2014-11-21 11:00:20 -05:00
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<!---
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Copyright (C) 2014 LoVullo Associates, Inc.
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Permission is granted to copy, distribute and/or modify this
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document under the terms of the GNU Free Documentation License,
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Version 1.3 or any later version published by the Free Software
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Foundation; with no Invariant Sections, no Front-Cover Texts, and no
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Back-Cover Texts. A copy of the license is included the file
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COPYING.FDL.
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-->
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2014-11-21 10:53:19 -05:00
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# Higher-Order XSL
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hoxsl is a library for XSLT 2.0, written in pure XSLT, introducing
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various types of higher-order logic; this includes higher-order
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functions and XSLT templates that take XSLT as input and produce XSLT
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as output.
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The system is fully tested---see the test cases for additional
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documentation as this project gets under way.
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## Higher-Order Functions
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[Higher-order functions][xslt-30-ho] are a part of XSLT 3.0, but
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implementations that support them (such as Saxon) hide it behind
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proprietary versions of their software. Others still may wish to
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continue using XSLT 2.0.
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There are various approaches/kluges for this problem in earlier
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versions of XSLT; this implementation is motivated by Dimitre
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Novatchev's work on [higher-order functions in FXSL][nova-ho].
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For example, consider an implementation of a filter function that
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accepts a node set and a predicate:
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```xml
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<xsl:function name="my:filter" as="xs:element()*">
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<xsl:param name="nodes" as="xs:element()*" />
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<xsl:param name="pred" />
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<xsl:for-each select="$nodes">
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<xsl:if test="f:apply( $pred, . )">
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<xsl:sequence select="." />
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</xsl:if>
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</xsl:for-each>
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</xsl:function>
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<xsl:function name="my:pred" as="xs:boolean">
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<xsl:param name="node" as="element()" />
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<xsl:sequence select="$node/@foo = 'true'" />
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</xsl:function>
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```
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We could then apply a filter using this predicate like so:
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```xml
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<sequence select="my:filter( $nodes, my:pred() )" />
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```
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hoxsl takes this a step further by providing a stylesheet to generate
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the boilerplate necessary for functions to be able to be applied using
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`f:apply`, as shown above. Applying `tranform/apply-gen.xsl` to the
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XSL stylesheet containing the above function definitions would produce
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output that can be directly imported (as a stylesheet); no additional
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work is needed. This can be included as part of a build process, and
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the output included within a distribution.
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## License
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This program is free software: you can redistribute it and/or modify it
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under the terms of the GNU General Public License as published by the Free
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Software Foundation, either version 3 of the License, or (at your option)
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any later version.
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This program is distributed in the hope that it will be useful, but WITHOUT ANY
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WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
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PARTICULAR PURPOSE. See the GNU General Public License for more details.
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[nova-ho]: http://conferences.idealliance.org/extreme/html/2006/Novatchev01/EML2006Novatchev01.html
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[xslt-30-ho]: http://www.w3.org/TR/xslt-30/#dt-higher-order-operand
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