Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!samuel
From: samuel@bifrost.cs.cornell.edu (Samuel M. Weber)
Newsgroups: comp.lang.misc
Subject: Re: swap(x,y) in Algol 60
Message-ID: <31728@cornell.UUCP>
Date: 2 Sep 89 20:43:17 GMT
References: <31690@cornell.UUCP>
Sender: nobody@cornell.UUCP
Reply-To: samuel@cs.cornell.edu (Samuel M. Weber)
Organization: Cornell Univ. CS Dept, Ithaca NY
Lines: 16
Keywords: swap, algol

In article <31690@cornell.UUCP> bard@cs.cornell.edu (Bard Bloom) writes:
>I have seen a folk theorem that it is impossible to define swap(x,y) in
>Algol, where swap is supposed to be a procedure with two call-by-name integer
>arguments which exchanges their values.
>
>I've asked around MIT and some at Cornell, and nobody seems to have a
>reference to the statement or proof.  Anyone remember where this came from?

On page 428 of the new Dragon book it says "A correctly working version
of swap apparently cannot be written if call-by-name is used.", and
references Fleck, A. C. "The impossibility of content exchange through
the by-name parameter transmission technique." SIGPLAN Notices 11:11
(November 1976)

    -- Sam Weber
       samuel@cs.cornell.edu