XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (251 page)

If your stylesheet processes multiple source documents of different types, similar considerations apply. In this case it might be clearer to use explicit namespace prefixes for everything.

See Also

on page 495

xsl:template

The

element defines a template for producing output. It may be invoked either by matching nodes against a pattern, or explicitly by name.

Changes in 2.0

It is now possible to define a template rule that matches in multiple modes.

The new
as
attribute allows the type of the result to be defined. Moreover, the new

instruction means that a template can now return atomic values and references to existing nodes; it is no longer limited to constructing new nodes.

The syntax for patterns has been extended so that a template rule can now match nodes according to their schema-defined type. The match pattern may also now contain references to global variables or parameters.

Nonsensical combinations of attributes are now considered to be errors; for example, specifying
priority
or
mode
when there is no
match
attribute. In XSLT 1.0, such attributes were ignored.

Format

Position

is a declaration, which means that it always appears as a child of the

element.

Attributes

Name	Value	Meaning
match optional	Pattern	A pattern that determines which nodes are eligible to be processed by this template. If this attribute is absent, there must be a name attribute.
name optional	Lexical QName	The name of the template. If this attribute is absent, there must be a match attribute.
priority optional	Number	A number (positive or negative, integer or decimal) that denotes the priority of this template and is used when several templates match the same node.
mode optional	Whitespace-separated list of mode names, or #all	The mode or modes to which this template rule applies. When is used to process a set of nodes, the only templates considered are those with a matching mode.
as optional	SequenceType	The type of the sequence produced when this template is evaluated. A type error is reported if the result does not match this type.

The construct
Pattern
is defined in Chapter 12, while
SequenceType
is defined in Chapter 11.

The
mode
and
priority
attributes must not be specified unless the
match
attribute is also specified.

Content

Zero or more

elements, followed by a sequence constructor.

Effect

There must be either a
match
attribute or a
name
attribute, or both.

• If there is a
  match
  attribute, the
  
  element defines a template rule that can be invoked using the
  
  instruction.
  
• If there is a
  name
  attribute, the
  
  element defines a named template that can be invoked using the
  
  instruction.
  
• If both attributes are present, the template can be invoked in either of these ways.
  

The match Attribute

The match attribute is a
Pattern
, as defined in Chapter 12. The pattern is used to define which nodes this template rule applies to.

When

is used to process a selected set of nodes, each node is processed using the best-fit template rule for this node, as described under

on page 240.

A template is only considered a candidate if the node matches the pattern supplied in the
match
attribute and if the value of the
mode
attribute matches the
mode
attribute of the

instruction (as described on page 242).

If more than one template rule meets these criteria, they are first considered in order of import precedence (as described under

on page 359), and only templates with the highest import precedence are considered further.

If there is still more than one template rule (in other words, if two template rules that both match the node have the same import precedence), they are next considered in order of priority. The priority is either given by the value of the
priority
attribute, described below, or is a default priority that depends on the
match
pattern. The rules for determining the default priority of any pattern are given in Chapter 12, on page 686.

If this leaves one pattern with a numerically higher priority than all the others, this one is chosen. If there are several with the same priority, which is higher than all the others, the XSLT processor has the choice of reporting an error or choosing from the remaining templates the one that appears last in the stylesheet. Several processors in practice report a warning, which you can ignore if you wish. In my experience, however, this condition often indicates that the stylesheet author has overlooked something.

XSLT 2.0 allows the pattern to contain a reference to a global variable or parameter. This allows a pattern such as
match=“part[@number=$param]”
, which means that the same pattern will match different nodes on different runs of the stylesheet. But there are rules to prevent circular definitions: evaluating the variable must not invoke an

instruction, either directly or indirectly.

There are special rules concerning the handling of errors that occur when matching a node against a pattern. Theoretically, when you execute an

instruction, the processor could evaluate every pattern against the selected nodes. This means that failures can occur if a pattern is inappropriate to the type of node: for example the pattern
B[.=3]
would give a type error when matching an element containing a date. The specification allows the processor to ignore this error and simply treat the pattern as not matching that particular node. This causes few problems in practice, but it creates the danger that real errors go unnoticed. It's best to keep patterns simple enough that runtime errors cannot occur while evaluating them.