Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!hp4nl!kunivv1!ge
From: ge@kunivv1.sci.kun.nl (Ge' Weijers)
Newsgroups: comp.object
Subject: Re: Continuations
Message-ID: <581@kunivv1.sci.kun.nl>
Date: 12 Dec 89 16:01:39 GMT
References: <2664@bingvaxu.cc.binghamton.edu> <9624@pyr.gatech.EDU> <1623@odin.SGI.COM> <1989Nov28.183816.15252@odi.com> <9964@june.cs.washington.edu> <34657@cornell.UUCP> <48742@ricerca.UUCP>
Organization: University of Nijmegen, The Netherlands.
Lines: 38

chase@Ozona.orc.olivetti.com (David Chase) writes:
>However, the languages listed differ in their treatment of nested
>procedures:

>C         forbids them  (no closures).

>Pascal    allows them, but (in Pascal Classic, at least) forbids
>          their assignment to variables, use as parameters to
>	  procedures, or use in a RETURN.  (no closures, because it
>	  turns out that a display can be used).

Not quite true, in the original Pascal procedures were allowed as parameters,
though type-safety goes out of the window, as the parameters were not indicated.

PROCEDURE example (PROCEDURE error);
BEGIN
	IF NOT (assertion)
	THEN error(4711);
END example;

I believe something like this was actually legal. In more current versions of
the language the procedure must be typed.
mr. Wirth probably did not dare to throw away the Algol-60 call by name
at the time, as some people thought is was useful. They still are.

>Modula-3  allows them, but forbids their assignment to variables or
>          their use in a RETURN.  (closures needed, but must obey
>	  stack lifetime rules).  (I hope this example isn't too
>	  obscure -- I'm rather familiar with it at the moment).

Just like in Algol 68. Only the lifetime rule was very tricky there:
the lifetime of a procedure is the lifetime of the 'youngest' non-local
name accessed. The lifetime of a function not referencing any globals was
therefore infinite, whereever it was declared.
Ge' Weijers                                    Internet/UUCP: ge@cs.kun.nl
Faculty of Mathematics and Computer Science,   (uunet.uu.net!cs.kun.nl!ge)
University of Nijmegen, Toernooiveld 1         
6525 ED Nijmegen, the Netherlands              tel. +3180612483 (UTC-2)