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

[Gareth Oakes <goakes@gpslsolutions.com>]

Maybe it's fine to have IDs as long as you never reference them? :)


-Gareth

On 4/02/2014 8:23 am, "Michael Sokolov" <msokolov@safaribooksonline.com>
wrote:

>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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>
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