GNU bug report logs - #17474
Making *unspecified* equivalent to (values) would seem convenient

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Message #54 received at 17474 <at> debbugs.gnu.org (full text, mbox):

From: David Kastrup <dak <at> gnu.org>
To: Mark H Weaver <mhw <at> netris.org>
Cc: 17474 <at> debbugs.gnu.org
Subject: Re: bug#17474: Ping?
Date: Mon, 01 Jun 2015 16:04:25 +0200

Mark H Weaver <mhw <at> netris.org> writes:

> David Kastrup <dak <at> gnu.org> writes:
>> So you think that it will be more "lightweight" if (values) does not
>> have an immediate representation but rather creates a multiple-values
>> object on the heap?
>
> I don't have time to continue this discussion, but I wanted to respond
> to this one point: there should be a single global
> statically-allocated instance of the multiple-values object containing
> zero values, and the procedures that create multiple-values objects
> would always use that one.

So we are in agreement that a single object (I don't see a meaningful
distinction between "immediate" and "single global statically-allocated"
that is consistently being used) with SCM_NO_VALUES semantics makes
sense.

Where we don't agree is about making it the same as SCM_UNSPECIFIED.  It
is my contention that the concepts for SCM_NO_VALUES and SCM_UNSPECIFIED
overlap too much to offer the user a meaningful distinction allowing him
to make a qualified choice between returning one of the two.

I consider it a good hint that the REPL already considers *unspecified*
and (values) similar enough that it prints exactly the same for either
input, namely nothing.

So I'm pretty convinced that the user is better off without having to
deal with two separate representations of "no useful value".

-- 
David Kastrup

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