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{-# LANGUAGE CPP #-}
#if __GLASGOW_HASKELL__ >= 702
{-# LANGUAGE Safe #-}
#endif
#if __GLASGOW_HASKELL__ >= 710
{-# LANGUAGE AutoDeriveTypeable #-}
#endif
-----------------------------------------------------------------------------
-- |
-- Module : Control.Monad.Trans.Maybe
-- Copyright : (c) 2007 Yitzak Gale, Eric Kidd
-- License : BSD-style (see the file LICENSE)
--
-- Maintainer : R.Paterson@city.ac.uk
-- Stability : experimental
-- Portability : portable
--
-- The 'MaybeT' monad transformer extends a monad with the ability to exit
-- the computation without returning a value.
--
-- A sequence of actions produces a value only if all the actions in
-- the sequence do. If one exits, the rest of the sequence is skipped
-- and the composite action exits.
--
-- For a variant allowing a range of exception values, see
-- "Control.Monad.Trans.Except".
-----------------------------------------------------------------------------
module Control.Monad.Trans.Maybe (
-- * The MaybeT monad transformer
MaybeT(..),
mapMaybeT,
-- * Monad transformations
maybeToExceptT,
exceptToMaybeT,
-- * Lifting other operations
liftCallCC,
liftCatch,
liftListen,
liftPass,
) where
import Control.Monad.IO.Class
import Control.Monad.Signatures
import Control.Monad.Trans.Class
import Control.Monad.Trans.Except (ExceptT(..))
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), liftM)
#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 (Foldable(foldMap))
import Data.Maybe (fromMaybe)
import Data.Traversable (Traversable(traverse))
-- | The parameterizable maybe monad, obtained by composing an arbitrary
-- monad with the 'Maybe' monad.
--
-- Computations are actions that may produce a value or exit.
--
-- The 'return' function yields a computation that produces that
-- value, while @>>=@ sequences two subcomputations, exiting if either
-- computation does.
newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) }
instance (Eq1 m) => Eq1 (MaybeT m) where
liftEq eq (MaybeT x) (MaybeT y) = liftEq (liftEq eq) x y
{-# INLINE liftEq #-}
instance (Ord1 m) => Ord1 (MaybeT m) where
liftCompare comp (MaybeT x) (MaybeT y) = liftCompare (liftCompare comp) x y
{-# INLINE liftCompare #-}
instance (Read1 m) => Read1 (MaybeT m) where
liftReadsPrec rp rl = readsData $
readsUnaryWith (liftReadsPrec rp' rl') "MaybeT" MaybeT
where
rp' = liftReadsPrec rp rl
rl' = liftReadList rp rl
instance (Show1 m) => Show1 (MaybeT m) where
liftShowsPrec sp sl d (MaybeT m) =
showsUnaryWith (liftShowsPrec sp' sl') "MaybeT" d m
where
sp' = liftShowsPrec sp sl
sl' = liftShowList sp sl
instance (Eq1 m, Eq a) => Eq (MaybeT m a) where (==) = eq1
instance (Ord1 m, Ord a) => Ord (MaybeT m a) where compare = compare1
instance (Read1 m, Read a) => Read (MaybeT m a) where readsPrec = readsPrec1
instance (Show1 m, Show a) => Show (MaybeT m a) where showsPrec = showsPrec1
-- | Transform the computation inside a @MaybeT@.
--
-- * @'runMaybeT' ('mapMaybeT' f m) = f ('runMaybeT' m)@
mapMaybeT :: (m (Maybe a) -> n (Maybe b)) -> MaybeT m a -> MaybeT n b
mapMaybeT f = MaybeT . f . runMaybeT
{-# INLINE mapMaybeT #-}
-- | Convert a 'MaybeT' computation to 'ExceptT', with a default
-- exception value.
maybeToExceptT :: (Functor m) => e -> MaybeT m a -> ExceptT e m a
maybeToExceptT e (MaybeT m) = ExceptT $ fmap (maybe (Left e) Right) m
{-# INLINE maybeToExceptT #-}
-- | Convert a 'ExceptT' computation to 'MaybeT', discarding the
-- value of any exception.
