Re: issue #24 does an ixml processor have to match everything?

Coming to the issue for the first time I would assume “maximally consuming” means parsing the entire string.

I think this rather reinforces michael’s point!

_________________
Tomos Hillman
eXpertML Ltd
+44 7793 242058
On 3 Feb 2022, 6:06 PM +0000, C. M. Sperberg-McQueen <cmsmcq@blackmesatech.com>, wrote:
> At the end of the last call, we agreed that at our next call we would
> talk about issue #24, "Does ixml have to match the whole input?"
>
> In preparation for this topic, I would like to ask a procedural question
> and lay out a preliminary position in preparation for our discussion.
>
> ................................................................
>
> The procedural question is: why are we re-opening this question?
>
> We discussed this at some length [1] last April and reached a decision.
> Old decisions should of course be re-opened when new information comes
> to light; my question is: what do we know now that we did not know last
> April when we decided that as far as the definition of conformance to
> the ixml spec is concerned, the answer to the question is "yes"?
>
> [1] https://lists.w3.org/Archives/Public/public-ixml/2021Apr/0007.html
>
> Last April, as the record shows, I argued in favor of prefix-matching
> being conformant behavior but did not carry the day; none of the
> arguments in favor of prefix-matching were found persuasive by the
> majority in the group, and none of the scenarios in which
> prefix-matching is useful or necessary was held to be a scenario a
> conforming ixml processor needs to support.
>
> What has changed?
>
> What has changed for me is fairly simple: the discussion last April
> persuaded me that if we attempt to define prefix-matching as a
> conformant behavior, I will not be happy with the definition. My
> perception is that the group proved incapable of finding a coherent
> formulation, or even a coherent position to formulate, and I concluded
> it would be better to say nothing about prefix-matching than to say
> something incoherent. Recent discussions have not changed my mind; they
> are full of suggestions that the ixml spec prescribe all sorts of
> behaviors and conditions that I think should not be prescribed and which
> make impossible pretty much every single use case I mentioned as cases
> where prefix matching makes sense and is useful.
>
> ................................................................
>
> So my position on issue #24, going in to the discussion, is yes, a
> conforming ixml processor reports that the input, in its entirety, is a
> sentence in the language defined by the specified grammar, or that it is
> not.
>
> That has several consequences:
>
> - A processor that wishes to support input streams of indeterminate
> length will do so as a non-standard extension.
>
> At one point I believe I proposed that conforming processors which
> support extensions to ixml must provide a user option to turn off all
> extensions. I don't think that made it into the spec, but I still
> think it would be a good idea.
>
> - A processor that wishes to offer a mode of operation in which
> successively larger prefixes of the input are identified as sentences
> in the language will do so as a non-standard extension.
>
> - A processor that wishes to inform the user that while the input string
> as a whole is not a sentence in the language, a particular substring
> of the input *is* a sentence in the language can do so as part of its
> error diagnostics. If it falsely reports that the input as a whole is
> a sentence, it is not conforming.
>
> In all three cases, the rationale for the decision is the same: every
> attempt the CG has made to describe how conformance would work if the
> rule were changed has been unsatisfactory.
>
> ................................................................
>
> Since the idea that a processor should "maximally consume" the input
> keeps coming up, it may be worth addressing it directly. I do not think
> the phrase has a clear meaning, but from context I think the idea is to
> require that a processor identify the longest prefix of the input that
> satisfies the grammar. This proposal is problematic in several ways:
>
> 1 It is incoherent in the case of infinite input. It assumes that there
> is a longest prefix of that description, which is not guaranteed the
> case. So in what appears to be the single most compelling use case for
> prefix matching, it is ill defined.
>
> 2 It is counterproductive for those who want an exhaustive enumeration
> of prefixes of the input which are grammatical (similar to the behavior
> of the Prolog 'phrase' predicate described in the April discussion).
>
> 3 It makes the primary use case for ixml (parse this input against this
> grammar and give me the parse tree) into a special case that requires
> special wording (and so far every attempt at sketching that wording has
> been clumsy and inaccurate).
>
> If anyone who has proposed "maximal consumption" of the input meant
> something different, then (a) please explain what you meant, and (b)
> please note how dismally the phrase failed to convey what you meant. It
> would be helpful if you can find a better way to express your meaning.
>
>
>
> --
> C. M. Sperberg-McQueen
> Black Mesa Technologies LLC
> http://blackmesatech.com
>