DeepControl.Monad.Morph

This module extended mmorph package's Control.Monad.Morph module.

Documentation

module Control.Monad.Morph

Level-1

(|>|) :: (Monad m, MFunctor t) => (forall a. m a -> n a) -> t m b -> t n b infixl 4

Alias for hoist.

(|<|) :: (Monad m, MFunctor t) => t m b -> (forall a. m a -> n a) -> t n b infixr 4

Equivalent to (|>|) with the arguments flipped.

Level-2

(|>>|) :: (Monad m, Monad (t2 m), MFunctor t1, MFunctor t2) => (forall a. m a -> n a) -> t1 (t2 m) b -> t1 (t2 n) b infixl 4

(|<<|) :: (Monad m, Monad (t2 m), MFunctor t1, MFunctor t2) => t1 (t2 m) b -> (forall a. m a -> n a) -> t1 (t2 n) b infixr 4

Level-3

(|>>>|) :: (Monad m, Monad (t3 m), Monad (t2 (t3 m)), MFunctor t1, MFunctor t2, MFunctor t3) => (forall a. m a -> n a) -> t1 (t2 (t3 m)) b -> t1 (t2 (t3 n)) b infixl 4

(|<<<|) :: (Monad m, Monad (t3 m), Monad (t2 (t3 m)), MFunctor t1, MFunctor t2, MFunctor t3) => t1 (t2 (t3 m)) b -> (forall a. m a -> n a) -> t1 (t2 (t3 n)) b infixr 4

Level-2 example

Here is a monad-morph example, a level-2 monad-morph.

import DeepControl.Monad.Morph
import DeepControl.Monad.Trans.State
import DeepControl.Monad.Trans.Writer

-- i.e. :: StateT Int Identity ()
tick    :: State Int ()
tick = modify (+1)

tock                        ::                   StateT Int IO ()
tock = do
    generalize |>| tick     :: (Monad      m) => StateT Int m  ()
    lift $ putStrLn "Tock!" :: (MonadTrans t) => t          IO ()

-- λ> runStateT tock 0
-- Tock!
-- ((),1)

-- i.e. :: StateT Int (WriterT [Int] Identity) ()
save    :: StateT Int (Writer  [Int]) ()
save = do
    n <- get
    lift $ tell [n]

program ::                   StateT Int (WriterT [Int] IO) ()
program = replicateM_ 4 $ do
    lift |>| tock
        :: (MonadTrans t) => StateT Int (t             IO) ()
    generalize |>>| save
        :: (Monad      m) => StateT Int (WriterT [Int] m ) ()

-- λ> execWriterT (runStateT program 0)
-- Tock!
-- Tock!
-- Tock!
-- Tock!
-- [1,2,3,4]