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Re: New to Xproc Question : conditionnal "output port" definition?

From: Norman Walsh <ndw@nwalsh.com>
Date: Wed, 12 Oct 2011 06:59:18 -0400
To: XProc Dev <xproc-dev@w3.org>
Message-ID: <m2botmh3w9.fsf@nwalsh.com>
Matthieu Ricaud-Dussarget <matthieu.ricaud@igs-cp.fr> writes:
> Actually Calabash raises this error :
> GRAVE: It is a static error if two subpipelines in a p:choose declare 
> different outputs.

This is one of the inconvenient corners in XProc. You have to arrange for all
the branches of a p:choose (and a p:try/p:catch) to produce the same outputs.

You've also hit another corner case.

The WG decided that the most common case for a p:choose was one where
each branch produced a single output. In that case, putting in a
p:error branch was inconvenient because it didn't produce any output,
so you had to put some sort of a dummy step after it in order to make
sure that that branch conformed to the rule. But because that step
wasn't going to have any data-flow connection to the p:error, you also
had to wire it up explicitly. All of this was terribly inconvenient.

So the WG decided to give p:error a primary output port. Nothing will
ever appear on it, but it satisfies the rule about consistent outputs.

Except, in your case, the other branch *doesn't* produce any output.


Maybe we should have increased the burden on implementors instead and
stated that any branch that must end in a p:error doesn't count. But
we didn't.

Here's the fix:

> <p:choose name="stop_if_viewport_inconsistence">
> <p:when
> test="count(distinct-values(/igs:aggregate/doc/pages/page/@viewportX))!=1">
> <p:error name="viewport_err" code="viewport_err">
> <p:input port="source">
> <p:inline>
> <message>viewports inconsistence here !</message>
> </p:inline>
> </p:input>
> </p:error>


> </p:when>
> <p:otherwise>
> <p:xslt name="generateECF">
> <p:input port="source">
> <p:pipe step="add-viewport" port="result"></p:pipe>
> </p:input>
> <p:input port="stylesheet">
> <p:document href="../xslt/generateECF.xsl"/>
> </p:input>
> <p:input port="parameters"><p:empty/></p:input>
> </p:xslt>
> <p:store encoding="UTF-8" omit-xml-declaration="false" indent="true">
> <p:with-option name="href" 
> select="concat(/ecf:content/opf:metadata/dc:identifier[@id=parent::opf:metadata/@unique-identifier],'.ecf')"></p:with-option>
> </p:store>
> </p:otherwise>
> </p:choose>

That p:sink consumes the output of p:error and as a result, neither
branch has a primary output port and so the rules for p:choose are

BTW, you don't need to say:

  <p:output port="result">

If you're pipeline doesn't have any output, simply don't declare any output ports.

                                        Be seeing you,

Norman Walsh
Lead Engineer
MarkLogic Corporation
Phone: +1 413 624 6676

Received on Wednesday, 12 October 2011 10:59:52 UTC

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