[Chicken-users] Using the amb egg

[Jeronimo Pellegrini]

Hi,

I've installed the amb egg and could use it, but I'm a bit confused:

#;1> (use amb amb-extras)
...
#;2> (let ((a (amb 1 2 3)) (b (amb -1 -2 -3))) (required (= (+ a b) 0))
(list a b))
(1 -1)
#;3> (let ((a (amb 1 2 3)) (b (amb -1 -2 -3))) (required (= (+ a b) 10))
(list a b))
(2 -2)

So, out of the scope of the first LET I use amb and require again, and
it returned values that would work for the first time (but not the
second).

If I leave the interpreter and try the impossible one from the start,
it does the right thing and signals an error:

#;2> (let ((a (amb 1 2 3)) (b (amb -1 -2 -3))) (required (= (+ a b) 10))
(list a b))

Error: (amb) expression tree exhausted

    Call history:

    ...

What's the semantics of amb and require in this situation? Am I supposed
to do something between the two lets? (Does amb need a "restart" or
something?)

I'm using git master (4f3e7c7ca683d7c8fc208f9632c77cc08f2eee9b) and
amb 2.1.2.

Thanks,
J.