But when I try to create a more generic code like this which could work with trees who don't have empty nodes in grandchild level :
function Node a (Node b Tree Tree) (Node c Tree Tree )
The problem is that 'Tree' is a type, not a constructor. Someone correct me, if I'm mistaken (this is my first post to the mailing-list, yieah :-)), but what seems to cause the problem is that the pattern matcher needs constructors, so it can determine, whether a pattern can produce an input data. There are several solutions. If you don't need the further subtrees, leave them fully unspecified via the underscore: function (Node a (Node b _ _) (Node c _ _)) = ... or you could give them variable names like this: function (Node a (Node b bl br) (Node c cl cr)) = ... where bl, br, cl, and cr are variables of the type Tree. However, what you might want to accomplish is a recursive function over the recursive type to get a fully generic code. This usually looks something like this: -- a generic recursive function cFunction :: Tree -> a cFunction Empty = ... cFunction (Node i l r) = f i (cFunction l) (cFunction r) where f :: Integer -> a -> a -> a f int recLeft recRight = ... Hope that helped.