[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Scheme-reports] multiple values, and, or, when, unless
On Sun 06 Jan 2013 03:34, Alex Shinn <alexshinn@x> writes:
> On Sun, Jan 6, 2013 at 4:31 AM, Andy Wingo <wingo@x> 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?
Scheme-reports mailing list