Subject: Re: MSC-News: Proposed changes to MSC'96 grammar to make it parsable.
From: Oystein Haugen (etooha#eto.ericsson.se)
Date: Fri May 15 1998 - 09:19:13 GMT
The originator of this message is responsible for its content.
-----From Oystein Haugen <etooha#eto.ericsson.se> to mscnews -----
Jan Docekal wrote:
> The originator of this message is responsible for its content.
> -----From Jan Docekal <jan.docekal#telelogic.se> to mscnews -----
> Dear MSC friends,
> I hope that you will remain friends after you have read this letter.
Dear JanYou are still my friend after this, and I believe your work will
make MSC an even more friendly language.
> Problem 1: Substitutions
> The problem with this grammar is that an LALR(1) parser (and possibly
> human reader) has a problem with deciding when substitution elements
> that may contain commas end (This is due to the fact that <replace
> rules may contain commas and that <substitution list>s are separated
> commas. Consider the following substitution example:
When it comes to substitutions, we have the following challenges:1. The
grammar is not properly parsable as pointed out above, and it must be
2. The formal semantics of substitution is still not finalized. (i.e.
the current Annex B just standardized omits substitution)
3. The practical writing of substitutions is space consuming partly due
to the use of keywords "subst" and "by".
> Proposed solution:
> The keyword subst should be dropped. The following syntax should be
> for substitutions (examples analogous to the two substitutions above):
> reference R (a1, b1, c1 := a2, b2, c2; d1, e1, f1:= d2, e2, f2)
> reference R (a1, b1, c1 := a2, b2, c2, d1, e1; f1:= d2, e2, f2)
> No grammar will be given for this syntax proposal at this point since
> the syntax might be changed base on Your feedback. If the syntax is
> accepted by the MSC community, a grammar will naturally be provided.
I do not suspect that the work on the semantics of substitution will
affect this discussion, but in principle the semantic exploration may
result in minor modification desires as well.There is no doubt that we
should make the grammar parsable, but we need not do this as thoroughly
as Jan suggests. As far as I can see, parentheses can do the same job
and we may be backwards compatible with the original MSC-96, meaning
that the example would read:
reference R subst (a1,b1,c1) by (a2, b2,c2), (d1,e1,f1) by (d2,e2,f2) ;
The grammar is trivial to update.
One problem with my solution is that the size of the clause will
increase and not decrease. Still I think that the backward compatibility
and the general style of MSC point in the direction of using "subst" and
> Problem 2: Event "labels"
> Proposed solution:
Do whatever you find practical since it is not seen by real users.
-- ------------------------ Oystein Haugen, Ericsson as. , P.O. box 34, N-1361 Billingstad, Norway Tel: +47 66 84 23 46 Fax: +47 66 84 19 15 E-mail: etooha#eto.ericsson.se
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