Re: [Scheme-reports] "unspecified values"

[Andy Wingo <wingo@x>]

On Sat 21 May 2011 06:59, Alex Shinn <alexshinn@x> writes:

>   (define-syntax time
>     ((time expr)
>      (let* ((start (current-time))
>             (res expr))
>       (report-time 'expr start (current-time))
>       res)))

This is actually an incorrect program.  If `expr' returns two values,
you want to return them to the continuation.  You need:

  (let ((start (current-time)))
    (call-with-values (lambda () expr)
      (lambda vals
        (report-time 'expr start (current-time))
        (apply values vals))))

> Now, you may dislike these idioms, and argue in
> particular that the first doesn't work if expr returns
> multiple values, but it works fine for users who were
> not previously using MV in that context, or perhaps
> in any of their code.  And using MV at all is still a
> contentious issue for some people.  There's a
> difference between providing a feature, forcing
> people to use that feature in new code, and forcing
> them to rewrite their old code to support the feature.
> We debated this and voted strongly in favor of not
> breaking existing code.

If you like this idiom, you are not interested in portable code,
cf. values: if expr returns 0 values, the implementation is allowed to
crash.

I think it's OK to be uninterested in portable code.  But don't restrict
all the implementations that *do* want to change because users have
programs that aren't even valid for R5.

Andy
-- 
http://wingolog.org/

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