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   Re: [xml-dev] choice, sequence, all: 'easy' XML Schema question

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John Graybeal <graybeal@mbari.org> writes:

> If [W3C XML Schema can't require 1 a, 0 or 1 b, any number of c, in
> any order], can you offer a few words about why things were
> constrained in this way?

One reason was that there was disagreement about precedence -- that
is, does

  ( a & b? & c*)


  a c b c 

is allowed, or only

  a c c b


Another (perhaps, with hindsight, misguided) reason was to mitigate
the impact on parser writers.

And finally, there was perhaps a somewhat paternalistic (and therefore
again perhaps misguided) feeling that this is bad markup design - you
shouldn't want to do this.  It makes documents hard to read, and hard
to process.  Allowing something unique (e.g. your 'required_element')
to appear in the midst of a large collection of optional elements is
at best unhelpful.

It also seems likely to arise from a confusion between domain
modelling with document design -- if, as seems likely, order is not
significant (that is, where in the group a child occurs bears no
semantic weight), then good markup design is to choose an order and
require it.

Speaking only for myself,

 Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
                     Half-time member of W3C Team
    2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
            Fax: (44) 131 650-4587, e-mail: ht@inf.ed.ac.uk
                   URL: http://www.ltg.ed.ac.uk/~ht/
[mail really from me _always_ has this .sig -- mail without it is forged spam]


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