Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!bu.edu!inmet!stt
From: stt@inmet.inmet.com
Newsgroups: comp.lang.modula3
Subject: Re: Local exceptions
Message-ID: <370700001@inmet>
Date: 29 May 91 15:18:00 GMT
Lines: 41
Nf-ID: #R:<9105180946.AA24240@jumbo.pa.dec:-37:inmet:370700001:000:1531
Nf-From: inmet.inmet.com!stt    May 29 11:18:00 1991


Re: non-top-level exceptions
> /* Written 12:41 pm  May 21, 1991 by stolfi@src.dec.com */
> 
>     [Greg Nelson:] The question is, what do you want to be the result
>     of P(TRUE) after:
>     
>         PROCEDURE P(b: BOOLEAN) RAISES ANY = 
>           EXCEPTION E;
>           BEGIN
>             IF b THEN 
>               TRY P(NOT b) EXCEPT E: (* Skip *) END
>             ELSE
>               RAISE E
>             END
>           END P;
> 
> I suppose the most useful interpretation is that each *static*
> occurrence of "EXCEPTION Id" defines a new exception, irrespective
> of how many times it is "executed".  Then all occurrences of E in the
> above example refer to the same exception.
> 
> Admittedly this is not consistent with the principle of structural
> type equivalence, but is at least consistent with the semantics of
> BRANDED T.

Ada took this path to defining the meaning of exceptions declared
in a recursive procedure, namely that each "static" occurrence defines
a unique exception.  My recommendation:  Avoid the problem.  Stick
with top-level only.  It is of marginal use to allow local exceptions,
and the butchery done to the normal model of declarations is not worth
the trouble.

Furthermore, it really gets nasty with "generics" in Ada,
since each instantiation is considered a new "static" occurrence,
requiring replication of exceptions even if otherwise the code
can be shared between two generic instantiations.

S. Tucker Taft     stt@inmet.inmet.com
Intermetics, Inc.
Cambridge, MA  02138