{-# 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 { forall (m :: * -> *) a. MaybeT m a -> m (Maybe a)
runMaybeT :: m (Maybe a) }

instance (Eq1 m) => Eq1 (MaybeT m) where
    liftEq :: forall a b. (a -> b -> Bool) -> MaybeT m a -> MaybeT m b -> Bool
liftEq a -> b -> Bool
eq (MaybeT m (Maybe a)
x) (MaybeT m (Maybe b)
y) = forall (f :: * -> *) a b.
Eq1 f =>
(a -> b -> Bool) -> f a -> f b -> Bool
liftEq (forall (f :: * -> *) a b.
Eq1 f =>
(a -> b -> Bool) -> f a -> f b -> Bool
liftEq a -> b -> Bool
eq) m (Maybe a)
x m (Maybe b)
y
    {-# INLINE liftEq #-}

instance (Ord1 m) => Ord1 (MaybeT m) where
    liftCompare :: forall a b.
(a -> b -> Ordering) -> MaybeT m a -> MaybeT m b -> Ordering
liftCompare a -> b -> Ordering
comp (MaybeT m (Maybe a)
x) (MaybeT m (Maybe b)
y) = forall (f :: * -> *) a b.
Ord1 f =>
(a -> b -> Ordering) -> f a -> f b -> Ordering
liftCompare (forall (f :: * -> *) a b.
Ord1 f =>
(a -> b -> Ordering) -> f a -> f b -> Ordering
liftCompare a -> b -> Ordering
comp) m (Maybe a)
x m (Maybe b)
y
    {-# INLINE liftCompare #-}

instance (Read1 m) => Read1 (MaybeT m) where
    liftReadsPrec :: forall a.
(Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (MaybeT m a)
liftReadsPrec Int -> ReadS a
rp ReadS [a]
rl = forall a. (String -> ReadS a) -> Int -> ReadS a
readsData forall a b. (a -> b) -> a -> b
$
        forall a t.
(Int -> ReadS a) -> String -> (a -> t) -> String -> ReadS t
readsUnaryWith (forall (f :: * -> *) a.
Read1 f =>
(Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (f a)
liftReadsPrec Int -> ReadS (Maybe a)
rp' ReadS [Maybe a]
rl') String
"MaybeT" forall (m :: * -> *) a. m (Maybe a) -> MaybeT m a
MaybeT
      where
        rp' :: Int -> ReadS (Maybe a)
rp' = forall (f :: * -> *) a.
Read1 f =>
(Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (f a)
liftReadsPrec Int -> ReadS a
rp ReadS [a]
rl
        rl' :: ReadS [Maybe a]
rl' = forall (f :: * -> *) a.
Read1 f =>
(Int -> ReadS a) -> ReadS [a] -> ReadS [f a]
liftReadList Int -> ReadS a
rp ReadS [a]
rl

instance (Show1 m) => Show1 (MaybeT m) where
    liftShowsPrec :: forall a.
(Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> MaybeT m a -> ShowS
liftShowsPrec Int -> a -> ShowS
sp [a] -> ShowS
sl Int
d (MaybeT m (Maybe a)
m) =
        forall a. (Int -> a -> ShowS) -> String -> Int -> a -> ShowS
showsUnaryWith (forall (f :: * -> *) a.
Show1 f =>
(Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> f a -> ShowS
liftShowsPrec Int -> Maybe a -> ShowS
sp' [Maybe a] -> ShowS
sl') String
"MaybeT" Int
d m (Maybe a)
m
      where
        sp' :: Int -> Maybe a -> ShowS
sp' = forall (f :: * -> *) a.
Show1 f =>
(Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> f a -> ShowS
liftShowsPrec Int -> a -> ShowS
sp [a] -> ShowS
sl
        sl' :: [Maybe a] -> ShowS
sl' = forall (f :: * -> *) a.
Show1 f =>
(Int -> a -> ShowS) -> ([a] -> ShowS) -> [f a] -> ShowS
liftShowList Int -> a -> ShowS
sp [a] -> ShowS
sl

