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bug#17474: Making *unspecified* equivalent to (values) would seem conven
bug#17474: Making *unspecified* equivalent to (values) would seem convenient
Mon, 12 May 2014 21:49:43 +0200
Gnus/5.13 (Gnus v5.13) Emacs/24.4.50 (gnu/linux)
address@hidden (Ludovic Courtès) writes:
> David Kastrup <address@hidden> skribis:
>> address@hidden (Ludovic Courtès) writes:
>>> R5RS defines ‘values’ as:
>>> (define (values . things)
>>> (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.
> Oh indeed, I stand corrected.
>> 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.
> I’m not completely convinced it makes sense to “specify” the zero values
> case in this way, but I’d like to hear what others think.
The real awkwardness is not when feeding (values) to a single-value
continuation but rather the ability of single-value expressions being
able to pass zero values to a multi-value continuation.
In Guilev1, there was a lot of those around, and indeed
(call-with-values (lambda () (car (list (values)))) list) => ()
But in Guile 2.0.9, I actually get the same, and I don't think people
reported bugs for that.
This design does not win a beauty contest. But at least it fits several
parts of Guile and the API together in a less ugly and more satisfactory
manner than before while providing a really large amount of backward
I don't really know whether there are assumptions in the compiler it
might break. So this change would certainly also want testing with
large applications and should not be introduced shortly before a stable