for-each-group select="cities/city" group-by="substring(@name,1,1)">
          <xsl:sort select="substring(@name,1,1)"/>
          <h2>
            <xsl:value-of select="upper-case(substring(@name,1,1))"/>
            <xsl:text> (</xsl:text>
            <xsl:value-of select="count(current-group())"/>
            <xsl:text>)</xsl:text>
          </h2>
          <xsl:for-each select="current-group()">
            <p><xsl:value-of select="@name"/></p>
          </xsl:for-each>
        </xsl:for-each-group>
      </body>
    </html>

The next example identifies a group not by the presence of a common value, but rather by adjacency in document order. A group consists of an h2 element, followed by all the p elements up to the next h2 element.

Source XML document:

    <body>
      <h2>Introduction</h2>
      <p>XSLT is used to write stylesheets.</p>
      <p>XQuery is used to query XML databases.</p>
      <h2>What is a stylesheet?</h2>
      <p>A stylesheet is an XML document used to define a transformation.</p>
      <p>Stylesheets may be written in XSLT.</p>
      <p>XSLT 2.0 introduces new grouping constructs.</p>
    </body>

Desired output:

    <chapter>
      <section title="Introduction">
        <para>XSLT is used to write stylesheets.</para>
        <para>XQuery is used to query XML databases.</para>
      </section> 
      <section title="What is a stylesheet?">
        <para>A stylesheet is an XML document used to define a transformation.</para>
        <para>Stylesheets may be written in XSLT.</para>
        <para>XSLT 2.0 introduces new grouping constructs.</para>
      </section>
    </chapter> 

Solution:

	<xsl:template match="body">
	<chapter>
		<xsl:for-each-group select="*" group-starting-with="h2"	>
		    <section title="{self::h2}">
		       <xsl:for-each select="current-group()[self::p]">
		           <para><xsl:value-of select="."/></para>
		       </xsl:for-each> 
		    </section>
		</xsl:for-each-group>
	</chapter>
	</xsl:template>

The use of title="{self::h2}" rather than title="{.}" is to handle the case where the first element is not an h2 element.

In the final example, the membership of a node within a group is based both on adjacency of the nodes in document order, and on common values. In this case, the grouping key is a boolean condition, true or false, so the effect is that a grouping establishes a maximal sequence of nodes for which the condition is true, followed by a maximal sequence for which it is false, and so on.

Source XML document:

    <p>Do <em>not</em>:
    <ul>
    <li>talk,</li>
    <li>eat, or</li>
    <li>use your mobile telephone</li>
    </ul>
    while you are in the cinema.</p>

Desired output:

    <p>Do <em>not</em>:</p>
    <ul>
    <li>talk,</li>
    <li>eat, or</li>
    <li>use your mobile telephone</li>
    </ul>
    <p>while you are in the cinema.</p>

Solution:

This requires creating a p element around the maximal sequence of sibling nodes that does not include a ul or ol element.

This can be done by using group-adjacent, with a grouping key that is true if the element is a ul or ol element, and false otherwise:

    
    <xsl:template match="p">
        <xsl:for-each-group select="node()" 
                group-adjacent="self::ul or self::ol">
            <xsl:choose>
                <xsl:when test="self::ul or self::ol">
                    <xsl:copy-of select="current-group()"/>  
                </xsl:when>
                <xsl:otherwise>
                    <p>
                        <xsl:copy-of select="current-group()">
                    </p>
                </xsl:otherwise>  
            </xsl:choose>
        </xsl:for-each-group>
    </xsl:template>    

14.1 Multiple Source Documents

The document function, which in XSLT 1.0 was an additional function defined in the XSLT specification, is now a core function defined in XPath 2.0: see [Functions and Operators]. Details of the function have therefore been removed from this specification.

generate-id(document("foo.xml"))=generate-id(document("foo.xml"))

14.2 Reading Text Files

Function: sequence unparsed-text(sequence, string?)

The unparsed-text reads one or more external files and returns the contents of each one as a string.

The first argument is treated as a sequence. Each item in the sequence, when converted to a string, must be a URI. The URI must contain no fragment identifier, and must identify a resource that can be read as text. If the URI is a relative URI, then:

• If the item containing the URI is a node, the relative URI is resolved relative to the base URI of that node;
• If the item containing the URI is a simple value, the relative URI is resolved relative to the base URI of the stylesheet element containing the expression that includes the call to the unparsed-text function.

The second argument, if present, is the name of an encoding. This encoding is used to translate the contents of the file into a string. The values for this attribute follow the same rules as for the encoding attribute in an XML declaration. The only values which every implementation is obliged to recognize are utf-8 and utf-16. If the second argument is omitted, the implementation may attempt to infer the encoding of the file by using external information (such as an HTTP header) or by auto-recognition using techniques such as those described in [XML].

[ERR080] It is a dynamic error if a URI cannot be used to retrieve a resource containing text. The processor must either signal the error, or must recover by treating the URI as if it referenced a resource containing a zero-length string.

[ERR081] It is a dynamic error if a resource contains characters that are not valid XML characters. The processor must either signal the error, or must recover in a implementation-defined way; one possible outcome is that the processor will produce an output file that is not well-formed XML.

[ERR082] It is a dynamic error if a resource contains bytes that cannot be decoded into valid XML characters using the specified encoding. This includes the case where the processor does not support the requested encoding. The processor must signal the error.

[ERR083] It is a dynamic error if the second argument of the unparsed-text function is omitted and the processor cannot infer the encoding using external information.The processor must signal the error.

The result is a sequence of strings, containing one string for each URI in the sequence supplied as the first argument; each string holds the text of the resource retrieved using the URI in the corresponding position of the sequence.

NOTE: If the text file contains characters such as < and &, these will typically be output as < and & when the string is written to the result tree and serialized as XML or HTML. If these characters actually represent markup (for example, if the text file contains HTML), then the stylesheet can attempt to write them as markup to the output file using the disable-output-escaping attribute of the xsl:value-of instruction (see [18.5 Disabling Output Escaping]. Note, however, that implementations are not required to support this feature.

The following example attempts to read an HTML file and copy it, as HTML, to the serialized output file:

<xsl:output method="html"/>

<xsl:template match="/">
  <xsl:value-of select="unparsed-text('header.html', 'iso-8859-1')"
                disable-output-escaping="yes"/>
  <xsl:apply-templates/>
  <xsl:value-of select="unparsed-text('footer.html', 'iso-8859-1')"
                disable-output-escaping="yes"/>
</xsl:template>

14.3 Keys

Keys provide a way to work with documents that contain an implicit cross-reference structure. They make it easier to locate the nodes within a document that have a given value for a given attribute or child element, and they provide a hint to the implementation that certain access paths in the document need to be efficient.

//(MATCH)/self::node()
   [some $k in (for $kk in USE return cast as TYPE ($kk)),
         $v in (for $vv in $V return cast as TYPE ($vv))
         satisfies $k eq $v]

//(MATCH)/self::node()
   [some $k in (for $kk in USE return cast as xsd:string($kk)),
         $v in (for $vv in $V return cast as xsd:string($vv))
         satisfies xf:compare($k, $v, 'COLLATION') = 0]

<xsl:key name="idkey" match="div" use="@id"/>

<!ATTLIST div id ID #IMPLIED>

<prototype name="key" return-type="node-set">
<arg type="string"/>
<arg type="object"/>
</prototype>

<function>key</function>

<xsl:key name="func" match="prototype" use="@name"/>

<xsl:template match="function">
<b>
  <a href="#{generate-id(key('func',.))}">
    <xsl:apply-templates/>
  </a>
</b>
</xsl:template>

<xsl:template match="prototype">
<p><a name="{generate-id()}">
<b>Function: </b>
...
</a></p>
</xsl:template>

<entry name="XSLT">...</entry>

<xsl:key name="bib" match="entry" use="@name"/>

<xsl:template match="bibref">
  <xsl:variable name="name" select="."/>
  <xsl:apply-templates select="document('bib.xml')/key('bib',$name)"/>
</xsl:template>

14.4 Number Formatting

Function: string format-number(double, string, string?)

The format-number function formats a number as a string using the picture string specified by the second argument and the decimal-format named by the third argument, or the default decimal-format, if there is no third argument. The number is supplied as the value of the first argument. The required type of this argument is xsd:double; if the value is not of the right type, it is converted using the standard conversion rules. The decimal-format name must be a QName, which is expanded as described in [4.1 Qualified Names]. The result of the function is the formatted string representation of the supplied number.

