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From: stolfi (Jorge Stolfi)
Newsgroups: comp.lang.modula3
Subject: Re: Local exceptions
Message-ID: <9105211642.AA01692@jumbo.pa.dec.com>
Date: 21 May 91 16:41:56 GMT
Lines: 57
To: gnelson
Cc: m3
X-Folder-Carbon: lang-des


    [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.

By the way, here is another situation where the Report seems
a bit ambiguous about static versus dynamic scopes:

    MODULE M2 EXPORT Main;
    
      PROCEDURE MemeChose(n: INTEGER; p: PROCEDURE():INTEGER): BOOLEAN =
        
        PROCEDURE G(): INTEGER = 
          BEGIN RETURN n END G;
          
        BEGIN
          IF n = 0 THEN 
            RETURN p = G
          ELSE 
            RETURN MemeChose(n-1, G)
          END
        END F;

      PROCEDURE Irrelevant(): INTEGER = BEGIN RETURN 0 END P0;

    BEGIN
      <* ASSERT NOT MemeChose(1, Irrelevant) *>
    END M2.

Should the ASSERT fail or succeed?  

The question is whether all instances of G are the same procedural
value, i.e. whether the "environment" component of a procedure value
is a static scope or a dynamic scope.  To further confound the issue,
the Report calls G a procedure *constant*...

(The SRC compiler does the right thing, by the way.)

--jorge