instance (Eq1 m, Eq a) => Eq (MaybeT m a) where == :: MaybeT m a -> MaybeT m a -> Bool
(==) = forall (f :: * -> *) a. (Eq1 f, Eq a) => f a -> f a -> Bool
eq1
instance (Ord1 m, Ord a) => Ord (MaybeT m a) where compare :: MaybeT m a -> MaybeT m a -> Ordering
compare = forall (f :: * -> *) a. (Ord1 f, Ord a) => f a -> f a -> Ordering
compare1
instance (Read1 m, Read a) => Read (MaybeT m a) where readsPrec :: Int -> ReadS (MaybeT m a)
readsPrec = forall (f :: * -> *) a. (Read1 f, Read a) => Int -> ReadS (f a)
readsPrec1
instance (Show1 m, Show a) => Show (MaybeT m a) where showsPrec :: Int -> MaybeT m a -> ShowS
showsPrec = forall (f :: * -> *) a. (Show1 f, Show a) => Int -> f a -> ShowS
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 :: forall (m :: * -> *) a (n :: * -> *) b.
(m (Maybe a) -> n (Maybe b)) -> MaybeT m a -> MaybeT n b
mapMaybeT m (Maybe a) -> n (Maybe b)
f = forall (m :: * -> *) a. m (Maybe a) -> MaybeT m a
MaybeT forall b c a. (b -> c) -> (a -> b) -> a -> c
. m (Maybe a) -> n (Maybe b)
f forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall (m :: * -> *) a. MaybeT m a -> m (Maybe a)
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 :: forall (m :: * -> *) e a.
Functor m =>
e -> MaybeT m a -> ExceptT e m a
maybeToExceptT e
e (MaybeT m (Maybe a)
m) = forall e (m :: * -> *) a. m (Either e a) -> ExceptT e m a
ExceptT forall a b. (a -> b) -> a -> b
$ forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap (forall b a. b -> (a -> b) -> Maybe a -> b
maybe (forall a b. a -> Either a b
Left e
e) forall a b. b -> Either a b
Right) m (Maybe a)
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 :: forall (m :: * -> *) e a. Functor m => ExceptT e m a -> MaybeT m a
exceptToMaybeT (ExceptT m (Either e a)
m) = forall (m :: * -> *) a. m (Maybe a) -> MaybeT m a
MaybeT forall a b. (a -> b) -> a -> b
$ forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap (forall a c b. (a -> c) -> (b -> c) -> Either a b -> c
either (forall a b. a -> b -> a
const forall a. Maybe a
Nothing) forall a. a -> Maybe a
Just) m (Either e a)
m
{-# INLINE exceptToMaybeT #-}

instance (Functor m) => Functor (MaybeT m) where
    fmap :: forall a b. (a -> b) -> MaybeT m a -> MaybeT m b
fmap a -> b
f = forall (m :: * -> *) a (n :: * -> *) b.
(m (Maybe a) -> n (Maybe b)) -> MaybeT m a -> MaybeT n b
mapMaybeT (forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap (forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap a -> b
f))
    {-# INLINE fmap #-}

instance (Foldable f) => Foldable (MaybeT f) where
    foldMap :: forall m a. Monoid m => (a -> m) -> MaybeT f a -> m
foldMap a -> m
f (MaybeT f (Maybe a)
a) = forall (t :: * -> *) m a.
(Foldable t, Monoid m) =>
(a -> m) -> t a -> m
foldMap (forall (t :: * -> *) m a.
(Foldable t, Monoid m) =>
(a -> m) -> t a -> m
foldMap a -> m
f) f (Maybe a)
a
    {-# INLINE foldMap #-}

instance (Traversable f) => Traversable (MaybeT f) where
    traverse :: forall (f :: * -> *) a b.
Applicative f =>
(a -> f b) -> MaybeT f a -> f (MaybeT f b)
traverse a -> f b
f (MaybeT f (Maybe a)
a) = forall (m :: * -> *) a. m (Maybe a) -> MaybeT m a
MaybeT forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
<$> forall (t :: * -> *) (f :: * -> *) a b.
(Traversable t, Applicative f) =>
(a -> f b) -> t a -> f (t b)
traverse (forall (t :: * -> *) (f :: * -> *) a b.
(Traversable t, Applicative f) =>
(a -> f b) -> t a -> f (t b)
traverse a -> f b
f) f (Maybe a)
a
    {-# INLINE traverse #-}

