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- 19 reference pages under /xslt/functions/ covering: xsl:value-of, xsl:for-each, xsl:for-each-group, xsl:function, xsl:iterate, last(), position(), count(), concat(), contains(), substring(), sum(), key(), current(), string-join(), tokenize(), matches(), replace(), format-date() - Each page: description, parameters table, return value, 2 runnable examples (input XML + stylesheet + output), notes, cross-links - New Hugo section with reference-specific layout (version badges, syntax block, "Try in Playground" CTA, parameters table styles) - Menu entry "Reference" → /xslt/functions/ - CSS: .ref-badge, .ref-syntax, .ref-index-list, .ref-try + table styles Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
122 lines
4.5 KiB
Markdown
122 lines
4.5 KiB
Markdown
---
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title: "xsl:function"
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description: "Defines a named, callable stylesheet function available in XPath expressions throughout the stylesheet."
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date: 2026-04-18T00:00:00Z
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version: "2.0"
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versionLabel: "XSLT 2.0"
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category: "element"
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syntax: '<xsl:function name="prefix:name" as="return-type">'
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tags: ["xslt", "reference", "xslt2", "xpath"]
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---
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## Description
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`xsl:function` lets you define reusable functions directly inside a stylesheet. Once declared, the function can be called from any XPath expression in the same stylesheet — in `select` attributes, predicates, pattern expressions, or attribute value templates.
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Functions must have a **namespace-qualified name** (a prefix other than `xsl:`). The `as` attribute declares the return type using an XPath sequence type such as `xs:string`, `xs:integer`, `element()`, or `xs:string*`. Parameters are declared with `xsl:param` child elements, each optionally carrying an `as` type constraint.
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Unlike named templates (`xsl:call-template`), stylesheet functions return a value and can be used inline inside XPath expressions. They cannot produce result-tree nodes as a side effect — the function body must evaluate to a sequence that becomes the return value.
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## Parameters
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| Parameter | Type | Required | Description |
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|-----------|------|----------|-------------|
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| `name` | QName | Yes | The qualified name of the function (must include a non-`xsl:` namespace prefix). |
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| `as` | SequenceType | No | Declared return type. Saxon enforces type checking when specified. |
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| `visibility` | `"public"` \| `"private"` \| `"final"` \| `"abstract"` | No | Controls visibility in package/module scenarios (XSLT 3.0). Defaults to `"private"`. |
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| `override` | `"yes"` \| `"no"` | No | XSLT 2.0 only: whether this function overrides an imported function with the same name and arity. |
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### Child elements
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| Element | Description |
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|---------|-------------|
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| `xsl:param` | Declares a parameter. Add one per argument in signature order. |
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| *(body instructions)* | Any XSLT instructions whose final evaluated result is the function's return value. |
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## Return value
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The return value is whatever the last instruction in the function body evaluates to, or whatever is returned by `xsl:sequence`. The type is validated against the `as` attribute if present.
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## Examples
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### String utility function
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**Stylesheet:**
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```xml
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<?xml version="1.0" encoding="UTF-8"?>
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<xsl:stylesheet version="2.0"
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xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
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xmlns:fn="http://example.com/functions"
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xmlns:xs="http://www.w3.org/2001/XMLSchema"
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exclude-result-prefixes="fn xs">
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<!-- Custom function: capitalise the first letter of a string -->
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<xsl:function name="fn:capitalize" as="xs:string">
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<xsl:param name="input" as="xs:string"/>
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<xsl:sequence select="concat(upper-case(substring($input, 1, 1)), substring($input, 2))"/>
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</xsl:function>
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<xsl:template match="/names">
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<xsl:output method="xml" indent="yes"/>
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<result>
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<xsl:for-each select="name">
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<item><xsl:value-of select="fn:capitalize(.)"/></item>
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</xsl:for-each>
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</result>
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</xsl:template>
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</xsl:stylesheet>
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```
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**Input XML:**
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```xml
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<?xml version="1.0" encoding="UTF-8"?>
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<names>
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<name>alice</name>
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<name>bob</name>
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<name>carol</name>
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</names>
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```
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**Output:**
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```xml
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<result>
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<item>Alice</item>
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<item>Bob</item>
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<item>Carol</item>
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</result>
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```
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### Function using string-join
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**Stylesheet (join a sequence with a custom delimiter):**
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```xml
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<?xml version="1.0" encoding="UTF-8"?>
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<xsl:stylesheet version="2.0"
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xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
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xmlns:util="http://example.com/util"
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xmlns:xs="http://www.w3.org/2001/XMLSchema"
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exclude-result-prefixes="util xs">
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<xsl:function name="util:join-tags" as="xs:string">
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<xsl:param name="items" as="xs:string*"/>
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<xsl:sequence select="string-join($items, ' | ')"/>
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</xsl:function>
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<xsl:template match="/article">
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<xsl:output method="text"/>
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<xsl:value-of select="util:join-tags(tags/tag/string())"/>
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</xsl:template>
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</xsl:stylesheet>
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```
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## Notes
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- Function names must use a non-empty namespace prefix. `xsl:function name="myFunc"` is invalid.
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- Recursive calls are supported and commonly used for tree-walking functions.
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- XSLT 3.0 introduces higher-order functions — `xsl:function` can be referenced as a value with `function-lookup()` or passed as an argument.
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- Type annotations (`as` on `xsl:param` and `xsl:function`) improve both performance and early error detection in Saxon.
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## See also
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- [string-join()](../xpath-string-join)
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