I'm not sure whether your program is correct, but it's easy to see why you're getting the type error. You define p(_, k) = empty k, but empty has type BinTree t -> Bool, so clearly p has type (a, BinTree t) -> Bool. However, you use p in "until p f ([], Push CreateStack b)". Since until has type (a -> Bool) -> (a -> a) -> a -> a, this implies that p must have type ([a], Keller (BinTree t)) -> Bool. So you need to either fix the definition of p or fix how it is used.
By the way, you are missing the case (Bin _ _ _) == EmptyTree in the instance decl. Also, note that Keller t is isomorphic to [t], i.e. to the list data type. Specifically, CreateStack is [], Push is (:), top is head, and pop is tail. Hope this helps, -Paul Hudak > -- Binary Tree structure > data BinTree t = EmptyTree | Bin (BinTree t) t (BinTree t) > left (Bin b1 _ _) = b1 > right (Bin _ _ b2) = b2 > value (Bin _ v _) = v > empty EmptyTree = True > empty (Bin _ _ _) = False > > -- Give the Binary Tree == > instance Eq a => Eq (BinTree a) where > (Bin b v c) == (Bin d w e) = (v == w) && (b == d) && (c == e) > EmptyTree == EmptyTree = True > EmptyTree == Bin _ _ _ = False > > -- Stack structure > data Keller t = CreateStack | Push (Keller t) t > top (Push s x) = x > pop (Push s x) = s > > -- Depth-First search > tiefensuche b = fst (until p f ([], Push CreateStack b)) > where p(_, k) = empty k > f(erg, k) | (top k) == EmptyTree = (erg, pop k) > | otherwise = ([v] ++ erg, Push( Push( pop k) b2) b1) > where (Bin b1 v b2) = top k > > The error message says that p has a wrong types > Haskell thinks it is p :: (b, BinTree c) -> Bool, instead of p :: ([a], > Keller (BinTree a)) -> Bool _______________________________________________ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
