Hi Alex, On Sun 06 Jan 2013 03:34, Alex Shinn <[email protected]> writes:
> On Sun, Jan 6, 2013 at 4:31 AM, Andy Wingo <[email protected]> wrote: > > as the last expression [of an `and' expression] is in tail > context, the _values_ of the last expression are returned. Same > with `or', `when', and `unless'. > > Pluralized for `and'. I'm not sure about `or' - I think it would > always be a mistake to use MVs in an or expression. I agree it's probably a mistake in practice, but I think that there's no way you can avoid reducing (or #f (foo)) to (foo) -- and in that case all values returned from the call to (foo) are returned. > Contrary to their specifications and the "differences from r6rs" > appendix, the return value of `when' and `unless' is indeed > specified if > the body is evaluated, as the body is in tail position. > > > No, this was a mistake on R6RS's part. It is meaningless > to return values from `when' or `unless', so we explicitly state > that "the result of the when/unless expression is unspecified." I used to agree with you, but requiring the consequent to be in tail position means that the last expression is evaluated in the continuation of the "when" expression as a whole. I don't think you can get around this with a special "this is unspecified" exception -- you've already specified it. Is there any definition other than (if TEST (begin CONSEQUENT ...)) that actually fits the requirements? Andy -- http://wingolog.org/ _______________________________________________ Scheme-reports mailing list [email protected] http://lists.scheme-reports.org/cgi-bin/mailman/listinfo/scheme-reports
