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From: bdb@cl.cam.ac.uk (Brian Brunswick)
Newsgroups: comp.lang.icon
Subject: Re: Multiple value assignment in icon???
Message-ID: <1991Mar16.142726.5947@cl.cam.ac.uk>
Date: 16 Mar 91 14:27:26 GMT
Article-I.D.: cl.1991Mar16.142726.5947
References: <1991Mar13.180536.22613@cl.cam.ac.uk> <1991Mar15.013415.1499@midway.uchicago.edu>
Reply-To: bdb@cl.cam.ac.uk (Brian Brunswick)
Organization: U of Cambridge Comp Lab, UK
Lines: 67

In article <1991Mar15.013415.1499@midway.uchicago.edu> goer@quads.uchicago.edu (Richard L. Goerwitz) writes:
>Bdb@cl.cam.ac.uk (Brian Brunswick) writes (with regard to the problem
>of assigning a series of results to a series of variables):
>>
>>Something like every (x|y):=f() is no good, of course.
>>
>
>This is sooo close to working.... [explanation of what goes wrong]
>
>Aha!  We've suddenly run into one very good use for coexpressions.
>Here's what to do:
>
>    val := create f()
>    every (x|y) := @val

Yup, I'd spotted that this was possible, but was put off by the extra
temporary variable needed - I'd really like something on one line.
One can't even do things like ...@(val := create f()), since rhs
evaluated multiple times when lhs is resumed. I did experiment with
putting inteligence in a wrapper function, but it doesn't have enough
information to work safely enough to withstand mistakes.

>Now here's an excercize to see if you fully grasp how Icon de-
>references variables.  Why will the following NOT work the way you
>want it to?
>
>    var := create (x|y)
>    every val := f() do
>        var := val

Urble ... create makes its own copy of local variables ... but would
they even survive undereferenced (ug!) to be (pointlessly) used?  I
suppose globals would work.

>Referring to a solution offered (but not quoted here):

Which was using a wrapper of ![...] around the procedure return values
to turn them into anonymous variables, which enables the calling line
to use a multiple swapping assignment to do rotation amongst several
variables and accumulate results that way. ( every z:=:y:=:x:=:f() )

>
>> Am I missing something, or does this irritation spoil an otherwise
>> pretty language?
>
>Your solution isn't particularly idiomatic.  You clearly sensed this.
>Hence your posting.  Your question was really very good, though, be-
>cause you've clearly grasped precisely the sort of situation that makes
>coexpressions useful and elegant additions to the language.  You aren't
>missing a thing, but rather demonstrating an understanding of the sort
>of logic that led to the implementation of coexpressions....
>
>I hope this helps.
>
>-Richard Goerwitz (goer@sophist.uchicago.edu)

Hmm... I'm not so sure that I don't prefer my cludge to having to
introduce an extra intermediate variable. Of course, its only good so
long as its clearly recognised as an idiom by the reader, otherwise
its needless obfuscation.

Also, I hesitated somewhat at creating a co expression just to do
something that short that I'm likely to use quite a lot. Isn't that
likely to be quite expensive in terms of garbage produced? Or would
reusing the same temporary to hold it mean that reference counting or
something rescues things?
Brian.Brunswick@uk.ac.cam.cl  Disclaimer.  Short sig rules!