forallgadts

gistfile1.txt

{-# LANGUAGE GADTs, RankNTypes #-}

data Foo a where
  Bar :: Int -> Foo Int
  Baz :: String -> Foo String

newtype Foo2 a = Foo2 {runFoo2 :: forall f. (Int -> f Int) -> (String -> f String) -> f a}

foo2s :: Foo2 String
foo2s = Foo2 $ \bar baz -> baz "asd"

gistfile2.txt

{-# LANGUAGE RankNTypes #-}

newtype Stream a = Stream {runStream :: forall f. (a -> f a -> f a) -> f a}

fromn :: Int -> Stream Int
fromn n = Stream $ \c -> c n $ runStream (fromn (n+1)) c

tolist :: Stream a -> [a]
tolist s = runStream s (:)