I thought ID/IDREF was about expressing the integrity of links (IDREF
must have a matching ID), not about context-sensitivity, but then again
I never used it since my links were always to different documents, and
it seemed useless (rather than evil) so I could be wrong.
-Mike
On 02/03/2014 05:05 PM, Costello, Roger L. wrote:
> Hi Folks,
>
> In this message I will attempt to persuade you:
>
> 1. Do not use the ID/IDREF capability.
>
> 2. Use a layering approach:
>
> (a) Layer 1: express your XML as a context-free grammar.
>
> (b) Layer 2: express context-sensitive rules using Schematron.
>
> 3. The ID/IDREF capability is a context-sensitive rule.
>
> Now for my argument:
>
> First, let me persuade you that by using ID/IDREF you have introduced context-sensitive rules into your XML. Consider this XML, which does not use ID/IDREF:
>
> <Book>
> <Title>Principles of Programming</Title>
> <Author>M. A. Jackson</Author>
> </Book>
>
> To show XML's rule nature, let's express it like so:
>
> Book --> Title Author
> Title --> string
> Author --> string
>
> That's a context-free grammar.
>
> Now let's add an ID/IDREF:
>
> <Book seller="Amazon">
> <Title>Principles of Programming</Title>
> <Author>M. A. Jackson</Author>
> </Book>
>
> Assume that @seller is of type IDREF. I don't show the corresponding ID attribute.
>
> Let's express that XML using grammar rules. The rule for the Book element depends on the existence of a corresponding ID attribute; if there is none, the Book rule is invalid. So we may express Book's rule like so:
>
> Book Amazon --> Title Author
>
> Read that as:
>
> In the context of an Amazon symbol
> the Book element may be replaced
> by Title and Author.
>
> In other words, our grammar tells us that this a valid string
>
> Principles of Programming M. A. Jackson
>
> only if the symbol "Amazon" exists.
>
> See the context-sensitivity? Book is context-sensitive due to the ID/IDREF.
>
> Any time you use ID/IDREF in your XML document you have introduced a context-sensitive rule into your XML document.
>
> "So what?" you ask.
>
> Well, here's so what:
>
> All known parsing algorithms for context-sensitive
> grammars are either very inefficient or very complex.
>
> Reasoning about context-sensitive grammars is difficult.
>
> Proofs about context-sensitive grammars is difficult.
>
> Take cue from compiler developers: they separate
> context-sensitive processing into a separate pass.
>
> So don't use ID/IDREF.
>
> Of course, that doesn't mean you will never have data that has intra-data dependencies. What it means is that you should modularize your grammar rules: express your context-free rules in your XML document and express your context-sensitive rules (intra-data dependencies) in Schematron. That's a nice, clean separation-of-concerns. That's a modular data design.
>
> Let's recap:
>
> 1. ID/IDREF introduces context-sensitive rules into your XML grammar wherever there is an ID attribute and wherever there is an IDREF attribute.
>
> 2. Don't use ID/IDREF.
>
> 3. Modularize your rules: express context-free rules in XML and express context-sensitive rules in Schematron.
>
> Comments?
>
> /Roger
>
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