Cool polymorphism

poly.hs

{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, GADTs, RankNTypes #-}

class HasX a b | a -> b where getX :: a -> b

data Point2 where
  Point2 :: Int -> Int -> Point2
data Point3 where
  Point3 :: Float -> Float -> Float -> Point3

instance HasX Point2 Int   where getX (Point2 x _) = x
instance HasX Point3 Float where getX (Point3 x _ _) = x

--myIf :: (Show a1, Show b1, Show c1,
--         HasX a a1, HasX b b1, HasX c c1
--        ) => Bool -> a -> b -> c1
--myIf bool a b | bool      = getX a
--              | otherwise = getX b


--example = show res
--  where
--    res = myIf True (Point2 1 2) (Point3 0.0 0.1 0.2


myIf :: (Show a1, Show b1,
         HasX a a1, HasX b b1
        ) => Bool 
          -> a
          -> b
          -> (forall c c1. (HasX c c1, Show c1) => c
                                                -> r)
          -> r
myIf True  a b k = k a
myIf False a b k = k b

example = myIf False (Point2 1 2) (Point3 0.0 0.1 0.2) (show.getX)

{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, GADTs, RankNTypes #-}

class HasX a b | a -> b where getX :: a -> b

data Point2 where
  Point2 :: Int -> Int -> Point2
data Point3 where
  Point3 :: Float -> Float -> Float -> Point3

instance HasX Point2 Int   where getX (Point2 x _) = x
instance HasX Point3 Float where getX (Point3 x _ _) = x

--myIf :: (Show a1, Show b1, Show c1,
--         HasX a a1, HasX b b1, HasX c c1
--        ) => Bool -> a -> b -> c1
--myIf bool a b | bool      = getX a
--              | otherwise = getX b


--example = show res
--  where
--    res = myIf True (Point2 1 2) (Point3 0.0 0.1 0.2


myIf :: (Show a1, Show b1,
         HasX a a1, HasX b b1
        ) => Bool 
          -> a
          -> b
          -> (forall c. (HasX c c1, Show c1) => c
                                             -> r)
          -> r
myIf True  a b k = k a
myIf False a b k = k b

example = myIf False (Point2 1 2) (Point3 0.0 0.1 0.2) (show.getX)

This results in

GHCi, version 7.8.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( test.hs, interpreted )

test.hs:33:20:
    Could not deduce (HasX a c1) arising from a use of ‘k’
    from the context (Show a1, Show b1, HasX a a1, HasX b b1)
      bound by the type signature for
                 myIf :: (Show a1, Show b1, HasX a a1, HasX b b1) =>
                         Bool -> a -> b -> (forall c. (HasX c c1, Show c1) => c -> r) -> r
      at test.hs:(25,9)-(32,14)
    Possible fix:
      add (HasX a c1) to the context of
        the type signature for
          myIf :: (Show a1, Show b1, HasX a a1, HasX b b1) =>
                  Bool -> a -> b -> (forall c. (HasX c c1, Show c1) => c -> r) -> r
    In the expression: k a
    In an equation for ‘myIf’: myIf True a b k = k a

test.hs:34:20:
    Could not deduce (HasX b c1) arising from a use of ‘k’
    from the context (Show a1, Show b1, HasX a a1, HasX b b1)
      bound by the type signature for
                 myIf :: (Show a1, Show b1, HasX a a1, HasX b b1) =>
                         Bool -> a -> b -> (forall c. (HasX c c1, Show c1) => c -> r) -> r
      at test.hs:(25,9)-(32,14)
    Possible fix:
      add (HasX b c1) to the context of
        the type signature for
          myIf :: (Show a1, Show b1, HasX a a1, HasX b b1) =>
                  Bool -> a -> b -> (forall c. (HasX c c1, Show c1) => c -> r) -> r
    In the expression: k b
    In an equation for ‘myIf’: myIf False a b k = k b
Failed, modules loaded: none.

With help of @Saizan:

{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, GADTs, RankNTypes #-}

class HasX a b | a -> b where getX :: a -> b

data Point2 where
  Point2 :: Int -> Int -> Point2
data Point3 where
  Point3 :: Float -> Float -> Float -> Point3

instance HasX Point2 Int   where getX (Point2 x _) = x
instance HasX Point3 Float where getX (Point3 x _ _) = x


myIf :: (Show a1, Show b1,
         HasX a a1, HasX b b1
        ) => Bool 
          -> a
          -> b
          -> (forall c c1. (HasX c c1, Show c1) => c
                                                -> r)
          -> r
myIf True  a b k = k a
myIf False a b k = k b

example = myIf False (Point2 1 2) (Point3 0.0 0.1 0.2) (show.getX)