John Cowan wrote:

> >   * gselect: One may expect the following to yield (a d e g h), but
> >     the sample implementation yields (a d e h) instead (not
> >     unreasonably).  Regardless of which option is chosen, some
> >     clarification would be helpful.
> >
> >       (let ((g (list->generator '(a b c d e f g h))))
> >         (append
> >          (generator->list (gselect g
> >                                    (list->generator '(#t #f #f #t #t #f))))
> >          (generator->list g)))
>
> This expression is undefined by the rules of Scheme, because its result
> depends on the order of evaluation of the arguments of `append`.  The
> procedures aren't ambiguous, it's depending on the order of side effects
> to the generator without using `begin` to sequence them that's ambiguous.

Oops; you are right, of course!  In my haste to write convert my rough
testing history into a self-contained example, I goofed up.

However, I think the following modified version still exhibits the main
issue:

  (let* ((g (list->generator '(a b c d e f g h)))
         (x (generator->list
             (gselect g
                      (list->generator '(#t #f #f #t #t #f)))))
         (y (generator->list g)))
    (append x y))

Regards,

-chaw