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From: kwh@bentley.UUCP (KW Heuer)
Newsgroups: net.lang
Subject: Re: Multiple assignments
Message-ID: <825@bentley.UUCP>
Date: Thu, 15-May-86 22:30:54 EDT
Article-I.D.: bentley.825
Posted: Thu May 15 22:30:54 1986
Date-Received: Sat, 17-May-86 04:57:19 EDT
References: <1455@mmintl.UUCP>
Organization: AT&T Bell Laboratories, Liberty Corner
Lines: 38

In article <1455@mmintl.UUCP> franka@mmintl.UUCP (Frank Adams) writes:
>... Supposing that "$" is being used for this ["multi-assignment"] operator,
>then to set both "a" and "b" to zero, one would write "a $ b = 0", rather
>than "a = b = 0", as C does....  

Some dialects of BASIC use "LET A,B = 0" for this.

>On the other hand, if your language supports argument passing by address,
>it permits an output argument to be multiply assigned; something which
>assignment-with-value does not support.

What do you mean by this?  I don't see how this operator allows you to do
anything that simple assignment doesn't.  Examples?

I was going to suggest a valueless assignment operator ":=" (a := b = 0),
but now I think your idea is a little closer to what I wanted.  Instead of
"a $ b = 0" , let's write that symmetrically as
	a := }
	     } 0
	b := }
or (since R-to-L assignment is a historical wart)
	  { =: a
	0 {
	  { =: b
Now, to put that into a form that can be parsed,
	0 fork { =: a | =: b }
The advantage over "$" is that this allows the duplicated value to be used
in other contexts, like the C idiom "while ((c = getchar()) != EOF)".

The observant observer has noticed that what I have is quite close to the
FORTH syntax "0 dup a ! b !" ("!" is the store operator), except that I've
been explicit about the parallelism.

Btw (somebody is bound to point this out, may as well be me), the result of
"a = b = x" in C is not really "b = x; a = x" as implicitly assumed above;
it's "b = x; a = b".  (It makes a difference if a and b differ in width.)

Karl W. Z. Heuer (ihnp4!bentley!kwh), The Walking Lint