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Re: [Scheme-reports] "unspecified values"

On Fri, 20 May 2011, Alex Shinn wrote:

On Thu, May 19, 2011 at 10:10 AM, Andy Wingo <wingo@x> wrote:

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))

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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