Control.Monad.Co

Monads from Comonads

http://comonad.com/reader/2011/monads-from-comonads/

Co can be viewed as a right Kan lift along a Comonad.

In general you can "sandwich" a monad in between two halves of an adjunction. That is to say, if you have an adjunction F -| G : C -> D then not only does GF form a monad, but GMF forms a monad for M a monad in D. Therefore if we have an adjunction F -| G : Hask -> Hask^op then we can lift a Comonad in Hask which is a Monad in Hask^op to a Monad in Hask.

For any r, the Contravariant functor / presheaf (-> r) :: Hask^op -> Hask is adjoint to the "same" Contravariant functor (-> r) :: Hask -> Hask^op. So we can sandwhich a Monad in Hask^op in the middle to obtain w (a -> r-) -> r+, and then take a coend over r to obtain forall r. w (a -> r) -> r. This gives rise to Co. If we observe that we didn't care what the choices we made for r were to finish this construction, we can upgrade to forall r. w (a -> m r) -> m r in a manner similar to how ContT is constructed yielding CoT.

We could consider unifying the definition of Co and Rift, but there are many other arguments for which Rift can form a Monad, and this wouldn't give rise to CoT.

Monads from Comonads

type Co w = CoT w Identity

co :: Functor w => (forall r. w (a -> r) -> r) -> Co w a

runCo :: Functor w => Co w a -> w (a -> r) -> r

Monad Transformers from Comonads

newtype CoT w m a

Co w a ~ Rift w Identity a

Constructors

CoT

Fields

runCoT :: forall r. w (a -> m r) -> m r

Instances

(Comonad w, MonadState s m) => MonadState s (CoT w m)

(Comonad w, MonadReader e m) => MonadReader e (CoT w m)

(Comonad w, MonadError e m) => MonadError e (CoT w m)

(Comonad w, MonadWriter e m) => MonadWriter e (CoT w m)

Comonad w => MonadTrans (CoT w)

Comonad w => Monad (CoT w m)

Functor w => Functor (CoT w m)

Comonad w => Applicative (CoT w m)

(Comonad w, MonadIO m) => MonadIO (CoT w m)

Extend w => Apply (CoT w m)

Extend w => Bind (CoT w m)

Klesili from CoKleisli

liftCoT0 :: Comonad w => (forall a. w a -> s) -> CoT w m s

liftCoT0M :: (Comonad w, Monad m) => (forall a. w a -> m s) -> CoT w m s

lowerCoT0 :: (Functor w, Monad m) => CoT w m s -> w a -> m s

lowerCo0 :: Functor w => Co w s -> w a -> s

liftCoT1 :: (forall a. w a -> a) -> CoT w m ()

liftCoT1M :: Monad m => (forall a. w a -> m a) -> CoT w m ()

lowerCoT1 :: (Functor w, Monad m) => CoT w m () -> w a -> m a

lowerCo1 :: Functor w => Co w () -> w a -> a

diter :: Functor f => a -> (a -> f a) -> Density (Cofree f) a

dctrlM :: Monad m => (forall a. w a -> m (w a)) -> CoT (Density w) m ()

posW :: ComonadStore s w => CoT w m s

peekW :: ComonadStore s w => s -> CoT w m ()

peeksW :: ComonadStore s w => (s -> s) -> CoT w m ()

askW :: ComonadEnv e w => CoT w m e

asksW :: ComonadEnv e w => (e -> a) -> CoT w m a

traceW :: ComonadTraced e w => e -> CoT w m ()