module Control.Comonad.Trans.Stream
(
Stream, stream, runStream
, StreamT(..)
, tails
, unfolds
, unfoldsW
) where
import Control.Applicative
import Control.Comonad
import Control.Comonad.Trans.Class
import Data.Functor.Identity
import Data.Foldable
import Data.Monoid
import Data.Traversable
type Stream f = StreamT f Identity
stream :: a -> f (Stream f a) -> Stream f a
stream a as = StreamT (Identity (a, as))
runStream :: Stream f a -> (a, f (Stream f a))
runStream = runIdentity . runStreamT
data StreamT f w a = StreamT { runStreamT :: w (a, f (StreamT f w a)) }
instance (Functor w, Functor f) => Functor (StreamT f w) where
fmap f = StreamT . fmap (\(a, as) -> (f a, fmap f <$> as)) . runStreamT
instance (Comonad w, Functor f) => Comonad (StreamT f w) where
extract = fst . extract . runStreamT
duplicate = StreamT . extend (\w -> (StreamT w, duplicate <$> snd (extract w))) . runStreamT
extend f = StreamT . extend (\w -> (f (StreamT w), extend f <$> snd (extract w))) . runStreamT
instance Functor f => ComonadTrans (StreamT f) where
lower = fmap fst . runStreamT
instance (Foldable w, Foldable f) => Foldable (StreamT f w) where
foldMap f = foldMap (\(a, as) -> f a `mappend` foldMap (foldMap f) as) . runStreamT
instance (Traversable w, Traversable f) => Traversable (StreamT f w) where
traverse f (StreamT w) = StreamT <$> traverse (\(a, as) -> (,) <$> f a <*> traverse (traverse f) as) w
tails :: Comonad w => StreamT f w a -> f (StreamT f w a)
tails = snd . extract . runStreamT
unfolds :: Functor f => (a -> (b, f a)) -> a -> Stream f b
unfolds f a = let (h, t) = f a in stream h (unfolds f <$> t)
unfoldsW :: (Comonad w, Functor f) => (w a -> (b, f a)) -> w a -> StreamT f w b
unfoldsW f = StreamT . extend (\s -> let (h, t) = f s in (h, fmap (\a -> unfoldsW f (a <$ s)) t))