Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!att!alberta!ubc-cs!grads.cs.ubc.ca!manis
From: manis@grads.cs.ubc.ca (Vincent Manis)
Newsgroups: comp.lang.scheme
Subject: Re: "unspecified" and SET!
Message-ID: <2281@ubc-cs.UUCP>
Date: 20 Jun 89 01:48:58 GMT
References: <3902@kalliope.rice.edu> <19890619192610.2.ALAN@PIGPEN.AI.MIT.EDU>
Sender: news@cs.ubc.ca
Reply-To: manis@grads.cs.ubc.ca (Vincent Manis)
Organization: The Invisible City of Kitezh
Lines: 40

I've bitten my tongue on this, but I really can't see the point of
#!unspecified. Perhaps it would be nice in theory (especially if 

(define loop  
  (lambda (x)
      (loop (1+ x))))

returned #!unspecified, which would then be equivalent to `bottom' in
the Scott-Strachey world), but it seems pointless.

One can argue, as somebody has, that the existing behaviour
distinguishes between expressions and commands; however, Scheme already
has a strong notion of side effects (unlike, say, Miranda), and
whether a side-effecting procedure yields (), #f, an unspecified
result, or #!unspecified really makes no difference to anybody, except
in one place, which I'll mention in a moment. 

I regard `returns an unspecified result' as an injunction not to place
a call to a procedure in a value-returning position, e.g., the last
step in a lambda or begin. If I wrote a Scheme compiler, fascist that
I am, I would probably generate an error message for such a usage.
Going to the trouble to develop a calculus of unspecified values
strikes me as almost as silly as some of the things I did when I was
involved in writing an Algol 68 compiler.

The one place where #!unspecified makes some sense is in
Read-Eval-Print loops. One can use #!unspecified as a trigger that the
result is not to be printed. But then one does not need to change the
language: PC Scheme, for example, has *the-non-printing-object*. 

If it ain't broke, don't fix it.
____________  Vincent Manis                    | manis@cs.ubc.ca
___ \  _____  The Invisible City of Kitezh     | manis@cs.ubc.cdn
____ \  ____  Department of Computer Science   | manis%cs.ubc@relay.cs.net
___  /\  ___  University of British Columbia   | uunet!ubc-cs!manis
__  /  \  __  Vancouver, BC, Canada V6T 1W5    | (604) 228-2394
_  / __ \  _  "Theoretical computer science helps me convince people that
____________  my indecisiveness is really Nondeterminism, which sounds like
              a much more positive characteristic." -- a student