[ERR092] It is a dynamic error if the stylesheet does not contain a declaration of the decimal-format with the expanded-QName specified as the third argument. The processor must either signal the error, or must recover by ignoring the third argument.

NOTE: Stylesheets can use other facilities in XPath to control rounding.

14.6 Miscellaneous Additional Functions

<xsl:value-of select="current()"/>

<xsl:value-of select="."/>

<xsl:apply-templates select="//glossary/item[@name=current()/@ref]"/>

<xsl:apply-templates select="//glossary/item[@name=./@ref]"/>

<xsl:apply-templates select="//glossary/item[@name=@ref]"/>

Issue (terminate-as-avt): In xsl:message, should the terminate attribute be changed to be an attribute value template?.

16.1 Extension Functions

If the FunctionName used in a FunctionCall within an XPath expression is not an NCName (that is, if it contains a colon), and if the stylesheet contains no stylesheet function with a matching expanded-QName, then it is treated as a call to an extension function. The QName used as the FunctionName is expanded using the namespace declarations in scope at the point in the stylesheet where the expression appears.

<!-- Convert to
string with extension function --> <xsl:value-of
select="myext:print($myvar)"/> 

16.2 Extension Instructions

The extension instruction mechanism allows namespaces to be designated as extension namespaces. When a namespace is designated as an extension namespace and an element with a name from that namespace occurs in a content constructor, then the element is treated as an instruction rather than as a literal result element. The namespace determines the semantics of the instruction.

NOTE: Since an element that is a child of an xsl:stylesheet element is not occurring in a content constructor, non-XSLT top-level elements are not extension elements as defined here, and nothing in this section applies to them.

17.1 The Principal Result Tree

<!-- Category: declaration -->
<xsl:principal-result-document
  format = qname
  href = { uri-reference } />

The xsl:principal-result-document element is used to define the URI of the principal result tree, and optionally to specify the output format to be used for serializing this tree.

The value of the format attribute, if specified, must be a QName. The QName is expanded using the namespace declarations in scope for the xsl:principal-result-document element. The expanded-QName must match the expanded QName of a named output definition in the stylesheet. This identifies the xsl:output declaration that will control the serialization of the result tree (see [18 Serialization]), if the result tree is serialized. If the format attribute is omitted, the unnamed output definition is used to control serialization of the principal result tree.

[ERR108] It is a static error if the value of the format attribute is not a valid QName, or if it does not match the expanded-QName of an output definition in the stylesheet.

If there is more than one xsl:principal-result-document element at the top level of the stylesheet, the one with highest import precedence is used. [ERR109] It is a static error if there is more than one xsl:result-document element at the top level of the stylesheet with the same import precedence, unless there is also another xsl:result-document element at the top level of the stylesheet with a higher import precedence.

If there is no xsl:principal-result-document element at the top level of the stylesheet, the effect is the same as specifying a single top-level xsl:result-document element with no attributes.

The href attribute defines the URI of the principal result tree. The attribute value template is expanded using a singleton focus based on the document node of the principal source document. The effective value of the attribute must be a URI. There may be implementation-defined restrictions on the form of absolute URI that may be used, but the implementation is not required to enforce any restrictions. Any legal relative URI must be accepted.

If the effective value is a relative URI, then it is resolved relative to an implementation-defined base URI.

The href attribute may be omitted, in which case the URI of the principal result tree is implementation-defined.

NOTE: The main reason to specify a URI for the principal result tree is to allow it to be referenced by links from a secondary result tree.

17.2 Secondary Result Trees

<!-- Category: instruction -->
<xsl:result-document
  format = qname
  href = { uri-reference }>
  <!-- Content: content-constructor -->
</xsl:result-document>

The xsl:result-document instruction is used to create a secondary result tree. The content of the xsl:result-document element is a content constructor; this is evaluated to create a sequence of nodes. A document node is created with this sequence of nodes as its children. The tree rooted at this document node forms the secondary result tree.

The xsl:result-document instruction defines the URI of a secondary final result tree, and may optionally specify the output format to be used for serializing this tree.

The value of the format attribute, if specified, must be a QName. The QName is expanded using the namespace declarations in scope for the xsl:result-document element. The expanded-QName must match the expanded QName of a named output definition in the stylesheet. This identifies the xsl:output declaration that will control the serialization of the result tree (see [18 Serialization]), if the result tree is serialized. If the format attribute is omitted, the unnamed output definition is used to control serialization of the result tree.

