Numeric literals are special. Their type is (Num t) => t, so it can belong to any type that is instance of Num. Whereas Test belongs to Test type only so you cannot call bar on any instance of Foo. So your pattern constrains type signature of bar more then it is constrained by class declaration. On Wed, Dec 03, 2008 at 03:05:37PM -0800, Anatoly Yakovenko wrote:

module Test where --why does this work: data Test = Test

class Foo t where foo :: Num v => t -> v -> IO ()

instance Foo Test where foo _ 1 = print $ "one" foo _ _ = print $ "not one"

--but this doesn't?

class Bar t where bar :: Foo v => t -> v -> IO ()

instance Bar Test where bar _ Test = print $ "test" bar _ _ = print $ "not test" _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe