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RE: Let's write extension functions in XSLT itself

From: Michael Kay <mhkay@iclway.co.uk>
Date: Fri, 20 Jul 2001 07:37:30 +0100
To: "'Richard A. O'Keefe'" <ok@atlas.otago.ac.nz>, <xsl-editors@w3.org>
Message-ID: <003401c110e6$7346d930$0100007f@PCUKMKA>
(Unofficial response)

The requirement to write extension functions in XSLT is recognized, and is
included in the published requirements list for XSLT 2.0.

Meanwhile, independently of W3C, a group of users have put together a
specification of a similar facility at http://www.exslt.org/, and this has
been implemented in some vendors' XSLT 1.0 products.

Mike Kay

> -----Original Message-----
> From: w3c-xsl-wg-request@w3.org [mailto:w3c-xsl-wg-request@w3.org]On
> Behalf Of Richard A. O'Keefe
> Sent: 20 July 2001 06:03
> To: xsl-editors@w3.org
> Subject: Let's write extension functions in XSLT itself
> I downloaded the 12-Dec-2000 draft of the XSLT 1.1 recently.
> I notice that there is to be a standard way of defining functions
> that can be called in patterns and expressions, provided those
> definitions are in JavaScript or Java.  That's really really odd.
> You can write part of a style sheet in a language which the processor
> might NOT support, but you you can't write it in a language which
> the processor MUST support.
> I enclose a proposal for defining extension functions in XSLT itself.
> It would be absurd to call out to Javascript (non-portably!) just
> because some calculation can't be expressed in the expresison language
> for want of an 'if' construct.
> ---------------- cut here ----------------
> <!-- File   : xslt-funs.htm
>      Author : Richard A. O'Keefe
>      Updated: 2001.07.19
>      Purpose: Propose a way to define extension functions in
> XSLT itself.
> -->
> <HTML>
> <HEAD>
> <TITLE>Defining Extension Functions in XSLT Itself</TITLE>
> </HEAD>
> <BODY>
> <H1>Defining Extension Functions in XSLT Itself</H1>
> <P>Looking at the 12 December 2000 version of the
> XSL Transformations (XSLT) Version 1.1 specification,
> we see that there is to be a standard way of defining functions
> that can be called in patterns and expressions, provided those
> definitions are in JavaScript or Java.</P>
> <P>This is a very odd state of affairs.  You can write part of a style
> sheet in a language which the processor may <EM>not</EM> support.  But
> you can't write it in a language which the processor <EM>must</EM>
> support.</P>
> <P>Experience with XSLT has already shown that the inability to define
> functions in the expression language can result in code duplication.
> For example, you cannot say "a DIV1, ..., DIV4 are processed
> identically except for using H2, ..., H5 respectively".</P>
> <H2>What we can do about it</H2>
> <P>I propose a single new top level element in the XSLT namespace,
> with three new elements that can appear in it.  First I present the
> specification using DTD syntax, then I explain it.</P>
> <PRE>
> &lt;!ENTITY % type "(Boolean|number|string|node-set|object|any)"&gt;
> &lt;!ELEMENT xsl:function
> (xsl:required*,((xsl:case+,xsl:else)|xsl:value-of))&gt;
> &lt;!ATTLIST xsl:function
>   type %type;  #REQUIRED&gt;
> &lt;!ELEMENT xsl:required EMPTY&gt;
> &lt;!ATTLIST xsl:required
>   type %type;  #REQUIRED&gt;
> &lt;!ELEMENT xsl:case EMPTY&gt;
> &lt;!ATTLIST xsl:case
>   test   CDATA #REQUIRED
>   select CDATA #REQUIRED&gt;
> &lt;!ELEMENT xsl:else EMPTY&gt;
> &lt;!ATTLIST xsl:else
>   select CDATA #REQUIRED&gt;
> </PRE>
> <DL>
> <DT><CODE>xsl:function</CODE></DT>
> <DD>defines an extension function that can
> be used in the expression language.  It has a <CODE>name</CODE>, which
> is a QName.  If the name has a prefix, that prefix must have
> been declared
> as an extension prefix.  If the name does not have a prefix,
> <CODE>#default</CODE> must have been declared as an extension prefix.
> The result type says which of the XSLT types the function is
> supposed to
> return.</DD>
> <DT><CODE>xsl:required</CODE></DT>
> <DD>defines a required formal parameter of the function.  The
> association between formal parameters in a definition and actual
> parameters in a call is by position.  In this draft, there are no
> optional parameters; the number of parameters in a call must match the
> number of parameters declared.  Each parameter has a <CODE>name</CODE>
> and a <CODE>type</CODE>; each actual parameter is converted to the
> required type the same way that built-in functions in the expression
> language are converted to expected types.  The interpretation