[ERR110] It is a static error if the value of the format attribute is not a valid QName, or if it does not match the expanded-QName of an output definition in the stylesheet.

The href attribute is mandatory. The effective value of the attribute must be a URI. There may be implementation-defined restrictions on the form of absolute URI that may be used, but the implementation is not required to enforce any restrictions. Any legal relative URI must be accepted.

If the effective value is a relative URI, then it is resolved relative to the URI of the principal result tree.

A processor may allow a secondary result tree to be serialized, just as it may allow the principal result tree to be serialized. Serialization is described in [18 Serialization]. However, an implementation (for example, a processor running in an environment with no access to writable filestore) is not required to support the serialization of secondary result trees. An implementation that does not support the serialization of secondary result trees must ignore the format attribute. Such an implementation must provide the application with some means of access to the (un-serialized) result tree, using its URI to identify it.

For example, the following would create a principal result document specifying an HTML frameset with two frames, together with two secondary result documents, one for the contents of each frame:

<xsl:template match="/">
  <html>
    <head><title>Frame example</title></head>
    <frameset cols="20%, 80%">
      <frame src="toc.html"/>
      <xsl:result-document href="toc.html">
        <html>
          <head><title>Table of Contents</title></head>
          <body>
             <xsl:apply-templates mode="toc" select="*"/>
          </body>
        </html>
      </xsl:result-document>
      <frame src="body.html"/>
      <xsl:result-document href="body.html">
        <html>
          <head><title>Body</title></head>
          <body>
             <xsl:apply-templates select="*"/>
          </body>
        </html>
      </xsl:result-document>
    </frameset>
  </html>
</xsl:template>

[ERR111] It is a dynamic error to evaluate the xsl:result-document instruction when the current result tree is neither the principal result tree, nor a secondary result tree. The processor must signal the error.. This means, for example, that it is an error to use xsl:result-document when the current result tree is a temporary tree created using xsl:variable, or a tree created using xsl:message, xsl:attribute, etc.

[ERR112] It is a dynamic error for a transformation to generate two or more result trees with the same URI. The processor must signal the error.

Technically, the result of evaluating the xsl:result-document instruction is an empty sequence. This means it does not contribute any nodes to the result of the content constructor it is part of.

Issue (output-newlines): Do we want to say anything about the representation of line endings when using the text output method? Do we want to provide any user control in this area?

Issue (char-representation): Should we provide some control over the way characters are serialized, e.g. as numeric character references or HTML entity references?

18.1 XML Output Method

The xml output method outputs the result tree as an XML entity that should satisfy the rules for either a well-formed XML document entity, or a well-formed XML external general parsed entity, or both. If the document node of the result tree has a single element node child and no text node children, then the serialized output should be a well-formed XML document entity conforming to the XML Namespaces Recommendation [XML Names]. If the result tree does not take this form, then the serialized output should be an entity which, when referenced within a trivial XML document wrapper like this

<!DOCTYPE doc [
<!ENTITY e SYSTEM "entity-URI">
]>
<doc>&e;</doc>

where entity-URI is a URI for the entity, produces a document which should itself be a well-formed XML document conforming to the XML Namespaces Recommendation [XML Names].

In addition, the output should be such that if a new tree was constructed by parsing the XML document as specified in [3 Data Model], then the new tree would be the same as the result tree, with the following possible exceptions:

• If the document was produced by adding a document wrapper, as described above, then it will contain an extra doc element as the document element.

• The order of attribute and namespace nodes in the two trees may be different.

• The base URIs of nodes in the two trees may be different.

• The new tree may contain additional attributes resulting from the expansion of default values in its DTD or schema.

[ERR116] It is a serialization error to request the output of a document type declaration, or of a standalone attribute, if the result tree contains text nodes or multiple element nodes as children of the root node. The processor may signal the error, or may recover by ignoring the request to output a document type declaration or standalone attribute.

The version attribute specifies the version of XML to be used for outputting the result tree. If the processor does not support this version of XML, it should use a version of XML that it does support. The version output in the XML declaration (if an XML declaration is output) should correspond to the version of XML that the processor used for outputting the result tree. The value of the version attribute should match the VersionNum production of the XML Recommendation [XML]. The default value is 1.0.

