Re: [Chicken-users] Using the amb egg

[Ivan Shmakov]

>>>>> Jeronimo Pellegrini <address@hidden> writes:

 > 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).

        But note that the latter is the result of the first ‘let’, and
        not the second.

[…]

 > What's the semantics of amb and require in this situation?

        That's quite simple — ‘amb’ returns a value that won't be
        followed by a contradiction.  The first expression was followed
        by the second, which, in turn, has led to a contradiction.
        Therefore, first ‘amb’ was evaluated for one more time, giving
        the latter result.

 > Am I supposed to do something between the two lets? (Does amb need a
 > "restart" or something?)

[…]

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