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

[=?UTF-8?B?UGlvdHIgQmHFhHNraQ==?= <bansp@o2.pl>]

Hi Roger,

Politicians should learn from you :-)

You put two mechanisms into a black box, which, you argue, has 
context-sensitive properties when viewed from outside:

> 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.
(This point actually could use some scrutiny, given how effortlessly you 
conflate conditions on the presence of an attribute with conditions on 
its content, but let's assume it to be OK for the purpose of the argument)

And then you add a premise from complexity, which is true in particular 
but not applicable in the context:

> All known parsing algorithms for context-sensitive
> 	grammars are either very inefficient or very complex.
You are implying here that what you have dressed up as a black box is 
treated as such by XML parsers, but it's not: all that an IDREF(s) 
requires is presence of an identical ID anywhere in the document, 
period, no baroque rule-parsing needed for that.

I think you've missed on that particular point, but, as usual, congrats 
on stirring up a discussion on related topics. :-)

Best,

  Piotr


On 03/02/14 23:05, 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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