Paul Hudak wrote: > 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
Thanks, that really helped. My fault was that the depth-search algorithm expected an empty-method for the stack not for the binary tree. -- ciao dennis "I hear and I forget, I see and I remember, I do and I understand" _______________________________________________ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