instance (Functor m, Monad m) => Applicative (MaybeT m) where
    pure :: forall a. a -> MaybeT m a
pure = forall (m :: * -> *) a. m (Maybe a) -> MaybeT m a
MaybeT forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall (m :: * -> *) a. Monad m => a -> m a
return forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall a. a -> Maybe a
Just
    {-# INLINE pure #-}
    MaybeT m (a -> b)
mf <*> :: forall a b. MaybeT m (a -> b) -> MaybeT m a -> MaybeT m b
<*> MaybeT m a
mx = forall (m :: * -> *) a. m (Maybe a) -> MaybeT m a
MaybeT forall a b. (a -> b) -> a -> b
$ do
        Maybe (a -> b)
mb_f <- forall (m :: * -> *) a. MaybeT m a -> m (Maybe a)
runMaybeT MaybeT m (a -> b)
mf
        case Maybe (a -> b)
mb_f of
            Maybe (a -> b)
Nothing -> forall (m :: * -> *) a. Monad m => a -> m a
return forall a. Maybe a
Nothing
            Just a -> b
f  -> do
                Maybe a
mb_x <- forall (m :: * -> *) a. MaybeT m a -> m (Maybe a)
runMaybeT MaybeT m a
mx
                case Maybe a
mb_x of
                    Maybe a
Nothing -> forall (m :: * -> *) a. Monad m => a -> m a
return forall a. Maybe a
Nothing
                    Just a
x  -> forall (m :: * -> *) a. Monad m => a -> m a
return (forall a. a -> Maybe a
Just (a -> b
f a
x))
    {-# INLINE (<*>) #-}
    MaybeT m a
m *> :: forall a b. MaybeT m a -> MaybeT m b -> MaybeT m b
*> MaybeT m b
k = MaybeT m a
m forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
>>= \a
_ -> MaybeT m b
k
    {-# INLINE (*>) #-}

instance (Functor m, Monad m) => Alternative (MaybeT m) where
    empty :: forall a. MaybeT m a
empty = forall (m :: * -> *) a. m (Maybe a) -> MaybeT m a
MaybeT (forall (m :: * -> *) a. Monad m => a -> m a
return forall a. Maybe a
Nothing)
    {-# INLINE empty #-}
    MaybeT m a
x <|> :: forall a. MaybeT m a -> MaybeT m a -> MaybeT m a
<|> MaybeT m a
y = forall (m :: * -> *) a. m (Maybe a) -> MaybeT m a
MaybeT forall a b. (a -> b) -> a -> b
$ do
        Maybe a
v <- forall (m :: * -> *) a. MaybeT m a -> m (Maybe a)
runMaybeT MaybeT m a
x
        case Maybe a
v of
            Maybe a
Nothing -> forall (m :: * -> *) a. MaybeT m a -> m (Maybe a)
runMaybeT MaybeT m a
y
            Just a
_  -> forall (m :: * -> *) a. Monad m => a -> m a
return Maybe a
v
    {-# INLINE (<|>) #-}

instance (Monad m) => Monad (MaybeT m) where
#if !(MIN_VERSION_base(4,8,0))
    return = MaybeT . return . Just
    {-# INLINE return #-}
#endif
    MaybeT m a
x >>= :: forall a b. MaybeT m a -> (a -> MaybeT m b) -> MaybeT m b
>>= a -> MaybeT m b
f = forall (m :: * -> *) a. m (Maybe a) -> MaybeT m a
MaybeT forall a b. (a -> b) -> a -> b
$ do
        Maybe a
v <- forall (m :: * -> *) a. MaybeT m a -> m (Maybe a)
runMaybeT MaybeT m a
x
        case Maybe a
v of
            Maybe a
Nothing -> forall (m :: * -> *) a. Monad m => a -> m a
return forall a. Maybe a
Nothing
            Just a
y  -> forall (m :: * -> *) a. MaybeT m a -> m (Maybe a)
runMaybeT (a -> MaybeT m b
f a
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 :: forall a. String -> MaybeT m a
fail String
_ = forall (m :: * -> *) a. m (Maybe a) -> MaybeT m a
MaybeT (forall (m :: * -> *) a. Monad m => a -> m a
return forall a. Maybe a
Nothing)
    {-# INLINE fail #-}
#endif

