Keith is talking about a "comitted choice" style of nondeterminism, where one of the arguments is picked and the computation continues from there. If you want a computation with backtracking, or a list of all possibly results then you should use the list monad, or another monad that supports nondetermanism. The tutorial "All About Monads" has a nice discussion of the list monad in section two, another example ("StateT with List") in section three, and it's a good introduction to monads overall, if you need one. If you wanted to pick x from 1 2 3 and y from 3 4 5 so that x squared equals y you could write results :: [(Int,Int)] results = do x <- [1,2,3] y <- [3,4,5] guard $ x^2 == y return (x,y) then results will be [(2,4)]. Brandon On Wed, 7 Apr 2004, Keith Wansbrough wrote:

...

Hi, I think it's good idea to compute non-deterministic problems with the `amb' operator, just as in LISP/scheme. But how to implement it in haskell?

Do you mean "evaluate e1 and e2, and return the result of whichever returns first"?

Probably best to do this using threads.

--KW 8-) -- Keith Wansbrough http://www.cl.cam.ac.uk/users/kw217/ University of Cambridge Computer Laboratory.

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