Re: [Scheme-reports] multiple values, and, or, when, unless

On Sun, Jan 6, 2013 at 7:15 PM, Andy Wingo <wingo@x> wrote:

Hi Alex,

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?

It's not particularly useful to use when/unless in tail context

anyway, so I think we should just remove these from the list

of tail calls.

But if we do keep it, I see no contradiction stating that the

body is a tail context but the result unspecified.

Alex