instance (Monad m) => MonadPlus (MaybeT m) where
    mzero :: forall a. MaybeT m a
mzero = forall (m :: * -> *) a. m (Maybe a) -> MaybeT m a
MaybeT (forall (m :: * -> *) a. Monad m => a -> m a
return forall a. Maybe a
Nothing)
    {-# INLINE mzero #-}
    mplus :: forall a. MaybeT m a -> MaybeT m a -> MaybeT m a
mplus MaybeT m a
x MaybeT m a
y = forall (m :: * -> *) a. m (Maybe a) -> MaybeT m a
MaybeT forall a b. (a -> b) -> a -> b
$ do
        Maybe a
v <- forall (m :: * -> *) a. MaybeT m a -> m (Maybe a)
runMaybeT MaybeT m a
x
        case Maybe a
v of
            Maybe a
Nothing -> forall (m :: * -> *) a. MaybeT m a -> m (Maybe a)
runMaybeT MaybeT m a
y
            Just a
_  -> forall (m :: * -> *) a. Monad m => a -> m a
return Maybe a
v
    {-# INLINE mplus #-}

instance (MonadFix m) => MonadFix (MaybeT m) where
    mfix :: forall a. (a -> MaybeT m a) -> MaybeT m a
mfix a -> MaybeT m a
f = forall (m :: * -> *) a. m (Maybe a) -> MaybeT m a
MaybeT (forall (m :: * -> *) a. MonadFix m => (a -> m a) -> m a
mfix (forall (m :: * -> *) a. MaybeT m a -> m (Maybe a)
runMaybeT forall b c a. (b -> c) -> (a -> b) -> a -> c
. a -> MaybeT m a
f forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall a. a -> Maybe a -> a
fromMaybe forall {a}. a
bomb))
      where bomb :: a
bomb = forall a. HasCallStack => String -> a
error String
"mfix (MaybeT): inner computation returned Nothing"
    {-# INLINE mfix #-}

instance MonadTrans MaybeT where
    lift :: forall (m :: * -> *) a. Monad m => m a -> MaybeT m a
lift = forall (m :: * -> *) a. m (Maybe a) -> MaybeT m a
MaybeT forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall (m :: * -> *) a1 r. Monad m => (a1 -> r) -> m a1 -> m r
liftM forall a. a -> Maybe a
Just
    {-# INLINE lift #-}

instance (MonadIO m) => MonadIO (MaybeT m) where
    liftIO :: forall a. IO a -> MaybeT m a
liftIO = forall (t :: (* -> *) -> * -> *) (m :: * -> *) a.
(MonadTrans t, Monad m) =>
m a -> t m a
lift forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall (m :: * -> *) a. MonadIO m => IO a -> m a
liftIO
    {-# INLINE liftIO #-}

#if MIN_VERSION_base(4,4,0)
instance (MonadZip m) => MonadZip (MaybeT m) where
    mzipWith :: forall a b c.
(a -> b -> c) -> MaybeT m a -> MaybeT m b -> MaybeT m c
mzipWith a -> b -> c
f (MaybeT m (Maybe a)
a) (MaybeT m (Maybe b)
b) = forall (m :: * -> *) a. m (Maybe a) -> MaybeT m a
MaybeT forall a b. (a -> b) -> a -> b
$ forall (m :: * -> *) a b c.
MonadZip m =>
(a -> b -> c) -> m a -> m b -> m c
mzipWith (forall (f :: * -> *) a b c.
Applicative f =>
(a -> b -> c) -> f a -> f b -> f c
liftA2 a -> b -> c
f) m (Maybe a)
a m (Maybe b)
b
    {-# INLINE mzipWith #-}
#endif

