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From: throopw@dg_rtp.UUCP (Wayne Throop)
Newsgroups: net.lang.c
Subject: swap by name
Message-ID: <431@dg_rtp.UUCP>
Date: Thu, 3-Jul-86 16:55:27 EDT
Article-I.D.: dg_rtp.431
Posted: Thu Jul  3 16:55:27 1986
Date-Received: Fri, 4-Jul-86 04:47:03 EDT
References: <1201@brl-smoke.ARPA> <193@stracs.cs.strath.ac.uk> <733@ho95e.UUCP> <1836@brl-smoke.ARPA>
Lines: 30

> gwyn@brl-smoke.UUCP (Doug Gwyn (VLD/VMB) <gwyn>)
> It may be amusing and/or instructive to contemplate the fact that
> there is no way to write a function that exchanges the contents of
> two variables in a language where parameters are passed "by name".

Amusing, instructive.... and wrong.

I am assuming that you mean "Given a language which passes arguments to
functions with call-by-name semantics, it is impossible write an
'exchange argument pair' function, no matter what other facilities the
language provides".  This is clearly incorrect.  In fact, the C language
is a counterexample (assuming that the C language includes the
preprocessor).

Consider: macros act much like functions with by-name arguments.  The
trick is to utter the swap while mentioning the arguments only once.
Thus, one needs a bind-by-reference operation, which in C is nicely
provided by the '&' operator (with some restrictions, granted).

My conclusion: a swap function is easy to implement even with
pass-by-name functions, *provided* there is a bind-by-reference operator
in the language.

--
It is a lovely language, but it takes a very long time to say anything
in it, because we do not say anything in it, unless it is worth taking
a long time to say, and to listen to.
                                                ---     J. R. R. Tolkien
-- 
Wayne Throop      <the-known-world>!mcnc!rti-sel!dg_rtp!throopw