On 2020-06-14 21:43 +0200, Marc Nieper-Wißkirchen wrote:
> > Or perhaps it would be simplest to add that the result is unspecified
> > if the procedures passed to maybe / either-unfold have side-effects.
> >
>
> That would be definitely wrong. See my example with `generator-unfold' I
> gave to John.

I wasn't able to find this example in the previous emails.

> If they don't get it right, we should correct them, not introduce the same
> error here.

This seems like a very long road to go down, one which does not end
with unfold.  Very few non-mutating Scheme procedures which take
procedure arguments specify when or how often those procedures will
be called, from the perspective of calling those procedures for
side-effects.  You can, of course, *infer* from the spec of say,
map, that (map proc lis) will call proc (length lis) times, but
that's, um, a side effect of the specification.  Making this sort of
assumption concrete will mean changing a lot of specifications,
I fear.

Unfold expressions should satisfy certain theorems, but, if you
expect that assumptions about side-effecting behavior drawn from
SRFI 1 `unfold' will hold for FOO-unfold, I think you lose.

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

"Not all programming languages have a way to write definitions, but
most do.  Those that do not are for wimps."  --The Great Quux