Hi. I read the lectures from the CIS194 course (version fall 2014) and I wanted to have some comments on my solution to the Exercise 7 of the Homework 5. Here is the question: Exercise 7 (Optional) The distribute and squashMulId functions are quite similar, in that they traverse over the whole expression to make changes to specific nodes. Generalize this notion, so that the two functions can concentrate on just the bit that they need to transform. (You can see the whole homework here: http://www.seas.upenn.edu/~cis194/hw/05-type-classes.pdf) Here is my squashMulId function from Exercise 6: squashMulId :: (Eq a, Ring a) => RingExpr a -> RingExpr a squashMulId AddId = AddId squashMulId MulId = MulId squashMulId (Lit n) = Lit n squashMulId (AddInv x) = AddInv (squashMulId x) squashMulId (Add x y) = Add (squashMulId x) (squashMulId y) squashMulId (Mul x (Lit y)) | y == mulId = squashMulId x squashMulId (Mul (Lit x) y) | x == mulId = squashMulId y squashMulId (Mul x y) = Mul (squashMulId x) (squashMulId y) And here is my solution to Exercise 7: distribute :: RingExpr a -> RingExpr a distribute = transform distribute' where distribute' (Mul x (Add y z)) = Just $ Add (Mul x y) (Mul x z) distribute' (Mul (Add x y) z) = Just $ Add (Mul x z) (Mul y z) distribute' _ = Nothing squashMulId :: (Eq a, Ring a) => RingExpr a -> RingExpr a squashMulId = transform simplifyMul where simplifyMul (Mul x (Lit y)) | y == mulId = Just $ squashMulId x simplifyMul (Mul (Lit x) y) | x == mulId = Just $ squashMulId y simplifyMul _ = Nothing transform :: (RingExpr a -> Maybe (RingExpr a)) -> RingExpr a -> RingExpr a transform f e | Just expr <- f e = expr transform _ AddId = AddId transform _ MulId = MulId transform _ e@(Lit n) = e transform f (AddInv x) = AddInv (transform f x) transform f (Add x y) = Add (transform f x) (transform f y) transform f (Mul x y) = Mul (transform f x) (transform f y) Is there a better way of doing such generalization? Thanks for your help.