Hi! I'm pretty (not to say very) new to haskell and i've to create a new class named container that includes types that hold other values, more specific, i have "created" this two types: data Abb a b = Branch a b (Abb a b) (Abb a b) | Leaf and data ListAssoc a b = Node a b (ListAssoc a b) | Empty and implemented some common functions (add, search, delete, empty,etc). So when I was asked to create this new class I crashed against a big problem, or at least for me. My first attempt was: class Container c where empty :: c but when I had to implement "add" I tried add :: c -> a -> b -> c although I was almost sure that it wouldn't work, so I realized that I had to give Container not one but three parameters, the container and the types of values that the container saved so I started searching and I found similar cases, but the difference iis that in this cases the class was defined as follows: class Container c e | c->e where empty :: c insert :: c -> e -> c member :: c -> e -> Bool but my particular problem here is that the types I designed hold two values of different type and these holds just one value. Besides I knew nothing about functional dependency used in here and I keep knowing almost nothing about them, but I tried something like this: class Container c a b |c -> a, c -> b where empty :: c add :: c -> a -> c search :: c -> a -> Maybe b del :: c -> a -> c toListPair :: c -> [(a,b)] instance Container Abb a b where empty = Leaf add Leaf x y = Branch x y Leaf Leaf add arb@(Branch ni nd ri rd) x y |x == ni = arb |x > ni = Branch ni nd ri (add rd x y) |otherwise = Branch ni nd (add ri x y) rd search Leaf x = Nothing search (Branch ni nd ri rd) x |x == ni = Just nd |x > ni = search rd x |x < ni = search ri x but when I try to load it in WinHugs I get the following error message: - Instance is more general than a dependency allows *** Instance : Container Abb a b *** For class : Container a b c *** Under dependency : a -> b and as I have stated above my knowledge about dependencies is almost null, not to say null, so I don´t even have an idea where the error is. A suggestion that I've received was to change the type of Abb for data Abb (a,b) = Branch a b (Abb (a,b)) (Abb (a,b)) | Leaf and declare container class with just two parameters like I found in all pages I visited. I have not tried this yet, as I still have hope that what I intend to do is possible. Well if you have any suggestions I'd appreciate you send it to me and sorry for bothering you and my english, but i'm "spanish-speaker".