README.md

$ ghc a.hs
$ time ./a
./a  1.51s user 1.67s system 77% cpu 4.082 total
$ ghc b.hs
$ time ./b
./b  1.58s user 1.68s system 78% cpu 4.126 total

a.hs

{-# LANGUAGE NoImplicitPrelude  #-}
{-# LANGUAGE RankNTypes         #-}

import Prelude         hiding (Maybe(..))
import Control.Monad

type Maybe a
  = forall b.
    (a -> b)    -- Just
    -> b        -- Nothing
    -> b

just :: a -> Maybe a
just x y _ = y x

nothing :: forall a. Maybe a
nothing _ x = x

dat :: Maybe Int
dat = just 42

main :: IO ()
main = do
  forM_ [0..1000000] $ const $ do
    dat print (return ())

b.hs

{-# LANGUAGE NoImplicitPrelude  #-}

import Prelude         hiding (Maybe(..))
import Control.Monad

data Maybe a
  = Just a
  | Nothing

dat :: Maybe Int
dat = Just 42

main :: IO ()
main = do
  forM_ [0..1000000] $ const $ do
    case dat of
      Just x -> print x
      Nothing -> return ()