Lifter.hs

{-# LANGUAGE OverlappingInstances, FlexibleInstances, FlexibleContexts, MultiParamTypeClasses, UndecidableInstances, TypeOperators, Safe #-}

module Control.Monad.Lifter where

import Control.Monad.Trans
import Control.Monad.ST
import Control.Monad.Identity
import Control.Monad.Morph


-- | An automatic lifter. The idea of automatic lifting is due to Dan Piponi.
class Lifter m n where
	lf :: m t -> n t

instance Lifter (ST RealWorld) IO where
	lf = stToIO

instance Lifter IO IO where
	lf = id

instance Lifter (ST s) (ST s) where
	lf = id

instance Lifter Identity Identity where
	lf = id

instance (MonadTrans n, Monad x, Lifter m x) => Lifter m (n x) where
	lf = lift . lf

instance (Monad x, MFunctor m) => Lifter (m Identity) (m x) where
	lf = hoist (return . runIdentity)