From: David Kastrup <dak@gnu.org>
To: ludo@gnu.org (Ludovic Courtès)
Cc: 17474@debbugs.gnu.org
Subject: bug#17474: Making *unspecified* equivalent to (values) would seem convenient
Date: Mon, 12 May 2014 18:58:25 +0200 [thread overview]
Message-ID: <87iopbue6m.fsf@fencepost.gnu.org> (raw)
In-Reply-To: <8738gfyoxm.fsf@gnu.org>
ludo@gnu.org (Ludovic Courtès) writes:
> R5RS defines ‘values’ as:
>
> (define (values . things)
> (call-with-current-continuation
> (lambda (cont) (apply cont things))))
>
> Thus, a conforming implementation must raise a run-time error when the
> continuation of a (values) form expects one or more values.
No. From R5RS:
-- procedure: call-with-current-continuation proc
[...]
The escape procedure accepts the same number of arguments as the
continuation to the original call to
call-with-current-continuation. Except for continuations created
by the `call-with-values' procedure, all continuations take
exactly one value. The effect of passing no value or more than
one value to continuations that were not created by
call-with-values is unspecified.
Please reread the last sentence. "unspecified". In fact, passing more
than one value to continuations that were not created by
call-with-values already does not raise a runtime error but instead just
drops the additional values:
(+ (values 4 5) 5) => 9
This patch _provides_ a default value when 0 values are given. That's
filling in a different unspecified behavior than throwing an error, but
Guile already fills in a different unspecified behavior than throwing an
error for multiple values.
So this behavior is neither out of line, nor against the standard. It
is merely a more convenient behavior for a situation that the standard
left unspecified.
--
David Kastrup
next prev parent reply other threads:[~2014-05-12 16:58 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-05-12 11:40 bug#17474: Making *unspecified* equivalent to (values) would seem convenient David Kastrup
2014-05-12 15:53 ` Ludovic Courtès
[not found] ` <8738gfyoxm.fsf@gnu.org>
2014-05-12 16:58 ` David Kastrup [this message]
2014-05-12 19:21 ` Ludovic Courtès
2014-05-12 19:49 ` David Kastrup
2014-06-22 5:25 ` Mark H Weaver
2014-06-22 6:09 ` David Kastrup
2014-06-21 21:30 ` bug#17474: Another point David Kastrup
2014-06-22 5:17 ` Mark H Weaver
2014-06-22 5:45 ` David Kastrup
2014-08-09 9:17 ` bug#17474: Ping? David Kastrup
2014-08-10 19:12 ` Mark H Weaver
2014-08-10 20:26 ` David Kastrup
2014-08-10 21:48 ` Mark H Weaver
2014-08-10 22:00 ` Mark H Weaver
2015-06-01 14:04 ` David Kastrup
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