XML processing instructions - Duncan Hothersall via ntg-context

From: Duncan Hothersall via ntg-context <ntg-context@ntg.nl>
To: mailing list for ConTeXt users <ntg-context@ntg.nl>
Cc: Duncan Hothersall <dh@capdm.com>
Subject: XML processing instructions
Date: Mon, 2 May 2022 08:19:33 +0100	[thread overview]
Message-ID: <CAN8fp9XKyzDi2xXgBHnFdcTtJnyQHEttWx7hWhB=M_iGOUPeNA@mail.gmail.com> (raw)

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I have a big set of existing XML books (held in a derivative of DocBook)
which I'm looking to start processing directly with ConTeXt. (Up to now I
have a system which converts the XML into ConTeXt code which is then
processed, but this is inefficient and lots of the code is now unsupported.)

I've had some success producing output, but my first real sticking point
has come with processing instructions. The existing XML contains lots of
processing instructions of the form
<?capdm whatever?>, some of which can be conditional and introduce new data
etc. But I'd be happy at this stage if I could just process the most basic
one of them, which is used to introduce a line stop in a running paragraph
of text.

My best guess at how to do this was to use the lxml.preprocessor function
to convert the processing instruction into an element, and then process the
element as normal. But (a) my attempt didn't work, and (b) there may well
be a better way.

Minimal working example below, except that obviously the processing
instruction bit doesn't work!

Thanks for any help or insights.

Duncan

MWE:
------

\startbuffer[demo]
<book>
  <para>A paragraph with<?capdm force_line_stop?>a processing
instruction.</para>
</book>
\stopbuffer

\startxmlsetups xml:demo:base
 \xmlsetsetup{#1}{*}{xml:demo:*}
\stopxmlsetups
\xmlregisterdocumentsetup{demo}{xml:demo:base}

\startxmlsetups xml:demo:book
 \xmlflush{#1}
\stopxmlsetups

\startxmlsetups xml:demo:para
 \xmlflush{#1}\endgraf
\stopxmlsetups

\startluacode
 function lxml.preprocessor(data,settings)
  return string.find(data,"<?capdm *force_line_stop?>")
   and string.gsub(data,"<?capdm
*force_line_stop?>","<capdmlinestop></capdmlinestop/>")
   or data
 end
\stopluacode

\startxmlsetups xml:demo:capdmlinestop
 \crlf
 \xmlflush{#1}
\stopxmlsetups

\setupbodyfont[modern]
\starttext
\xmlprocessbuffer{demo}{demo}{}
\stoptext

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