Why does only tr2 work, although the only difference between tr1 and tr2 is the order of arguments? import Data.Tree (Tree(..)) data Type a where Tree :: Type a -> Type (Tree a) Int :: Type Int String :: Type String type Traversal1 = forall a.a -> Type a -> a type Traversal2 = forall a.Type a -> a -> a tr1 :: Traversal1 tr1 (Node _ (t:_)) (Tree Int) = Node 1 [t] tr1 n Int = n+n tr1 s String = s++s tr2 :: Traversal2 tr2 (Tree Int) (Node _ (t:_)) = Node 1 [t] tr2 Int n = n+n tr2 String s = s++s Couldn't match expected type `a' against inferred type `Tree Int' `a' is a rigid type variable bound by the type signature for `tr1' at tratest.hs:9:25 In the pattern: Node _ (t : _) In the definition of `tr1': tr1 (Node _ (t : _)) (Tree Int) = Node 1 [t]