Re: Proposal for Generic Extensions with Import Clauses

[Andrzej Tarlecki <tarlecki@mimuw.edu.pl>]

Dear Hubert,

We have just had another look at your proposal for generic extensions.

One issue arose: conservativity.

First, you do not mention what is the semantics of

        GEN-CONS-EXT ::= gen-cons-ext IMPORT? PARAMS SPEC+ 

We presume that the conservativity requirement applies here to the
subsequent extensions of PARAMS (by SPEC+), right? 

Another option would be to make this to apply also to the "first
step", ie, to the extension of IMPORT by PARAMS. However, instead of
proposing this, we suggest that perhaps we could consider a more
general solution, where we might have two forms of import, indicating
whether its further extension is required to be conservative or not.

Even though at least one of us (guess who) in general questions the
necessity of the conservativity requirements in structured specs, this
would seem uniform with the way conservativity may be imposed
elsewhere in CASL.

So perhaps we should have:

IMPORT               ::= CONSERVATIVE-IMPORT | JUST-IMPORT
JUST-IMPORT          ::= just-import SPEC
CONSERVATIVE-IMPORT  ::= conservative-import SPEC

[I've adjusted the names of the constructors above, for uniformity. --PDM]

Then there would be no conservativity requirement on the extension of
the import SPEC by PARAMS (either in GEN-EXTENSION or in
GEN-CONS-EXTN) for the JUST-IMPORT clause. If, however, the
CONSERVATIVE-IMPORT clause was used then this would imply that

a) in the generic extension (whether conservative or not), PARAMS
        would have to extend the import SPEC conservatively

b) at each instantiation of this generic spec (whether conservative
        or not), the actual parameters would have to extend the import
        SPEC conservatively as well.

We do not think that any further elaboration (like formal parameters
extend import conservatively but actual parameters need not to, or
vice versa) can be easily justified.

Any reactions?

Best regards,

Michel and Andrzej

[Pending documents: The revised concrete syntax for CASL should appear
 by Thu 18 Dec - including proposals for generic, structured, and
 architectural specifications.  A draft of version 0.99 of the CASL
 Summary should appear by Mon 5 Jan.  Sorry for the delays! --PDM]