[BLACKBOX] EBNF question

From: [at]} <Bob>
Date: Wed, 15 Feb 2012 23:23:19 -0000

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Evening all,

this is not strictly on topic, but since EBNF is one of Wirth's languages I
hope people won't feel too angry with me for raising it here.

I need to define an interface file structure which is intended to be a
simplified version of a really quite complex XML schema. I am sketching the
structure of the file using a variety of notations, including EBNF, to make
sure I understand it properly before I document it for programmers who will
be producing files with this structure.

An excerpt of the EBNF in question is broadly as follows:

activity = fixedPart mandatoryParts { option } .
fixedPart = <easy to define> .
option = <easy to define>

The mandatoryParts is what I'm seeking help with. It consists of several
different structure types which are all of 1..M cardinality - in ebnf:
structure { structure }.

The difficult aspect is that each structure type must occur at least once,
and they may occur in any order.

This means that I can't write:

mandatoryParts = a {a} b {b} c {c} ... z{z} .

because that imposes the order a, b, c, ..., z

I can't write

mandatoryParts = { a {a} | b {b} | c {c} | ... | z {z} } .
or the equivalent
mandatoryParts = { a | b | c | ... | z }

because that makes each one optional.

The best I've come up with is to enumerate all permutations, which is
impractical because there are so many. For example:

mandatoryParts = ( A B C | A C B | B C A | B A C | C A B | C B A ) { a | b |
c } .
A = a {a} .
B = b {b} .
C = c {c} .
...

As you can see, it would quickly become ridiculous as the number of
mandatory types increases.

Any ideas for something succinct and general which captures this?

Thanks,
Bob

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Received on Thu Feb 16 2012 - 00:23:19 UTC