Pass.hs

{-# LANGUAGE CPP #-}
{-# LANGUAGE RankNTypes #-}
-- | Lifting the 'pass' operation.
module Control.Monad.Trans.Lift.Pass
    ( LiftPass(..)
    , Pass
    , defaultLiftPass
    , module Control.Monad.Trans.Class
    ) where

#if __GLASGOW_HASKELL__ < 710
import Data.Monoid
#endif

import Control.Monad.Signatures
import Control.Monad.Trans.Class

import qualified Control.Monad.Trans.Except        as E
import qualified Control.Monad.Trans.Identity      as I
import qualified Control.Monad.Trans.Maybe         as M
import qualified Control.Monad.Trans.Reader        as R
import qualified Control.Monad.Trans.RWS.Lazy      as RWS.Lazy
import qualified Control.Monad.Trans.RWS.Strict    as RWS.Strict
import qualified Control.Monad.Trans.State.Lazy    as S.Lazy
import qualified Control.Monad.Trans.State.Strict  as S.Strict
import qualified Control.Monad.Trans.Writer.Lazy   as W.Lazy
import qualified Control.Monad.Trans.Writer.Strict as W.Strict

import Control.Monad.Trans.Lift.StT

-- | The class of monad transformers capable of lifting 'pass'.
class MonadTrans t => LiftPass t where
    -- | Lift the 'pass' operation.
    -- Should satisfy the uniformity property
    --
    -- * @'lift' . 'liftPass' = 'liftPass' . 'lift'@
    --
    liftPass :: Monad m => Pass w m (StT t a) -> Pass w (t m) a

-- | Default definition for the 'liftPass' method.
defaultLiftPass
    :: (Monad m, LiftPass n)
    => (forall x . n m x -> t m x)
    -- ^ Monad constructor
    -> (forall o x . t o x -> n o x)
    -- ^ Monad deconstructor
    -> Pass w m (StT n a)
    -> Pass w (t m) a
defaultLiftPass t unT pass m = t $ liftPass pass (unT m)

instance LiftPass (E.ExceptT e) where
    liftPass = E.liftPass

instance LiftPass I.IdentityT where
    liftPass = I.mapIdentityT

instance LiftPass M.MaybeT where
    liftPass = M.liftPass

instance LiftPass (R.ReaderT r) where
    liftPass = R.mapReaderT

instance LiftPass (S.Lazy.StateT s) where
    liftPass = S.Lazy.liftPass

instance LiftPass (S.Strict.StateT s) where
    liftPass = S.Strict.liftPass

instance Monoid w' => LiftPass (RWS.Lazy.RWST r w' s) where
    liftPass pass m = RWS.Lazy.RWST $ \r s -> pass $ do
        ~((a, f), w', s') <- RWS.Lazy.runRWST m r s
        return ((a, w', s'), f)

instance Monoid w' => LiftPass (RWS.Strict.RWST r w' s) where
    liftPass pass m = RWS.Strict.RWST $ \r s -> pass $ do
        ((a, f), w', s') <- RWS.Strict.runRWST m r s
        return ((a, w', s'), f)

instance Monoid w' => LiftPass (W.Lazy.WriterT w') where
    liftPass pass m = W.Lazy.WriterT $ pass $ do
        ~((a, f), w') <- W.Lazy.runWriterT m
        return ((a, w'), f)

instance Monoid w' => LiftPass (W.Strict.WriterT w') where
    liftPass pass m = W.Strict.WriterT $ pass $ do
        ((a, f), w') <- W.Strict.runWriterT m
        return ((a, w'), f)