l...@gnu.org (Ludovic Courtès) writes: > David Kastrup <d...@gnu.org> skribis: > >> l...@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. > > 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. > > Right. > > 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) => () in Guilev1. 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 compatibility. 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 release. -- David Kastrup
