Hi all,

there was an interesting question about Generators on StackOverflow:

http://stackoverflow.com/questions/43340007/yield-prolog-adaptation-for-smalltalk

It's about a pseudo-Prolog implementation using Generators and yield.

Our Generator *almost* works for this, except there is a little problem
with the timing of side-effects:

| x gen |
x := 0.

gen := Generator on: [:g |
x := 1.
g yield: nil.

x := 2.
g yield: nil.

x := 3.
g yield: nil].

gen do: [:i | Transcript cr; show: x].

This *should* print '1 2 3' but actually prints '2 3 3'. This is because
our Generator is written to support a streaming interface (using atEnd and
peek) so it always computes one yield ahead.

In other languages (e.g. JavaScript, Python, C#) this is different, the
execution is paused at the current yield statement, not 'before' the next
one. In general, a generator cannot know if there will be more "yields"
without executing more code. In Javascript, 'next' returns both a value and
a 'done' flag, and if the flag is true, the returned value must not be used.

IMHO we should fix this - see attached changeset Marcel and I came up with.
We did not fix the tests yet, but the behavior seems right.

Even though generators are in the image for a long time they are not used
widely yet (AFAIK) so maybe we can still change the behavior? What do
people think?

- Bert & Marcel -
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