On 2022-09-07 08:28 +0200, Marc Nieper-Wißkirchen wrote:
> > On Tue, Sep 6, 2022 at 5:39 PM Marc Nieper-Wißkirchen <xxxxxx@gmail.com> wrote:
> >
> >> there cannot be an
> >> implementation, portable or not, on every Scheme implementation that
> >> is allowed by R7RS. (As I tried to explain earlier, even access to
> >> something like "%true-promise" would not help.)
> >
> >
> > I continue to find your explanation incomprehensible, but I'll add a warning saying that on an unusual implementation of Scheme promises, it is not possible to treat promises as thunks; however, no such implementations are known.
>
> Wolfgang, have you understood my point? Maybe you can explain it
> differently and better.

If I'm understanding correctly, the problem is that, by
the standard, promises don't form a type, and it may be nonsense
to try to detect them with 'promise?', just it would be nonsense
to try to define an 'isPromise' predicate in Haskell.  Further,
the implementation may not be able to do any better because of
how it represents values/expressions ("there is no %true-promise").

So, for example, in an implicit-forcing implementation which
completely hides promises, the following would return 1, because
"everything is a promise":

(lazy-or-procedure 1 2)

But it should be a type error.  This is the kind of problem that
we run into if we try to distinguish a type that "isn't there".

Is this something like what you mean?

> In any case, I strongly suggest writing a SRFI dealing exclusively
> with lazy evaluation and combinators for promises instead of making
> SRFI 235 polymorphic.

I think I agree.

> I imagine that Wolfgang could be in the mood to write such a SRFI.

It'll need research, but yes, I'd be willing to do that at some point.

--
Wolfgang Corcoran-Mathe  <xxxxxx@sigwinch.xyz>