Path: utzoo!utgpu!water!watmath!clyde!ima!think!barmar
From: barmar@think.COM (Barry Margolin)
Newsgroups: comp.lang.misc
Subject: Re: The need for D-scussion (was Re: D Wishlist)
Message-ID: <18643@think.UUCP>
Date: 30 Mar 88 02:57:33 GMT
References: <12176@brl-adm.ARPA> <1988Mar11.215238.976@utzoo.uucp> <719@l.cc.purdue.edu> <10763@mimsy.UUCP> <3177@haddock.ISC.COM> <18580@think.UUCP> <Mar.28.12.43.35.1988.1511@porthos.rutgers.edu>
Sender: usenet@think.UUCP
Reply-To: barmar@fafnir.think.com.UUCP (Barry Margolin)
Organization: Thinking Machines Corporation, Cambridge, MA
Lines: 33

In article <Mar.28.12.43.35.1988.1511@porthos.rutgers.edu> gaynor@porthos.rutgers.edu (Silver) writes:
>barmar@think.COM (Barry Margolin) writes:
>> (setq z (multiple-value-bind (quotient remainder)
>> 	      (floor x y)
>> 	    (+ (* quotient quotient) (* remainder remainder))))
>  How would Scheme's
>continuations fit into the area of multiple return values?

Continuations would have to be extended to be able to take multiple
arguments.  In order to precisely duplicate Common Lisp multiple-value
semantics, the above MULTIPLE-VALUE-BIND would call FLOOR with a
continuation of

(lambda (continuation &optional quotient remainder &rest ignore)
  (continuation (+ (* quotient quotient) (* remainder remainder))))

(I'm using CL lambda-list syntax because I'm not very familiar with
Scheme syntax).  This duplicates the feature of allowing the function
to return a different number of values than those explicitly
requested, where extra values are ignored and values not provided are
defaulted to NIL.

(values ...) is then equivalent to (continuation ...) and (values-list
list) is (apply continuation list).

Converting the rest of the multiple-value forms is similarly
straightforward.

Barry Margolin
Thinking Machines Corp.

barmar@think.com
uunet!think!barmar