{-# LANGUAGE FlexibleInstances #-}
import Data.Set (Set,elems,cartesianProduct,fromList,empty,insert,singleton,
union,unions)
data CombinatorialProblem a = CP
{
bound :: Set a,
close :: Set a -> Set a
}
enumerate :: CombinatorialProblem a -> Set a
enumerate cp = iterate' f null (empty, singleton (close cp empty))
where
f (old, new) = (union old new, fromList fresh)
where
fresh = [close cp (insert a s) | a <- elems (bound cp), s <- elems new]
iterate' :: ((a,a) -> (a,a)) -> (a -> Bool) -> (a,a) -> a
iterate' f p x = let (x0,x1) = iterate'' x in x0
where
iterate'' x = if p (let (x0,x1) = x in x1) then x else iterate'' (f x)
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