Path: utzoo!mnetor!uunet!mcvax!ukc!its63b!aiva!jeff
From: jeff@aiva.ed.ac.uk (Jeff Dalton)
Newsgroups: comp.lang.misc
Subject: Re: Multiple values
Message-ID: <313@aiva.ed.ac.uk>
Date: 31 Mar 88 22:47:38 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> <8121@sol.ARPA>
Reply-To: jeff@uk.ac.ed.aiva (Jeff Dalton)
Organization: Dept. of AI, Univ. of Edinburgh, UK
Lines: 40

Gaynor@porthos asks
:How would Scheme's continuations fit into the area of multiple
:return values?

In article <8121@sol.ARPA> quiroz@cs.rochester.edu (Cesar Quiroz) writes:

>In a capsule:
>1- Returning == Calling a continuation
>so
>2- Returning multiple values == Calling a continuation with multiple
>arguments.

>I believe Scheme doesn't have this class of continuations.  (Scheme's
>continuations take only one argument). However, that this modification
>is possible has been known for a long time [...], so I wouldn't be
>surprised if at least one implementation has supported multiple values.

T does more or less what you suggest.  You can write

     (receive-values receiver sender)

to call receiver with the values returned by sender.  Sender should
be a "thunk" that returns the right number of values when called.
Return is used to return multiple values.  So

     (receive-values
       (lambda (a b c) (list a b c))
       (lambda () (return 1 2 3)))     =>  (1 2 3)

Then,

     (reveive (<variable>*) <values-expr> <expr>*)
         ==
            (receive-values
              (lambda (<variable>*) <expr>*)
              (lambda () <values-expr>))

Jeff Dalton,                      JANET: J.Dalton@uk.ac.ed             
AI Applications Institute,        ARPA:  J.Dalton%uk.ac.ed@nss.cs.ucl.ac.uk
Edinburgh University.             UUCP:  ...!ukc!ed.ac.uk!J.Dalton