[SCL] Representing constraints that go beyond EBNF

[Murray Altheim]

John F. Sowa wrote:
> Pat,
> 
> EBNF can be defined as a subset of Horn-clause logic, which is
> a subset of FOL.  The constraints that go beyond EBNF can be
> defined in any version of FOL, including SCL.  That would give
> a complete specification of SCL in SCL.
> 
> JS> In mathematics, a common representation for a graph consists
>  > of a set of nodes and a set of arcs.  Since sets are by definition
>  > unordered, no further comment is needed.
> 
> PH> That kind of an answer isn't going to be acceptable to the Perl
>  > hackers, though. They want a machine-readable syntax for specifying
>  > it, which is what I was also looking for.

Me too.

> There are machine executable implementations of Z.  So it would be
> possible to specify everything in Z (assuming that EBNF was first
> translated to Z).  But I doubt that Z is what the average Perl hacker
> would like to see.
> 
> PH> A grammar written in ISO EBNF *with some additions* is not written
>> in ISO EBNF.
[...]

Agreed. And obviously, it would be far better to use EBNF without any
extension. But still better to use EBNF and an extension than either
not provide a *mostly* machine-readable grammar at all. While EBNF can
be fed directly into a automated parser generator, I've still written
parsers by hand using "EBNF+".

I've not been following this discussion very closely because this has
been a particularly busy week for me, but it *seems* that the argument
here goes a bit further than is necessary in stating the difference
between EBNF and what is required for SCL. Am I correct in thinking
that the only thing that EBNF doesn't provide in your minds is unordered
sets? The reason I ask is that if that's the case, then the very simple
addition of maybe one grammatical element to EBNF might be enough, *if*
that's the case.

Thing is, I *thought* you could already do this in EBNF. If, for example,
you look at the W3C Recommendation for XML, it has an EBNF grammar for
XML included in the spec. There certainly seems to me (and I could be
wrong, but I've been doing this stuff for a long time) as having the
ability to create unordered collections of tokens (e.g., elements in a
document). The XML spec even provides the EBNF for the DTD grammar,
which again, has both ordered (e.g., '+','*') and unordered (',') syntax.

    http://www.w3.org/TR/REC-xml

I'm guessing I'm missing something here. What is it that EBNF can't
provide?

Murray

----
PS. I have had a somewhat lengthy private discussion with Steve Newcomb,
and while they aren't providing an EBNF for the Topic Map Reference Model,
my understanding of his response is that he doesn't see any reason why it
couldn't be created. They've employed a mathematician to write up a set-
theoretical model of the assertion model of the RM, to be included (I
believe) in the ISO spec. But the echoes between what SCL and the RM are
trying to accomplish are to my mind very similar, strikingly so.
...........................................................................
Murray Altheim                         http://kmi.open.ac.uk/people/murray/
Knowledge Media Institute
The Open University, Milton Keynes, Bucks, MK7 6AA, UK                    .

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