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In article <INFO-M2%87093023534491@UCF1VM> Info-Modula2 Distribution List <INFO-M2%UCF1VM.bitnet@jade.berkeley.edu> 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 ;
>	  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.
>

Maybe this is why Wirth states emphatically (Programming in Modula-2 ed. 3,
pp. 82):

    To conclude, we emphasize the restriction that procedures that are assigned
    to variables or are passed as parameters, must not be declared local to any
    other procedure.

Therefore any procedure represented by a procedure variable will reference only
local variables or global variables.

Note that in Pascal, which only has procedure parameters, this problem does
not exist, since that at the point where the procedure is passed as a parameter
to a subroutine, if the procedure is accessable then all the activation records
it needs must exist and will continue to exist at least until the return from
the subroutine. Therefore the restriction Wirth mentions regarding Modula-2 does
not exist in Pascal.

(I purposely used different terms, i.e. procedure and subroutine to
differentiate between the called procedure and the procedure passed as a
parameter). 


	Avi Bloch
National Semiconductor (Israel)
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Herzlia 46104, Israel
(011-972) 52-522263
avi%taux01@nsc