From: David Kastrup <dak@gnu.org>
To: guile-devel@gnu.org
Subject: Re: when and unless
Date: Wed, 07 Dec 2011 10:23:25 +0100 [thread overview]
Message-ID: <87liqoivle.fsf@fencepost.gnu.org> (raw)
In-Reply-To: 20111206230556.GA14717@yarrow.destinee.acro.gen.nz
"Chris K. Jester-Young" <cky944@gmail.com> writes:
> On Tue, Dec 06, 2011 at 11:08:08PM +0100, David Kastrup wrote:
>> > Have you considered using `(values)' as your way of saying, "I'm not
>> > returning any values"?
>>
>> Testing for that is not all that much fun. It is also rather useless
>> since pretty much all of the call-for-effect functions of Guile return
>> *unspecified* rather than (values).
>
> You're not really supposed to test for it.
Lilypond is not Scheme but has a more complex syntax. You can use
Scheme in a lot of places with different implications on the grammar
depending on the type of the expression. It would not be feasible to
create a separate Scheme calling operator for every possible type of
expression. And "called just for action" is one such type.
>> It is not clear to me why (values) can't just evaluate to a single
>> *unspecified* just like '() evaluates to null. Outside of
>> call-with-values, I don't see much need to treat it special.
>
> Implementing that would pretty much either require CPS transforms all
> around (then you'd look at the arity of the continuation), or else
> you'd have to be keeping track of the arity of the current
> continuation some other way. Is it just me, or does that smell like
> Perl's wantarray?
Well, you'd need to have
(call-with-values (lambda () *unspecified*) (lambda x (length x))) => 0
That's the actual clincher I presume? If one takes a look at the Scheme
language definition, one finds for one thing:
(define (values . things)
(call-with-current-continuation
(lambda (cont) (apply cont things))))
And, more importantly:
Except for continuations created by the call-with-values procedure,
all continuations take exactly one value.
That means that one _only_ needs to consider the implications on
call-with-values continuations.
And it is not like Guile has a problem distinguishing content and
package itself (at least Guile 1.8):
guile> (write *unspecified*)
#<unspecified>guile> (write (values))
#<values ()>guile>
So I still don't quite see the problem that would arise from making
(eq? *unspecified* (values)) => #t
--
David Kastrup
next prev parent reply other threads:[~2011-12-07 9:23 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-06-30 10:44 when and unless Andy Wingo
2011-06-30 21:46 ` Ludovic Courtès
2011-07-01 7:50 ` Andy Wingo
2011-07-01 12:47 ` Ludovic Courtès
2011-12-05 20:23 ` Andy Wingo
2011-12-06 7:48 ` Marijn
2011-12-06 8:29 ` Alex Shinn
2011-12-06 11:17 ` David Kastrup
2011-12-06 16:25 ` Andy Wingo
2011-12-06 16:42 ` David Kastrup
2011-12-06 17:35 ` Andy Wingo
2011-12-06 22:08 ` David Kastrup
2011-12-06 23:05 ` Chris K. Jester-Young
2011-12-07 9:23 ` David Kastrup [this message]
2011-12-07 15:58 ` Chris K. Jester-Young
2011-12-08 8:42 ` David Kastrup
2011-12-08 15:34 ` Chris K. Jester-Young
2011-12-08 16:10 ` David Kastrup
2011-12-08 18:10 ` Ian Price
2011-12-07 16:10 ` Chris K. Jester-Young
2011-12-06 19:05 ` Chris K. Jester-Young
2011-12-06 19:33 ` Andy Wingo
2011-12-06 14:39 ` Ludovic Courtès
2011-12-07 14:19 ` Ludovic Courtès
2011-12-07 14:27 ` David Kastrup
2012-01-07 0:16 ` Andy Wingo
2012-01-07 22:36 ` Ludovic Courtès
2012-01-20 20:19 ` Andy Wingo
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