Trans.hs

-----------------------------------------------------------------------------
-- |
-- Module      :  Data.Functor.Bind.Trans
-- Copyright   :  (C) 2011 Edward Kmett,
-- License     :  BSD-style (see the file LICENSE)
--
-- Maintainer  :  Edward Kmett <ekmett@gmail.com>
-- Stability   :  provisional
-- Portability :  portable
--
----------------------------------------------------------------------------
module Data.Functor.Bind.Trans ( 
  BindTrans(..)
  ) where

-- import _everything_
import Control.Category
import Control.Monad.Instances
import Control.Monad.Trans.Class
import Control.Monad.Trans.Cont
-- import Control.Monad.Trans.Error
import Control.Monad.Trans.Identity
-- import Control.Monad.Trans.Maybe
import Control.Monad.Trans.Reader
-- import Control.Monad.Trans.List
import qualified Control.Monad.Trans.RWS.Lazy as Lazy
import qualified Control.Monad.Trans.State.Lazy as Lazy
import qualified Control.Monad.Trans.Writer.Lazy as Lazy
import qualified Control.Monad.Trans.RWS.Strict as Strict
import qualified Control.Monad.Trans.State.Strict as Strict
import qualified Control.Monad.Trans.Writer.Strict as Strict
import Data.Functor.Bind
import Data.Semigroup hiding (Product)
import Prelude hiding (id, (.))

-- | A subset of monad transformers can transform any 'Bind' as well.
class MonadTrans t => BindTrans t where
  liftB :: Bind b => b a -> t b a

instance BindTrans IdentityT where
  liftB = IdentityT

instance BindTrans (ReaderT e) where
  liftB = ReaderT . const 

instance (Semigroup w, Monoid w) => BindTrans (Lazy.WriterT w) where
  liftB = Lazy.WriterT . fmap (\a -> (a, mempty))

instance (Semigroup w, Monoid w) => BindTrans (Strict.WriterT w) where
  liftB = Strict.WriterT . fmap (\a -> (a, mempty))

instance BindTrans (Lazy.StateT s) where
  liftB m = Lazy.StateT $ \s -> fmap (\a -> (a, s)) m 

instance BindTrans (Strict.StateT s) where
  liftB m = Strict.StateT $ \s -> fmap (\a -> (a, s)) m 

instance (Semigroup w, Monoid w) => BindTrans (Lazy.RWST r w s) where
  liftB m = Lazy.RWST $ \ _r s -> fmap (\a -> (a, s, mempty)) m
  
instance (Semigroup w, Monoid w) => BindTrans (Strict.RWST r w s) where
  liftB m = Strict.RWST $ \ _r s -> fmap (\a -> (a, s, mempty)) m

instance BindTrans (ContT r) where
  liftB m = ContT (m >>-)