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RE: choice, sequence, all: 'easy' XML Schema question

• To: "'Don Bate'" <donbate@i...>,"'Rick Jelliffe'" <ricko@a...>,<xml-dev@l...>
• Subject: RE: choice, sequence, all: 'easy' XML Schema question
• From: "Michael Kay" <mike@s...>
• Date: Wed, 16 Feb 2005 22:52:39 -0000
• In-reply-to: <p06002000be3950a60495@[192.168.1.42]>
• Thread-index: AcUUYIMrXGcfw2UESUa3nP6kRil6IAAGIbYg

> Huh? Grammars aren't good at expressing position independence? I'm 
> having troubling thinking of any grammars for any language (not just 
> XML) that don't handle constraints similar to the one that the poster 
> describes.

It's common to have such constraints in a language, but they aren't
generally part of the grammar.

For example, a grammar can say that you can have zero or more attributes,
but it can't express the constraint that their names must be distinct.
Similarly, a constraint that you can have any sequence of A, B, and C
elements provided that there's at least one A and not more than three Cs is
not a grammatical constraint.

> XML Schema was a classic case of design by committee with individual 
> features grafted onto it by the different participants. 

Ad hominem arguments always weaken your case. In this case, one could argue
the opposite criticism: that the group was too wedded to the conceptual
integrity of the theoretical framework they had chosen (namely, to design a
grammar-based constraint language), and not flexible enough to bend the
rules to meet practical user requirements.

Michael Kay
http://www.saxonica.com/

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