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From: ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe)
Newsgroups: comp.lang.misc
Subject: Re: Dynamic typing (part 3)
Message-ID: <5388@goanna.cs.rmit.oz.au>
Date: 24 Apr 91 08:21:53 GMT
References: <1366@optima.cs.arizona.edu> <1991Apr15.183805.10544@maths.nott.ac.uk>
Organization: Comp Sci, RMIT, Melbourne, Australia
Lines: 37

In article <1991Apr15.183805.10544@maths.nott.ac.uk>,
 anw@maths.nott.ac.uk (Dr A. N. Walker) writes [about Algol 60]:
> In any case, dynamic
> *own* arrays are a pig to compile;  again, they were [?universally]
> omitted from implementations.

If I remember correctly, Burroughs Algol supported them.
There never was any particular problem in compiling them;
the problem was in chosing from several interpretations of what
they should mean (not unlike C++, just like a head cold is not
altogether unlike the Plague).  The interpretation which was
adopted for Algol 60.1 is that the bounds are determined the
first time that the declaration is elaborated, and don't change
after that.  So
	for n := 1, n + 2 while n < 13 do begin
	    own integer array a[1:n];
	    ...
	end;
would have the array declaration compiled to something like
	    static int already_done = 0;
	    static struct Int_1D a;
	    if (!already_done) {
		a.lb1 = 1;
		a.ub1 = n;
		a.data = malloc((a.ub1-a.lb1+1) * sizeof *(a.data));
		already_done = 1;
	    }
This is a pig to compile?
Of course, if the loop is
	for n := 11, n - 2 while n > 0 do begin
	    own integer array a[1:n];
	    ...
	end;
then you may be in trouble ...
-- 
Bad things happen periodically, and they're going to happen to somebody.
Why not you?					-- John Allen Paulos.