Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!bard
From: bard@gjalp.cs.cornell.edu (Bard Bloom)
Newsgroups: comp.lang.misc
Subject: swap(x,y) in Algol 60
Keywords: swap, algol
Message-ID: <31690@cornell.UUCP>
Date: 1 Sep 89 19:18:03 GMT
Sender: nobody@cornell.UUCP
Reply-To: bard@cs.cornell.edu (Bard Bloom)
Organization: Cornell Univ. CS Dept, Ithaca NY
Lines: 10

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.  It's not exactly clear what the
statement of the theorem should be when (say) x and y have side effects.

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?

Thanks,
  Bard Bloom