Re: [Scheme-reports] 5. Program Structure

[Andy Wingo <wingo@x>]

Hi,

I don't have much more to say about the r7rs, but there is this one
point.

On Sun 06 Jan 2013 02:53, Alex Shinn <alexshinn@x> writes:

>     Also it's not true, unfortunately!  Because in the next section,
>     "5.3.3
>     Multiple-value definitions", we see the introduction of
>     `define-values'
>     with an optimistically short specification and no corresponding
>     expansion.
>
> Define-values is just derived syntax, it doesn't need special
> treatment.

Ah, I hadn't seen the expansion in the appendix.  It does seem to be
broken, though; var0 will hold the list of values, not the first value.

How about this expansion:

  (define-syntax define-values
    (syntax-rules ()
      ((_ (var ... . var*) expr)
       (begin
         (define values
           (call-with-values (lambda () expr) list))
         (define var
           (let ((x (car values)))
             (set! values (cdr values))
             x))
         ...
         (define var* values)))
      ((_ (var ...) expr)
       (begin
         (define values
           (call-with-values (lambda () expr) list))
         (define var
           (let ((x (car values)))
             (set! values (cdr values))
             x))
         ...))))

No error checking though.

Also, if used at the top-level, the above and the report expansion may
or may not bind an unrenamed variable ("dummy" or "values"), depending
on the implementation.  Not so great.

>      I think you _cannot_ introduce define-values without
>     `letrec-values' and `letrec*-values' and corresponding semantics
>
> I disagree.  I'd love to remove all three though.

To be clear, neither letrec-values nor letrec*-values are present in the
report.  Thus there is nothing to remove, there.

I still think that define-values should be accompanied with
letrec-values / letrec*-values or not present at all, but given that you
have an expansion this is not as crucial.

Andy
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