Friends This is a message for people who use RULES, to ask your opinion. Have a look at http://hackage.haskell.org/trac/ghc/ticket/2600 and add your comments if you want. The intro to the ticket appears below, so you can get an idea of whether you are interested. Simon Roman writes: I found an example I came across while working on the recycling paper and which I subsequently forgot about. Suppose we have: {-# LANGUAGE Rank2Types #-} module T where class T t where to :: [a] -> t a from :: t a -> [a] tmap :: (a -> a) -> t a -> t a {-# RULES "myrule" forall f x. from (tmap f (to x)) = map f (from (to x)) #-} Alas, this fails with: Ambiguous type variable `t' in the constraint: `T t' arising from a use of `to' at T.hs:12:40-43 Probable fix: add a type signature that fixes these type variable(s) Of course, I'd like the t on the rhs to be the same as on the lhs but I don't see how to tell this to GHC. Is it actually possible? The only solution I've found was to add a dummy argument to 'to': to' :: t a -> [a] -> t a to = to' undefined {-# RULES "myrule" forall f t x. from (tmap f (to' t x)) = map f (from (to' t x)) #-} That's ugly, of course. Am I missing something or is this just impossible to do with the current system?