Xref: utzoo comp.arch:10642 comp.lang.misc:3077
Path: utzoo!attcan!utgpu!watmath!watdragon!watyew!dwtill
From: dwtill@watyew.waterloo.edu (Dave Till)
Newsgroups: comp.arch,comp.lang.misc
Subject: Multiple Return Values From Functions (was Re: Language Tenets)
Keywords: multiple-valued expressions, function parameter lists
Message-ID: <15141@watdragon.waterloo.edu>
Date: 14 Jul 89 15:38:56 GMT
References: <1207@quintus.UUCP>
Sender: daemon@watdragon.waterloo.edu
Reply-To: dwtill@watyew.waterloo.edu (Dave Till)
Followup-To: comp.lang.misc
Organization: Computer Science Department, University Of Waterloo
Lines: 45

In article <1207@quintus.UUCP> pds@quintus.UUCP (Peter Schachte) writes:
>All is not lost, though.  In C, you can have
>
>	divrem(top, bottom, &quotient, &remainder)
>
>and something similar is possible with VAR parameters in Pascal (and
>Modula-2, I think).  This is symmetrical and reasonably elegant.  More
>elegant, I think, than complicating a language with multiple value
>expressions without adding enough power to do anything other than assign
>them to something.

I think it would be more elegant to allow the returning of multiple
values, as in

	[quotient,remainder] = divrem(top,bottom);

because then it would be clear that "quotient" and "remainder" are return
values, not input parameters or in-out parameters.
Using multiple return values also allows
the user to remove unnecessary dereferencing operations from the
called function.

>Basically, there's not much you can do with a multiple-value expression
>except ignore all but the first value (which happens if you nest a MV
>expression within a function call) and assign its results to variables.

I may have misunderstood what you are saying here, but I disagree:  I
believe it is possible to use a multi-valued expression as part of a
parameter list for a function call.  In fact, I believe that a parameter
list for a function call *is* a multiple-value expression.

I am trying to determine the applications of multiple-value expressions
and multiple return values from functions
as part of my master's thesis work, and would be happy to discuss this
by e-mail with anyone who is interested.  (I have written a C preprocessor
which implements constructs such as the above, and am trying to figure
out how useful these constructs are.)

Followups directed to comp.lang.misc, since this isn't really an
architecture-related issue any more.
--
--Dave Till, Computer Science Department, University of Waterloo
(dwtill@watyew.waterloo.edu)  ...<known universe>!watmath!watyew!dwtill
"Bottle, say after me, I must learn the difference between a lion and
a spider." -- Eccles to Bluebottle, the Goon Show, "Ned's Atomic Dustbin"