Hi Angus, > testconcat [(x:xs), (y:ys)] = x : xs : y : ys When you're replacing the x, xs, y, and ys with types you're getting: a : [a] : a : [a] The type of (:) is: a -> [a] -> [a] Can you see the problem? Hint, the problem is the (:) in the middle. Greetings, Daniel