Re: [xml-dev] How to represent mixed content in JSON and JSON Schema?

["Norman Gray" <norman@astro.gla.ac.uk>]

Amy, hello!

On 13 Jul 2018, at 2:29, Amelia A Lewis wrote:

> Hmmmm.
>
> On Fri, 13 Jul 2018 01:01:17 +0100, Norman Gray wrote:
> > On 12 Jul 2018, at 15:58, Liam R. E. Quin wrote:
> > > Yes. We saw this also back in Perl days, with some XML libraries 
> > > using
> > > a mix of an array for contents and a hash for attributes,
> [snip]
> > Entertainingly, the XML spec does not in fact explicitly require that
> > elements be presented to the application in document order.  But it
> > omits that requirement on the grounds (and I think I can cite chapter
> > and verse on this) that such a requirement is so screamingly obvious
> > that it would be bloody silly to spell it out.
> Section 3.2.1? The specification of the contents of a document type
> definition provides productions for 'choice' and for 'seq', and note:
> "Any content particle in a choice list may appear in the element
> content at the location where the choice list appears in the grammar;
> content particles occurring in a sequence list must each appear in the
> element content in the order given in the list."
Ah no, you're not getting away that easily.  That specifies that order 
is constrained _in the source document_, but it is magisterially silent 
on the order in which those elements are presented to the processing 
application.  If the parser wants to save up all of the <strong> 
elements and deliver them in a big reveal at the end, with trumpets, 
then who are we to deny it its god-given right to do so?!  The quality 
of syntax is not strained.  It droppeth as the gentle rain from 
tag-salad, upon the application beneath.

> It is, I suppose, merely an implication that a documented validated by
> DTD would be presented to the application in the same order that it 
> had
> to be presented to the validator or validating parser, but since the
> validator can be conceived as an application, I think it's a fairly
> strong implication.
But is that written down?! (plus a couple more !! for good luck)

Of course, at this point I start to feel like the rather desperate 
lawyer saying 'but your honour, the legislation doesn't actually _say_ 
that drink-driving remains illegal if my... if a driver were at the same 
time wearing a clown wig and playing the national anthem on the kazoo 
(and whilst I appreciate my client's latest address to the court may 
have been too slurred to be readily intelligible, your honour should 
infer no animadversions from the admittedly unfortunate gestures which 
accompanied it)'

> A somewhat stronger doubt might be thrown by suggesting that since DTD
> validation is optional, non-validating parsers need not present 
> content
> in order, but here the mere existence of section 3.2.2, and the 
> concept
> of mixed content, pretty much mandates that children (elements and 
> text
> nodes) have to be presented in the order they are encountered, or
> significant information is lost.
I think that the 'problem' here is that the XML spec is almost entirely 
concerned with the syntax of the source document, and says remarkably 
little about its semantics.  That's because the semantics of a parse are 
so obvious, and the semantics of a pointy-bracket parse so immediately 
in the SGML background of most folk reading the XML spec at that time, 
that it would be merely obfuscatory to rehearse them.  I think XML was 
trying to hum a different practical tune from the high-minded legalism 
of ISO 8879 and friends.

> [...] and who insisted
> that the Namespaces in XML 1.0 specification had simply misspoken when
> it distinguished in namespace handling for attributes and elements, 
> and
> therefore implemented his code to treat attributes identically to
> elements). I can *hear* your colleague insisting "But it doesn't *say*
> that, does it? Ha!"
What can one say?  Except: loading-bay -- bare-knuckles -- now!

> I can't imagine how the Perl code you describe handled mixed content,
> though. Did it just not support it? Concatenate all the text nodes (or
> better: throw away all the text nodes after the first, or replace the
> m_text members value with each new text node, effectively discarding
> all but the last) and set them as an m_text member, separate from the
> m_children hash, which contained only elements? And how did it
> distinguish between replacing a child versus multiple children of the
> same name? Oh, well ... long ago, in a different country, and the code
> is dead, I suppose.
I think I have cleansed my head of the memory of what it actually did, 
but I suspect that it was originally intended for parsing XML 
configuration files (let's not start...), and thus used in contexts 
where document order didn't matter.

As so often happens, exasperation found expression in a wall of text, 
which resulted in <http://text.nxg.me.uk/2010/1yfs>.  That, as so often 
happens, "fell deadborn from the press, without reaching such 
distinction as even to excite a murmur among the zealots."

Best wishes,

Norman


--
Norman Gray  :  https://nxg.me.uk