RE: Equality constraint type mismatch

| type family T a :: * | my_id :: f a -> f a; my_id = id | x :: T a -> T a; x = my_id | | | IMHO, this was not clear from the documentation or from the error | message (It certainly _looks_ like "T a" should match "f a"...). | Thanks for the explanation. Did you read this? http://haskell.org/haskellwiki/GHC/Type_families#Injectivity.2C_type_inferen... Please edit it to improve the clarity, or add a whole new sub-section if you think that would be clearer. It's a wiki. Simon