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From: steven@mcvax.uucp (Steven Pemberton)
Newsgroups: net.lang
Subject: Re: nested procedures.
Message-ID: <7030@boring.mcvax.UUCP>
Date: Mon, 4-Aug-86 11:57:41 EDT
Article-I.D.: boring.7030
Posted: Mon Aug  4 11:57:41 1986
Date-Received: Tue, 5-Aug-86 03:51:18 EDT
References: <335@graffiti.UUCP> <149@opus.UUCP> <7026@boring.mcvax.UUCP> <337@ubc-cs.UUCP>
Reply-To: steven@boring.uucp (Steven Pemberton)
Organization: CWI, Amsterdam
Lines: 34
Apparently-To: rnews@mcvax


In article <337@ubc-cs.UUCP> ludemann@ubc-cs.UUCP (Peter Ludemann) writes:
[ Example program from original posting ]
> This is a good example of why nested procedures should NOT exist.  
> The procedure writeElement has a side-effect: modifying the variable 
> "first".  Because this example is small, it is easily comprehended, 
> but a larger example could be quite incomprehensible.

I'm unclear about this point. Procedures by definition have side-effects.
Maybe you mean that it modifies a non-local variable, but that is another
question altogether. Anyway, if the example is comprehensible, it stands.
Any use of procedures could become incomprehensible, but that's no argument
against procedures.

> I've written lots of these kinds of systems using a language with no 
> nested procedures.  There is a straightforward way of handling such 
> cases: pass the control information explicitely to the "action" 
> procedure.  This requires an extra initialisation procedure: 
> 
> 	procedure forEach(seq: value; 
> 			  procedure actionInit(var control: bool);
> 			  procedure action(var control: bool; elem: value));
> 	begin
> 	    var control: bool;
> 	    actionInit(control);
> 	    { For each element e of sequence seq, call action(control, e) }
> 	end;

It also requires a new version of forEach for every different type of action
procedure you need to call. Even the example program requires two: one for
writeElement which has a boolean parameter, and another for loop which has a
'location' parameter.

Steven Pemberton, CWI, Amsterdam; steven@mcvax.uucp
Lrf, vg'f ebg guvegrrarq!