gistfile1.txt

{-# LANGUAGE MultiParamTypeClasses, FlexibleContexts, UndecidableInstances, FlexibleInstances, OverlappingInstances, FunctionalDependencies #-}
class Cast a b | a -> b where
  to :: a -> b

data Point2 = Point2 Int Int
            deriving (Show)

data Point3 = Point3 Int Int Int

instance Cast Point3 Point2 where
  to (Point3 x y _) = Point2 x y

class ToString a where
  toString :: a -> String

instance ToString Point2 where
  toString = show

instance (Cast a Point2) => ToString a where
  toString x = toString $ to x

instance ToString (Point3) where
  toString x = "yolo"

foo :: (ToString a) => a -> String
foo x = (toString x) ++ "X"1