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For certain classes of grammars, what you say is true, but it is not
a true claim in general. XML Schema's "all" construct opens up the
problem to more than just context-free grammars and simple look-ahead
grammars. Once they opened up that box why didn't they set the
problem space up to include the ability to to specify other useful
grammars. If they had remained pure then I could accept that defense,
but since they didn't, I won't. The lack of purity is one of the
hallmarks of the design-by-committee process where no single point of
view is dominant. It has very little to do with the ability or
motives of the individuals involved and was not meant as a slam
against them individually. It was offered as a warning that users
will find areas that are there for reasons that are incoherent to an
outsider who wasn' t there at the time.
At 10:52 PM +0000 2/16/05, Michael Kay wrote:
> > 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
>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.
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