#if MIN_VERSION_base(4,12,0)
instance Contravariant m => Contravariant (MaybeT m) where
    contramap :: forall a' a. (a' -> a) -> MaybeT m a -> MaybeT m a'
contramap a' -> a
f = forall (m :: * -> *) a. m (Maybe a) -> MaybeT m a
MaybeT forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall (f :: * -> *) a' a.
Contravariant f =>
(a' -> a) -> f a -> f a'
contramap (forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap a' -> a
f) forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall (m :: * -> *) a. MaybeT m a -> m (Maybe a)
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 :: forall (m :: * -> *) a b.
CallCC m (Maybe a) (Maybe b) -> CallCC (MaybeT m) a b
liftCallCC CallCC m (Maybe a) (Maybe b)
callCC (a -> MaybeT m b) -> MaybeT m a
f =
    forall (m :: * -> *) a. m (Maybe a) -> MaybeT m a
MaybeT forall a b. (a -> b) -> a -> b
$ CallCC m (Maybe a) (Maybe b)
callCC forall a b. (a -> b) -> a -> b
$ \ Maybe a -> m (Maybe b)
c -> forall (m :: * -> *) a. MaybeT m a -> m (Maybe a)
runMaybeT ((a -> MaybeT m b) -> MaybeT m a
f (forall (m :: * -> *) a. m (Maybe a) -> MaybeT m a
MaybeT forall b c a. (b -> c) -> (a -> b) -> a -> c
. Maybe a -> m (Maybe b)
c forall b c a. (b -> c) -> (a -> b) -> a -> c
. forall a. a -> Maybe a
Just))
{-# INLINE liftCallCC #-}

-- | Lift a @catchE@ operation to the new monad.
liftCatch :: Catch e m (Maybe a) -> Catch e (MaybeT m) a
liftCatch :: forall e (m :: * -> *) a.
Catch e m (Maybe a) -> Catch e (MaybeT m) a
liftCatch Catch e m (Maybe a)
f MaybeT m a
m e -> MaybeT m a
h = forall (m :: * -> *) a. m (Maybe a) -> MaybeT m a
MaybeT forall a b. (a -> b) -> a -> b
$ Catch e m (Maybe a)
f (forall (m :: * -> *) a. MaybeT m a -> m (Maybe a)
runMaybeT MaybeT m a
m) (forall (m :: * -> *) a. MaybeT m a -> m (Maybe a)
runMaybeT forall b c a. (b -> c) -> (a -> b) -> a -> c
. e -> MaybeT m a
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 :: forall (m :: * -> *) w a.
Monad m =>
Listen w m (Maybe a) -> Listen w (MaybeT m) a
liftListen Listen w m (Maybe a)
listen = forall (m :: * -> *) a (n :: * -> *) b.
(m (Maybe a) -> n (Maybe b)) -> MaybeT m a -> MaybeT n b
mapMaybeT forall a b. (a -> b) -> a -> b
$ \ m (Maybe a)
m -> do
    (Maybe a
a, w
w) <- Listen w m (Maybe a)
listen m (Maybe a)
m
    forall (m :: * -> *) a. Monad m => a -> m a
return forall a b. (a -> b) -> a -> b
$! forall (f :: * -> *) a b. Functor f => (a -> b) -> f a -> f b
fmap (\ a
r -> (a
r, w
w)) Maybe a
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 :: forall (m :: * -> *) w a.
Monad m =>
Pass w m (Maybe a) -> Pass w (MaybeT m) a
liftPass Pass w m (Maybe a)
pass = forall (m :: * -> *) a (n :: * -> *) b.
(m (Maybe a) -> n (Maybe b)) -> MaybeT m a -> MaybeT n b
mapMaybeT forall a b. (a -> b) -> a -> b
$ \ m (Maybe (a, w -> w))
m -> Pass w m (Maybe a)
pass forall a b. (a -> b) -> a -> b
$ do
    Maybe (a, w -> w)
a <- m (Maybe (a, w -> w))
m
    forall (m :: * -> *) a. Monad m => a -> m a
return forall a b. (a -> b) -> a -> b
$! case Maybe (a, w -> w)
a of
        Maybe (a, w -> w)
Nothing     -> (forall a. Maybe a
Nothing, forall a. a -> a
id)
        Just (a
v, w -> w
f) -> (forall a. a -> Maybe a
Just a
v, w -> w
f)
{-# INLINE liftPass #-}