Re: [xml-dev] ID/IDREF is evil

[Michael Kay <mike@saxonica.com>]

I think most people would classify the ID/IDREF constraint as extra-grammatical, rather than saying that it makes the grammar context sensitive.

I can think of plenty of reasons not to use ID/IDREF without that one. #1 being that you need one of those ghastly DTDs.

Michael Kay
Saxonica


On 3 Feb 2014, at 22:05, Costello, Roger L. <costello@mitre.org> 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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