{-# OPTIONS -fglasgow-exts #-}
import Maybe import Data.Typeable
data Nil = Nil deriving (Eq,Typeable,Show)
class (Typeable t) => List a t where init :: (t -> b) -> (forall y . (List a y) => y -> b) init f z = fromJust $ do x <- cast z return $ f x
instance List a Nil where
Could not deduce (List a1 y) from the context (List a Nil, Typeable Nil, List a y) arising from use of `Main.$dminit' at Main.hs:21:0 Probable fix: add (List a1 y) to the class or instance method `Main.init' In the definition of `init': init = Main.$dminit In the definition for method `Main.init' In the instance declaration for `List a Nil' but copying and pasting the code from init to the instance declaration works fine. Jim