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{-# LANGUAGE CPP #-}
#if __GLASGOW_HASKELL__ >= 702
{-# LANGUAGE Safe #-}
#endif
#if __GLASGOW_HASKELL__ >= 706
{-# LANGUAGE PolyKinds #-}
#endif
#if __GLASGOW_HASKELL__ >= 710
{-# LANGUAGE AutoDeriveTypeable #-}
#endif
-----------------------------------------------------------------------------
-- |
-- Module : Control.Monad.Trans.Identity
-- Copyright : (c) 2007 Magnus Therning
-- License : BSD-style (see the file LICENSE)
--
-- Maintainer : R.Paterson@city.ac.uk
-- Stability : experimental
-- Portability : portable
--
-- The identity monad transformer.
--
-- This is useful for functions parameterized by a monad transformer.
-----------------------------------------------------------------------------
module Control.Monad.Trans.Identity (
-- * The identity monad transformer
IdentityT(..),
mapIdentityT,
-- * Lifting other operations
liftCatch,
liftCallCC,
) where
import Control.Monad.IO.Class (MonadIO(liftIO))
import Control.Monad.Signatures
import Control.Monad.Trans.Class (MonadTrans(lift))
import Data.Functor.Classes
#if MIN_VERSION_base(4,12,0)
import Data.Functor.Contravariant
#endif
import Control.Applicative
import Control.Monad (MonadPlus(mzero, mplus))
#if MIN_VERSION_base(4,9,0)
import qualified Control.Monad.Fail as Fail
#endif
import Control.Monad.Fix (MonadFix(mfix))
#if MIN_VERSION_base(4,4,0)
import Control.Monad.Zip (MonadZip(mzipWith))
#endif
import Data.Foldable
import Data.Traversable (Traversable(traverse))
import Prelude hiding (foldr, foldr1, foldl, foldl1, null, length)
-- | The trivial monad transformer, which maps a monad to an equivalent monad.
newtype IdentityT f a = IdentityT { runIdentityT :: f a }
instance (Eq1 f) => Eq1 (IdentityT f) where
liftEq eq (IdentityT x) (IdentityT y) = liftEq eq x y
{-# INLINE liftEq #-}
instance (Ord1 f) => Ord1 (IdentityT f) where
liftCompare comp (IdentityT x) (IdentityT y) = liftCompare comp x y
{-# INLINE liftCompare #-}
instance (Read1 f) => Read1 (IdentityT f) where
liftReadsPrec rp rl = readsData $
readsUnaryWith (liftReadsPrec rp rl) "IdentityT" IdentityT
instance (Show1 f) => Show1 (IdentityT f) where
liftShowsPrec sp sl d (IdentityT m) =
showsUnaryWith (liftShowsPrec sp sl) "IdentityT" d m
instance (Eq1 f, Eq a) => Eq (IdentityT f a) where (==) = eq1
instance (Ord1 f, Ord a) => Ord (IdentityT f a) where compare = compare1
instance (Read1 f, Read a) => Read (IdentityT f a) where readsPrec = readsPrec1
instance (Show1 f, Show a) => Show (IdentityT f a) where showsPrec = showsPrec1
instance (Functor m) => Functor (IdentityT m) where
fmap f = mapIdentityT (fmap f)
{-# INLINE fmap #-}
instance (Foldable f) => Foldable (IdentityT f) where
foldMap f (IdentityT t) = foldMap f t
{-# INLINE foldMap #-}
foldr f z (IdentityT t) = foldr f z t
{-# INLINE foldr #-}
foldl f z (IdentityT t) = foldl f z t
{-# INLINE foldl #-}
foldr1 f (IdentityT t) = foldr1 f t
{-# INLINE foldr1 #-}
foldl1 f (IdentityT t) = foldl1 f t
{-# INLINE foldl1 #-}
#if MIN_VERSION_base(4,8,0)
null (IdentityT t) = null t
length (IdentityT t) = length t
#endif
instance (Traversable f) => Traversable (IdentityT f) where
traverse f (IdentityT a) = IdentityT <$> traverse f a
{-# INLINE traverse #-}
instance (Applicative m) => Applicative (IdentityT m) where
pure x = IdentityT (pure x)
{-# INLINE pure #-}
(<*>) = lift2IdentityT (<*>)
{-# INLINE (<*>) #-}
(*>) = lift2IdentityT (*>)
{-# INLINE (*>) #-}
(<*) = lift2IdentityT (<*)
{-# INLINE (<*) #-}
instance (Alternative m) => Alternative (IdentityT m) where
empty = IdentityT empty
{-# INLINE empty #-}
(<|>) = lift2IdentityT (<|>)
{-# INLINE (<|>) #-}
instance (Monad m) => Monad (IdentityT m) where
#if !(MIN_VERSION_base(4,8,0))
return = IdentityT . return
{-# INLINE return #-}
#endif
m >>= k = IdentityT $ runIdentityT . k =<< runIdentityT m
{-# INLINE (>>=) #-}
#if !(MIN_VERSION_base(4,13,0))
fail msg = IdentityT $ fail msg
{-# INLINE fail #-}
#endif
#if MIN_VERSION_base(4,9,0)
instance (Fail.MonadFail m) => Fail.MonadFail (IdentityT m) where
fail msg = IdentityT $ Fail.fail msg
{-# INLINE fail #-}
#endif
instance (MonadPlus m) => MonadPlus (IdentityT m) where
mzero = IdentityT mzero
{-# INLINE mzero #-}
mplus = lift2IdentityT mplus
{-# INLINE mplus #-}
instance (MonadFix m) => MonadFix (IdentityT m) where
mfix f = IdentityT (mfix (runIdentityT . f))
{-# INLINE mfix #-}
instance (MonadIO m) => MonadIO (IdentityT m) where
liftIO = IdentityT . liftIO
{-# INLINE liftIO #-}
#if MIN_VERSION_base(4,4,0)
instance (MonadZip m) => MonadZip (IdentityT m) where
mzipWith f = lift2IdentityT (mzipWith f)
{-# INLINE mzipWith #-}
#endif
instance MonadTrans IdentityT where
lift = IdentityT
{-# INLINE lift #-}
#if MIN_VERSION_base(4,12,0)
instance Contravariant f => Contravariant (IdentityT f) where
contramap f = IdentityT . contramap f . runIdentityT
{-# INLINE contramap #-}
#endif
-- | Lift a unary operation to the new monad.
mapIdentityT :: (m a -> n b) -> IdentityT m a -> IdentityT n b
mapIdentityT f = IdentityT . f . runIdentityT
{-# INLINE mapIdentityT #-}
-- | Lift a binary operation to the new monad.
lift2IdentityT ::
(m a -> n b -> p c) -> IdentityT m a -> IdentityT n b -> IdentityT p c
lift2IdentityT f a b = IdentityT (f (runIdentityT a) (runIdentityT b))
{-# INLINE lift2IdentityT #-}
-- | Lift a @callCC@ operation to the new monad.
liftCallCC :: CallCC m a b -> CallCC (IdentityT m) a b
liftCallCC callCC f =
IdentityT $ callCC $ \ c -> runIdentityT (f (IdentityT . c))
{-# INLINE liftCallCC #-}
-- | Lift a @catchE@ operation to the new monad.
liftCatch :: Catch e m a -> Catch e (IdentityT m) a
liftCatch f m h = IdentityT $ f (runIdentityT m) (runIdentityT . h)
{-# INLINE liftCatch #-}
|
