CoData.hs

{-# LANGUAGE Rank2Types, GADTs, LambdaCase #-}

data Stream a res where
    Head :: Stream a a
    Tail :: Stream a (CoData (Stream a))

newtype CoData con = CoD {unCoD :: forall r. con r -> r}

appendList :: [a] -> CoData (Stream a) -> CoData (Stream a)
appendList [] s = s
appendList (x:xs) s = CoD $ \case
                                    Head -> x
                                    Tail -> appendList xs s

cycle :: [a] -> CoData (Stream a)
cycle xs = let r = appendList xs r in r

toList :: CoData (Stream a) -> [a]
toList (CoD f) = f Head : toList (f Tail)