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From: MSRS002@ECNCDC.BITNET.UUCP
Newsgroups: comp.lang.modula2
Subject: Procedure Types & Variables
Message-ID: <INFO-M2%87093023534491@UCF1VM>
Date: Wed, 30-Sep-87 23:15:00 EDT
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Posted: Wed Sep 30 23:15:00 1987
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not be using any normal I/O modules, indeed, it may be some burried
program for which the term I/O doesn't really apply.  Anyhow, the
module which was called outside its environment would take some sort
of exception.

It would be perfectly acceptable to allow this sort of call if the
procedure being called did not reference any variables global to itself,
except perhaps variables defined in the module itself.

Any thoughts on this matter?

                                    R
                                    Rec
                                    Recurs
                                    Recursive
                                    Recursively yours,
                                          Tom Ruby
                                       MSRS002@ECNCDC
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 ;

  BEGIN
    B ;
    P ;
  END A ;

Now, I realize this wouldn't compile because I forgot to say END B, but
once that was finished, what would you expect to happen at run time?
When procedure C assigns the value 17 to X, X does not exist because the
activation record for B does not exist.

I haven't actually tried this on any compilers, but I would expect the
same sort of "crash" as you get when you assign a value to a dereferenced
pointer after the pointer has been de-allocated, that is, you set some
word in memory to 17, and nobody really knows what word that is.

I can think of a device to catch this error, if it is considered an error.
Each procedure would be assigned a unique number by the compiler, and this
number would be placed in the activation record at run time.  When a
procedure is entered, either directly or through a variable, it would check
the activation records for procedure numbers to check if it is in the proper
environment.  If not, it would issue some sort of helpful ( please )
message, then terminate the program.  Of course, in Modula, you're not
always sure what "Issue a Message" means since the application program may