exceptToMaybeT :: (Functor m) => ExceptT e m a -> MaybeT m a
exceptToMaybeT (ExceptT m) = MaybeT $ fmap (either (const Nothing) Just) m
{-# INLINE exceptToMaybeT #-}
instance (Functor m) => Functor (MaybeT m) where
fmap f = mapMaybeT (fmap (fmap f))
{-# INLINE fmap #-}
instance (Foldable f) => Foldable (MaybeT f) where
foldMap f (MaybeT a) = foldMap (foldMap f) a
{-# INLINE foldMap #-}
instance (Traversable f) => Traversable (MaybeT f) where
traverse f (MaybeT a) = MaybeT <$> traverse (traverse f) a
{-# INLINE traverse #-}
instance (Functor m, Monad m) => Applicative (MaybeT m) where
pure = MaybeT . return . Just
{-# INLINE pure #-}
mf <*> mx = MaybeT $ do
mb_f <- runMaybeT mf
case mb_f of
Nothing -> return Nothing
Just f -> do
mb_x <- runMaybeT mx
case mb_x of
Nothing -> return Nothing
Just x -> return (Just (f x))
{-# INLINE (<*>) #-}
m *> k = m >>= \_ -> k
{-# INLINE (*>) #-}
instance (Functor m, Monad m) => Alternative (MaybeT m) where
empty = MaybeT (return Nothing)
{-# INLINE empty #-}
x <|> y = MaybeT $ do
v <- runMaybeT x
case v of
Nothing -> runMaybeT y
Just _ -> return v
{-# INLINE (<|>) #-}
instance (Monad m) => Monad (MaybeT m) where
#if !(MIN_VERSION_base(4,8,0))
return = MaybeT . return . Just
{-# INLINE return #-}
#endif
x >>= f = MaybeT $ do
v <- runMaybeT x
case v of
Nothing -> return Nothing
Just y -> runMaybeT (f y)
{-# INLINE (>>=) #-}
#if !(MIN_VERSION_base(4,13,0))
fail _ = MaybeT (return Nothing)
{-# INLINE fail #-}
#endif
#if MIN_VERSION_base(4,9,0)
instance (Monad m) => Fail.MonadFail (MaybeT m) where
fail _ = MaybeT (return Nothing)
{-# INLINE fail #-}
#endif
instance (Monad m) => MonadPlus (MaybeT m) where
mzero = MaybeT (return Nothing)
{-# INLINE mzero #-}
mplus x y = MaybeT $ do
v <- runMaybeT x
case v of
Nothing -> runMaybeT y
Just _ -> return v
{-# INLINE mplus #-}
instance (MonadFix m) => MonadFix (MaybeT m) where
mfix f = MaybeT (mfix (runMaybeT . f . fromMaybe bomb))
where bomb = error "mfix (MaybeT): inner computation returned Nothing"
{-# INLINE mfix #-}
instance MonadTrans MaybeT where
lift = MaybeT . liftM Just
{-# INLINE lift #-}
instance (MonadIO m) => MonadIO (MaybeT m) where
liftIO = lift . liftIO
{-# INLINE liftIO #-}
#if MIN_VERSION_base(4,4,0)
instance (MonadZip m) => MonadZip (MaybeT m) where
mzipWith f (MaybeT a) (MaybeT b) = MaybeT $ mzipWith (liftA2 f) a b
{-# INLINE mzipWith #-}
#endif
#if MIN_VERSION_base(4,12,0)
instance Contravariant m => Contravariant (MaybeT m) where
contramap f = MaybeT . contramap (fmap f) . runMaybeT
{-# INLINE contramap #-}
#endif
-- | Lift a @callCC@ operation to the new monad.
liftCallCC :: CallCC m (Maybe a) (Maybe b) -> CallCC (MaybeT m) a b
liftCallCC callCC f =
MaybeT $ callCC $ \ c -> runMaybeT (f (MaybeT . c . Just))
{-# INLINE liftCallCC #-}
-- | Lift a @catchE@ operation to the new monad.
liftCatch :: Catch e m (Maybe a) -> Catch e (MaybeT m) a
liftCatch f m h = MaybeT $ f (runMaybeT m) (runMaybeT . h)
{-# INLINE liftCatch #-}
-- | Lift a @listen@ operation to the new monad.
liftListen :: (Monad m) => Listen w m (Maybe a) -> Listen w (MaybeT m) a
liftListen listen = mapMaybeT $ \ m -> do
(a, w) <- listen m
return $! fmap (\ r -> (r, w)) a
{-# INLINE liftListen #-}
-- | Lift a @pass@ operation to the new monad.
liftPass :: (Monad m) => Pass w m (Maybe a) -> Pass w (MaybeT m) a
liftPass pass = mapMaybeT $ \ m -> pass $ do
a <- m
return $! case a of
Nothing -> (Nothing, id)
Just (v, f) -> (Just v, f)
{-# INLINE liftPass #-}
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