> and scope
> of argument names is the same as the interpretation and scope of
> <CODE>xsl:param</CODE> names; however there are no optional
> or "keyword"
> parameters for XSLT-in-XSLT extension functions.</DD>
> <DT>xsl:case</DT>
> <DD>is a construct we wouldn't need if only the designers of XPath had
> included an "<CODE>if</CODE>" construction.  Each
> <CODE>xsl:case</CODE>
> element is tested in document order until a <CODE>test</CODE>
> expression
> comes out true, and then the value of the corresponding
> <CODE>select</CODE>
> expression is the result of the function.</DD>
> <DT>xsl:else</DT>
> <DD>must appear at the end of a conditional construction; its
> <CODE>select</CODE> attribute specifies what result to return
> if none of
> the <CODE>xsl:case</CODE> tests succeeds.</DD>
> <DT>xsl:value-of</DT>
> <DD>is used when the body of the expression is unconditional.</DD>
> </DL>
> <H2>Some examples</H2>
> <P>Absolute value</P>
> <PRE>
> &lt;xsl:function name="my:abs" type="number"&gt;
>   &lt;xsl:required name="x" type="number"&gt;
>   &lt;xsl:case test="$x < 0" select="-$x"&gt;
>   &lt;xsl:else select="$x"&gt;
> &lt;/xsl:function&gt;
> </PRE>
> <P>Converting DIV to H</P>
> <PRE>
> &lt;xsl:function name="my:div-to-h" type="string"&gt;
>   &lt;xsl:required name="node" type="node-set"&gt;
>   &lt;xsl:case test="$node[name()='div1'" select="'h2'"&gt;
>   &lt;xsl:case test="$node[name()='div2'" select="'h3'"&gt;
>   &lt;xsl:case test="$node[name()='div3'" select="'h4'"&gt;
>   &lt;xsl:case test="$node[name()='div4'" select="'h5'"&gt;
>   &lt;xsl:else select="h6"&gt;
> &lt;/xsl:function&gt;
> </PRE>
> <P>Computing a label from a node</P>
> <PRE>
> &lt;xsl:function name="my:label" type="string"&gt;
>   &lt;xsl:required name="node" type="node-set"&gt;
>   &lt;xsl:case test="$node/@id" select="concat('diff-',$node/id)"&gt;
>   &lt;xsl:else select="concat('diff-',generated-id($node))"&gt;
> &lt;/xsl:function&gt;
> </PRE>
> <H2>Discussion</H2>
> <P>This is basically XML syntax for wrapping "if" around a bunch of
> expressions.  It does not appear to require any fundamentally new
> constructions in an XSLT processor; an entire definition should be
> translatable into the internal form used for XSLT expressions.</P>
> <P>There was a note at the end of XSLT 1.0 that conditional
> expressions
> should be considered for a future version of XSLT.  If that were done,
> then these function definitions could be simplified by leaving out
> <CODE>xsl:case</CODE> and <CODE>xsl:else</CODE>.</P>
> <P>Having a type in the <CODE>xsl:function</CODE> element means that
> even should the different cases have different "natural" types, they
> will be converted to a common form, unless the function is explicitly
> typed as "<CODE>any</CODE>".</P>
> <P>The <CODE>data-type</CODE> attribute used in sorting has a
> sufficiently
> different range of options that it did not seem wise to call the
> <CODE>type</CODE> attribute used here <CODE>data-type</CODE>.</P>
> <P>This feature would mean that style sheets could be more portable.
> There are very strong reasons <EM>not</EM> to support Java;
> measurements
> with existing free XSLT processors shows Java versions running 2 to 5
> times slower than a C version, and taking a great deal of memory for
> what seem like simple tasks.  It would be as inappropriate for XSLT
> in practice to <EM>demand</EM> Java support as it would be for XSLT
> to <EM>forbid</EM> Java support.  It would be especially regrettable
> if XSLT were to encourage people to write non-portable scripts simply
> in order to perform calculations that should be within its
> own scope.</P>
> <P>Whether <CODE>xsl:value-of</CODE> and
> <CODE>xsl:else</CODE> should be
> distinct elements is arguable.</P>
> <P>Optional parameters are not in the present design, just to
> keep it simple.
> Another alternative would have</P>
> <PRE>
> &lt;!ELEMENT xsl:function (xsl:required*,xsl:optional*,
>                         ((xsl:case+,xsl:else) | xsl:value-of))&gt;
> &lt;!ATTLIST xsl:optional
>   type   %type;  #REQUIRED
>   select CDATA   #REQUIRED&gt;
> </PRE>
> <H2>The End</H2>
> <P>If you want to discuss this, my e-mail address is
> <A HREF="mailto:ok@cs.otago.ac.nz">ok@cs.otago.ac.nz</A>.</P>
> <p><a href="http://validator.w3.org/check/referer"><img border="0"
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> </BODY>
> </HTML>
> ---------------- cut here ----------------
Received on Friday, 20 July 2001 02:33:53 GMT

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