[plt-scheme] "Standard" coroutine library suitable for building filters as well as generators

Scott McLoughlin wrote:
> (let ((pa (print-all)))   ; "instantiates" the coroutine, here with no
> initialization arguments
>    (resume pa 4)         ; this pumps values to a pending "receive"
> within the coroutine
>    (resume pa 2)
>    (resume pa 6))

I would have expected (yield) to be just your (resume), but it turns out
not to be. Attached is a variant on generator/yield that permits
bidirectional communication with a generator. Your print-all example
becomes:

(define (print-all)
  (display "FIRST")
  (newline)
  (do ()
    (#f)
    (display (yield))
    (newline)))

(let ((pa (generator () (print-all))))
  (pa)
  (pa 1)
  (pa 2)
  (pa 3))

Be warned, I hacked it together just now, and while it's received light
testing and looks approximately like the right kind of thing to have
written, I wouldn't put it into production without some proper test cases.

Tony
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