bug#59140: 29.0.50; iter-yield from lambda

[Michael Heerdegen <michael_heerdegen@web.de>]

Hello,

In emacs-help (Okamsn <okamsn@protonmail.com>) it has been pointed to
the issue that `iter-yield' (from generator.el) is not supported in some
contexts - contexts where yielding happens from inside a lambda
expression or directly (like here when mapping over a sequence), e.g.

     (setq iter-maker (iter-lambda (x)
                        (seq-doseq (item x)
                          (iter-yield item))))
     (setq my-iter (funcall iter-maker '(1 2 3)))
     (iter-next my-iter)

  ~~> (void-function cps-internal-yield)

(seq-doseq expands to more or less just mapc+lambda) or

     (setq iter-maker (iter-lambda (x)
                        (seq-do #'iter-yield x)))
     (setq my-iter (funcall iter-maker '(1 2 3)))
     (iter-next my-iter)
  ~~> (error "‘iter-yield’ used outside a generator")

These examples are written correctly but seem to hit an undocumented
limitation of the implementation in generator.el

I quote Stefan Monnier answering what should be done:

> IIRC, one of the main limitations of `generator.el` is that it doesn't
> handle `lambda` (and neither should you use `#'iter-yield`, IIRC).
> 
> I don't really know how to go about fixing it.
> 
> A good first step would be to make sure it emits an error (or a warning)
> when you use `#'iter-yield` or when you call `#'iter-yield` from with
> a lambda expression.

TIA,

Michael.