Andy Wingo <wi...@pobox.com> writes: > which is strictly unspecified, as it is returning an unexpected number > of values to a continuation. > > Guile 1.8: > > guile> (eqv? (values) (values)) > #f > > Guile 2.0: > > scheme@(guile-user)> (eqv? (values) (values)) > ERROR: In procedure values: > ERROR: Throw to key `vm-error' with args `(vm-run "Zero values returned > to single-valued continuation" ())'. > > Not a nicely printed error, but oh well. > > Guile 2.0 returns a canonical unspecified value in this situation. I > would like to consider returning 0 values instead in the future, but > figuring out how to do so without breaking the world is tricky. It's > useful to hear about your experiences with *unspecified*. > >> I am working on a language where returning values in certain contexts >> might at one point of time might lead to the values being used. So I >> need to implement warnings to that effect in order to find out calls >> _not_ returning *unspecified*... > > 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). 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. -- David Kastrup
