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Re: sxml simple, sxml->xml and namespaces

From: Ricardo Wurmus
Subject: Re: sxml simple, sxml->xml and namespaces
Date: Tue, 21 Jun 2016 22:36:18 +0200
User-agent: mu4e 0.9.16; emacs 24.5.1

Andy Wingo <address@hidden> writes:

> On Mon 20 Jun 2016 12:52, Ricardo Wurmus <address@hidden> writes:
>> Andy Wingo <address@hidden> writes:
>>> Apologies for the long delay here.  I'm with you regarding namespaces
>>> and sxml->xml.  In the past I made sure to always get the namespaces
>>> attached to the root element via the @ xmlns attributes, and then have
>>> namespaced uses just be local names, not qnames, and that way sxml->xml
>>> works fine.  But, perhaps that doesn't cover all cases in a nice way.
>>> Do you still have thoughts on this patch?  Is the right thing for you?
>>> In any case we need better documentation in the manual about how to deal
>>> with namespaces and SXML, in practice, with examples.
>> Here is another proposal, mirroring what is done in “xml->sxml”:
> Neat!  Can you elaborate on how it is supposed to work?  In a final form
> it would need documentation, tests, and an update to the docstring, but
> I'd be interested in some xml->sxml->xml round trips as an example.

This is the same patch I sent to the discussion of bug#20339 about a
year ago.

The patch is not very ambitious: it only gives the user a way around the
error by letting them pass an alist of namespaces.  The patched
“sxml->xml” does not attempt to be smart about anything.  It will still
fail if it encounters an undeclared namespace.  My primary goal was to
get around the error.  Maybe “sxml->xml” really should be smarter than

What follows is a copy of my original message:

    >> Since xml->sxml accepts a namespace alist I suppose it would make sense
    >> to extend sxml->xml to do the same.

    Attached is a minimal patch to extend "sxml->xml" such that it accepts an
    optional keyword argument "namespaces" with an alist of prefixes to
    URLs, analogous to "xml->sxml".

    When the namespaces alist is provided, "xmlns:prefix=url" attributes are
    prepended to the element's list of attributes.

        ;; Define SVG document with namespaces
        (define the-svg "<svg xmlns=''
        <rect x='5' y='5' width='20' height='20'
              stroke-width='2' stroke='purple' fill='yellow'
              id='rect1' />
        <rect x='30' y='5' width='20' height='20'
              ry='5' rx='8' stroke-width='2' stroke='purple' fill='blue'
              xlink:href='#rect1' />

        ;; Define alist of namespaces
        (define ns '((svg . "";)
                     (xlink . "";)))

        ;; Convert to SXML, abbreviate namespaces according to ns alist
        (define the-sxml (xml->sxml the-svg #:namespaces ns))

        ;; Convert back to XML
        (sxml->xml the-sxml #:namespaces ns)

        => <svg:svg xmlns:svg="";
             <svg:rect y="5" x="5"
                       fill="yellow" />
             <svg:rect xlink:href="#rect1"
                       y="5" x="30"
                       ry="5" rx="8"
                       fill="blue" />

~~ Ricardo

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