exist.hs

{-# LANGUAGE ExistentialQuantification #-}

data Adder = forall a. Adder
    { increment :: a -> a
    , render :: a -> String
    , value :: a
    }

main :: IO ()
main = do
    let adder = Adder
            { increment = (+ 1)
            , render = show
            , value = 0 :: Int
            }
    runAdder adder

runAdder :: Adder -> IO ()
runAdder (Adder increment render value) =
    putStrLn . render . increment . increment $ value