[plt-scheme] srfi-42, values, named let, etc.

From: David Einstein (deinst at gmail.com)
Date: Sat Sep 22 16:05:47 EDT 2007

In my neverending quest to get a :nested generator for srfi-42 I have
managed to get to the following

(define-generator (:nested stx)
  (syntax-case stx ()
    [(:nested (var ...) gen ...)
     #'(:do (let-values ([(fun-in done-in var ...)
                          (let/cc ret
                            (do-ec gen ... (let/cc enter
(call-with-values (lambda () (values enter #f var ...)) ret)))
                            (let ([var #t] ...)
                              (call-with-values (lambda () (values #f
#t var ...)) ret)))]))
              ((fun fun-in)(done done-in)(var var) ...)
              (not done)
              (let ())

This kinda-sorta-almost works, it is good enough for my purposes, but
there is one place that it collapses, and that is in expressions like
(:parallel (:nested <generator>+) <generator>+)

The reason for this is the code expands to something like

(let-values ([(fun done a b)
             (let/cc ret
               (do-ec <generators>
                      (let/cc enter (call-with-values (lambda ()
(values enter #f a b)) ret)))
               (call-with-values (lambda () (values #f #t #f #f)) ret))])
 (let loop ([fun fun][done done][a a][b b])
   (if (not done)
         <Do stuff>
         (loop (fun))))))

The problems occur with (loop (fun)) where loop is the function
created by the named let, and fun is a function that returns multiple
values.  The problem exists when we embed this into the :parallel
generator, which expects the loop function to be called with a list of
 parameters, and operates by concatenating the parameter lists.
Unfortunately we did not use a parameter list.

I have a few questions.

Is calling the named let function as I do legal scheme in general, or
is it a feature of PLT scheme (if I stare at it long enough it appears
to be magic).

Is there some way of modifying it so that it plays nice with
:parallel?  Alternatively, is there some way of modifying :parallel so
that it plays nice with the above ?

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