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Re: when and unless

From: Andy Wingo
Subject: Re: when and unless
Date: Tue, 06 Dec 2011 18:35:16 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.3 (gnu/linux)

On Tue 06 Dec 2011 17:42, David Kastrup <address@hidden> writes:

> Andy Wingo <address@hidden> writes:
>> On Tue 06 Dec 2011 12:17, David Kastrup <address@hidden> writes:
>>> I've actually wondered if it would not make sense to return
>>> *unspecified* in the case of the plain else-less if even if the
>>> condition is true, namely when you write (if #t #t).
>> A first (and probably worthwhile) step would be to warn if such
>> a statement is processed for value.
> Well, is it being processed for value if what I do with the value is
> calling unspecified? on it in order to find out whether I should warn
> about a function returning a value when it shouldn't?

Yes, it would be.

Note, in R5RS scheme, this should be true:

  (eqv? (if #f #f) (if #f #f))

In R6RS (and R7RS, I think) scheme it does not have any meaning --
implementations are free to do what they like.

The reason is that (if #f #f) returns "unspecified values" rather than a
canonical "unspecified value".  So the implementation may treat it like:

  (eqv? (values) (values))

which is strictly unspecified, as it is returning an unexpected number
of values to a continuation.

Guile 1.8:

    guile> (eqv? (values) (values))

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"?


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