Re: [Scheme-reports] "unspecified values"

[Andre van Tonder <andre@x>]

On Fri, 20 May 2011, Alex Shinn wrote:

> On Thu, May 19, 2011 at 10:10 AM, Andy Wingo <wingo@x> wrote:
> > Unfortunate.
> >
> > I do not agree with the note that permitting any number of values to be
> > returned from `set!' et al is incompatible.  It is not incompatible with
> > implementations, as it widens the scope of what they may do.  It is not
> > incompatible with existing programs, as I do not expect existing
> > implementations to switch -- most will do what they have been doing.
>
> Yes, it is incompatible with existing programs.  One common example is:
>
>  (define-syntax time
>    ((time expr)
>     (let* ((start (current-time))
>            (res expr))
>      (report-time 'expr start (current-time))
>      res)))
>
> This will work fine for arbitrary expressions, including
> *set! and I/O operations, so long as they return a single
> value as they do in R5RS.

Arbitrary expressions do not return a single value in R5RS, so
I would say the above macro is buggy even in R5RS.  It fails
for something as simple as

  (time (values 1 2))

/in R5RS/.

So your example is IMO invalid, unless what you really want is to
return to R4RS.  I wonder sometimes... ;)

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