return type polymorphism

rtp.hs

class WithInt c where
  combinate :: Integral i => i -> c -> [(i,c)]

class RTP c where
  fromNothing :: Integral i => i -> [(i,c)]


data Wut = Wut deriving (Show)
data Foo = Foo deriving (Show)

instance WithInt Wut where 
  combinate i w = if (i == 0)
                    then []
                    else (i,w):combinate (i - 1) w

instance RTP Wut where
  fromNothing i = if (i == 0)
                    then []
                    else (i,Wut):fromNothing (i - 1)
  
instance RTP Foo where
  fromNothing i = [(8, Foo)]


main = let wutlist :: [(Int, Wut)]
           wutlist = fromNothing 7
           foolist :: [(Int, Foo)]
           foolist = fromNothing 7
       in do
         putStrLn $ show wutlist
         putStrLn $ show foolist