The encoding attribute specifies the preferred encoding to use for outputting the result tree. Processors are required to respect values of UTF-8 and UTF-16. [ERR117] A serialization error occurs when an output encoding other than UTF-8 or UTF-16 is requested, if the implementation does not support that encoding. The processor may signal the error, or may recover by using UTF-8 or UTF-16 instead. The processor must not use an encoding whose name does not match the EncName production of the XML Recommendation [XML]. If no encoding attribute is specified, then the processor should use either UTF-8 or UTF-16.

It is possible that the result tree will contain a character that cannot be represented in the encoding that the processor is using for output. In this case, if the character occurs in a context where XML recognizes character references (i.e. in the value of an attribute node or text node), then the character should be output as a character reference. [ERR118] A serialization error occurs if such a character appears in a context where character references are not allowed (for example if the character occurs in the name of an element). The processor should signal the error.

If the indent attribute has the value yes, then the xml output method may output whitespace in addition to the whitespace in the result tree (possibly based on whitespace stripped from either the source document or the stylesheet) in order to indent the result nicely; if the indent attribute has the value no, it should not output any additional whitespace. The default value is no. The xml output method should use an algorithm to output additional whitespace that ensures that the result if whitespace were to be stripped from the output using the process described in [3.4 Whitespace Stripping] with the set of whitespace-preserving elements consisting of just xsl:text would be the same when additional whitespace is output as when additional whitespace is not output.

NOTE: It is usually not safe to use indent="yes" with document types that include element types with mixed content.

The cdata-section-elements attribute contains a whitespace-separated list of QNames. Each QName is expanded into an expanded-QName using the namespace declarations in effect on the xsl:output element in which the QName occurs; if there is a default namespace, it is used for QNames that do not have a prefix. The expansion is performed before the merging of multiple xsl:output elements into a single effective xsl:output element. If the expanded-QName of the parent of a text node is a member of the list, then the text node should be output as a CDATA section. For example,

<xsl:output cdata-section-elements="example"/>

would cause a literal result element written in the stylesheet as

<example>&lt;foo></example>

or as

<example><![CDATA[<foo>]]></example>

to be output as

<example><![CDATA[<foo>]]></example>

If the text node contains the sequence of characters ]]>, then the currently open CDATA section should be closed following the ]] and a new CDATA section opened before the >. For example, a literal result element written in the stylesheet as

<example>]]&gt;</example>

would be output as

<example><![CDATA[]]]]><![CDATA[>]]></example>

If the text node contains a character that is not representable in the character encoding being used to output the result tree, then the currently open CDATA section should be closed before the character, the character should be output using a character reference or entity reference, and a new CDATA section should be opened for any further characters in the text node.

CDATA sections should not be used except where they have been explicitly requested by the user, either by using the cdata-section-elements attribute, or by using some other implementation-defined mechanism.

NOTE: This is phrased to permit an implementor to provide an option that attempts to preserve CDATA sections present in the source document.

The xml output method should output an XML declaration unless the omit-xml-declaration attribute has the value yes. The XML declaration should include both version information and an encoding declaration. If the standalone attribute is specified, it should include a standalone document declaration with the same value as the value as the value of the standalone attribute. Otherwise, it should not include a standalone document declaration; this ensures that it is both an XML declaration (allowed at the beginning of a document entity) and a text declaration (allowed at the beginning of an external general parsed entity).

The omit-xml-declaration attribute should be ignored if the standalone attribute is present, or if the encoding attribute specifies a value other than UTF-8 or UTF-16.

If the doctype-system attribute is specified, the xml output method should output a document type declaration immediately before the first element. The name following <!DOCTYPE should be the name of the first element. If doctype-public attribute is also specified, then the xml output method should output PUBLIC followed by the public identifier and then the system identifier; otherwise, it should output SYSTEM followed by the system identifier. The internal subset should be empty. The doctype-public attribute should be ignored unless the doctype-system attribute is specified.

The media-type attribute is applicable for the xml output method. The default value for the media-type attribute is text/xml.

18.2 XHTML Output Method

The xhtml output method serializes the result tree as XML, using the HTML compatibility guidelines defined in the XHTML specification.

It is entirely the responsibility of the stylesheet author to ensure that the result tree conforms to the [XHTML 1.0] or [XHTML 1.1] specification. It is not an error if the result tree is invalid XHTML. Equally, it is entirely under the control of the stylesheet author whether the output conforms to XHTML Strict, XHTML Transitional, XHTML Frameset, or XHTML Basic.

The serialization of the result tree follows the same rules as for the xml output method, with the exceptions noted below. These differences are based on the HTML compatibility guidelines published in Appendix C of [XHTML 1.0], which are designed to ensure that as far as possible, XHTML is rendered correctly on user agents designed originally to handle HTML.

• The serializer should include a space before the trailing / and > of empty elements, e.g. <br  />, <hr  /> and <img  src="karen.jpg"  alt="Karen"  />. Also, the serializer should use the minimized tag syntax for empty elements, e.g. <br  />, as the alternative syntax <br></br> allowed by XML gives uncertain results in many existing user agents.

• Given an empty instance of an element whose content model is not EMPTY (for example, an empty title or paragraph) the serializer should not use the minimized form (e.g. use <p> </p> and not <p  />).

• The serializer should avoid outputting line breaks and multiple whitespace characters within attribute values. These are handled inconsistently by user agents.

• The serializer should avoid use of the entity reference ' which, although legal in XML and therefore in XHTML, is not defined in HTML and is not recognized by all HTML user agents.

  Issue (apos-in-XHTML): The entity reference &apos; should probably be avoided when producing output using the XHTML output method.

• If there is a head element, then unless the include-content-type attribute is present and has the value "no", the xhtml output method should add a meta element immediately after the start-tag of the head element specifying the character encoding actually used. For example,

  <head>
  <meta http-equiv="Content-Type" content="text/html; charset=EUC-JP">
  ...

  The content type should be set to the value given for the media-type attribute; the default value for XHTML 1.0 is text/html. The value application/xhtml+xml, registered in [RFC2336], may also be used.

• Unless the escape-uri-attributes attribute is present and has the value no, the html output method should escape non-ASCII characters in URI attribute values using the method recommended in [RFC2396] (section 2.4.1).

18.3 HTML Output Method

The html output method outputs the result tree as HTML; for example,

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

<xsl:output method="html"/>

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

...

</xsl:stylesheet>

The version attribute indicates the version of the HTML. The default value is 4.0, which specifies that the result should be output as HTML conforming to the HTML 4.0 Recommendation [HTML].

The html output method should not output an element differently from the xml output method unless the expanded-QName of the element has a null namespace URI; an element whose expanded-QName has a non-null namespace URI should be output as XML. If the expanded-QName of the element has a null namespace URI, but the local part of the expanded-QName is not recognized as the name of an HTML element, the element should output in the same way as a non-empty, inline element such as span.

The html output method should not output an end-tag for empty elements. For HTML 4.0, the empty elements are area, base, basefont, br, col, frame, hr, img, input, isindex, link, meta and param. For example, an element written as <br/> or <br></br> in the stylesheet should be output as <br>.

The html output method should recognize the names of HTML elements regardless of case. For example, elements named br, BR or Br should all be recognized as the HTML br element and output without an end-tag.

The html output method should not perform escaping for the content of the script and style elements. For example, a literal result element written in the stylesheet as

<script>if (a &lt; b) foo()</script>

or

<script><![CDATA[if (a < b) foo()]]></script>

should be output as

<script>if (a < b) foo()</script>

The html output method should not escape < characters occurring in attribute values.

If the indent attribute has the value yes, then the html output method may add or remove whitespace as it outputs the result tree, so long as it does not change how an HTML user agent would render the output. The default value is yes.

Unless the escape-uri-attributes attribute is present and has the value no, the html output method should escape non-ASCII characters in URI attribute values using the method recommended in [RFC2396] (section 2.4.1).

The html output method may output a character using a character entity reference, if one is defined for it in the version of HTML that the output method is using.

The html output method should terminate processing instructions with > rather than ?>.

The html output method should output boolean attributes (that is attributes with only a single allowed value that is equal to the name of the attribute) in minimized form. For example, a start-tag written in the stylesheet as

<OPTION selected="selected">

should be output as

<OPTION selected>

The html output method should not escape a & character occurring in an attribute value immediately followed by a { character (see Section B.7.1 of the HTML 4.0 Recommendation). For example, a start-tag written in the stylesheet as

<BODY bgcolor='&amp;{{randomrbg}};'>

should be output as

<BODY bgcolor='&{randomrbg};'>

The encoding attribute specifies the preferred encoding to be used. If there is a HEAD element, then unless the include-content-type attribute is present and has the value "no", the html output method should add a META element immediately after the start-tag of the HEAD element specifying the character encoding actually used. For example,

<HEAD>
<META http-equiv="Content-Type" content="text/html; charset=EUC-JP">
...

The content type should be set to the value given for the media-type attribute; the default value is text/html.

[ERR119] It is possible that the result tree will contain a character that cannot be represented in the encoding that the processor is using for output. In this case, if the character occurs in a context where HTML recognizes character references, then the character should be output as a character entity reference or decimal numeric character reference; otherwise (for example, in a script or style element or in a comment), the processor should signal a serialization error.

If the doctype-public or doctype-system attributes are specified, then the html output method should output a document type declaration immediately before the first element. The name following <!DOCTYPE should be HTML or html. If the doctype-public attribute is specified, then the output method should output PUBLIC followed by the specified public identifier; if the doctype-system attribute is also specified, it should also output the specified system identifier following the public identifier. If the doctype-system attribute is specified but the doctype-public attribute is not specified, then the output method should output SYSTEM followed by the specified system identifier.

The media-type attribute is applicable for the html output method. The default value is text/html.

18.4 Text Output Method

The text output method outputs the result tree by outputting the string-value of every text node in the result tree in document order without any escaping.

The media-type attribute is applicable for the text output method. The default value for the media-type attribute is text/plain.

The encoding attribute identifies the encoding that the text output method should use to convert sequences of characters to sequences of bytes. The default is system-dependent. [ERR120] If the result tree contains a character that cannot be represented in the encoding that the processor is using for output, the implementation should signal a serialization error.

18.5 Disabling Output Escaping

Normally, the xml output method escapes & and < (and possibly other characters) when outputting text nodes. This ensures that the output is well-formed XML. However, it is sometimes convenient to be able to produce output that is almost, but not quite well-formed XML; for example, the output may include ill-formed sections which are intended to be transformed into well-formed XML by a subsequent non-XML-aware process. For this reason, XSLT provides a mechanism for disabling output escaping. An xsl:value-of or xsl:text element may have a disable-output-escaping attribute; the allowed values are yes or no; the default is no; if the value is yes, then a text node generated by evaluating the xsl:value-of or xsl:text element should be output without any escaping. For example,

<xsl:text disable-output-escaping="yes">&lt;</xsl:text>

should generate the single character <.

[ERR121] It is a dynamic error for output escaping to be disabled for a text node that is used for something other than a text node in the result tree. Thus, it is an error to disable output escaping for an xsl:value-of or xsl:text element that is used to generate the string-value of a comment, processing instruction or attribute node; it is also a dynamic error to convert a node to a number or a string if the node is, or contains, a text node for which escaping was disabled. In both cases, the processor must either signal the error, of must recover by ignoring the disable-output-escaping attribute.

When a text node is copied, either by using xsl:copy or by applying xsl:copy-of to that node or one of its ancestors, and when escaping was disabled for some or all of the characters within that text node, then escaping should also be disabled for the resulting copy of those characters. For example

<xsl:variable name="x">
<xsl:text disable-output-escaping="yes">&lt;</xsl:text>
</xsl:variable>
<xsl:copy-of select="$x"/>

should output < not <.

Text nodes for which escaping is disabled are subject to merging with text nodes that are adjacent in the result tree in the same way as text nodes for which is escaping is not disabled. An element or document node in the result tree never has two consecutive text node children. Thus, it is possible for escaping to be disabled for some but not all of the characters in a text node.

The disable-output-escaping attribute may be used with the html output method as well as with the xml output method. The text output method ignores the disable-output-escaping attribute, since it does not perform any output escaping.

A processor will only be able to disable output escaping if it controls how the result tree is output. This might not always be the case. For example, the result tree might be used as the source tree for another XSLT transformation instead of being output. It is implementation-defined whether (and under what circumstances) disabling output escaping is supported. [ERR122] It is a serialization error if an xsl:value-of or xsl:text instruction specifies that output escaping should be disabled and the implementation does not support this. The processor must either signal the error, of must recover by not disabling output escaping.