UHCのドキュメントにあるexistentialsの例をGHCでやってみた

Existentials.hs

{-# LANGUAGE ExistentialQuantification #-}
module Main where
import Data.Char (ord)

data Exists r = forall a. Exists a (a -> r)

x2 :: Exists Int
x2 = Exists (3 :: Int) id

xapp :: Exists a -> a
xapp (Exists v f) = f v

x2app :: Int
x2app = xapp x2

mkx :: Bool -> Exists Int
mkx b = if b then x2 else Exists 'a' ord

y1, y2 :: Exists Int
y1 = mkx True
y2 = mkx False

{-
-- type error
mixy = case y1 of
         Exists v1 f1 ->
           case y2 of
             Exists v2 f2 -> f1 v2
-}

main :: IO ()
main = do putStrLn $ show $ xapp y1
          putStrLn $ show $ xapp y2