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From: wille@ethz.UUCP (M. Wille)
Newsgroups: comp.lang.modula2
Subject: Re: procedure variables ...
Message-ID: <208@bernina.UUCP>
Date: Mon, 5-Oct-87 03:48:28 EDT
Article-I.D.: bernina.208
Posted: Mon Oct  5 03:48:28 1987
Date-Received: Fri, 9-Oct-87 03:19:00 EDT
Reply-To: wille@ethz.UUCP (M. Wille)
Distribution: world
Organization: ETH Zuerich, Switzerland
Lines: 35

MSRS002@ECNCDC.BITNET writes
>Modula allows a variable to be a procedure type, and that variable to
>be assigned the "value" of different procedures.  This is a very usefull
>thing when making programs such as operating systems or I/O drivers, but
>it leads to one question.  Consider the following scrap of code:
 
 PROCEDURE A;
   VAR P: PROC;
   PROCEDURE B;
     VAR X: CARDINAL;
     PROCEDURE C;
       BEGIN
         X := 17
       END C;
     BEGIN
       P := C;
     END B;
   BEGIN
     B; P
   END A ;
  
Do you really think that your code scrap makes any sense. Calling 'C'
outside of its environment does not work in Modula-2. Our compiler
rejects this piece of code, saying that the procedure 'C' in the
assignment 'P := C' is not declared on level 0. Hopefully, other
compilers do the same.

Finally, I will provide you with Wirth's solution of this problem. He
simply restricted the use of assigned procedures to non-local procedures
(see Programming in Modula-2 3rd. Ed., p. 151).
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   Matthias Wille             |  CSNET: wille@ifi.ethz.ch
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