What is the difference between using a class context and deriving in data type declaration? Are there certain situations in which one or the other is preferred? data Eq a => Set1 a = NilSet1 | ConsSet1 a (Set1 a) data Set2 a = NilSet2 | ConsSet2 a (Set2 a) deriving Eq (NilSet1) == (NilSet1) -- no instance, error (NilSet2) == (NilSet2) -- True -- seems OK type Type1 = Set1 Integer type Type2 = Set2 Integer -- seems OK data Data1 = Set1 | Set2 -- Seems to have same type :t ConsSet2 1 NilSet2 ConsSet2 1 NilSet2 :: forall t. (Num t) => Set2 t :t ConsSet1 1 NilSet1 ConsSet1 1 NilSet1 :: forall t. (Num t) => Set1 t -- Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie