MaybeT

MaybeT

newtype MaybeT (m :: * -> *) a

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.

Constructors

MaybeT

Fields

• runMaybeT :: m (Maybe a)

Instances

MonadTrans MaybeT

Defined in Control.Monad.Trans.Maybe

Methods

lift :: Monad m => m a -> MaybeT m a

MMonad MaybeT

Defined in Control.Monad.Morph

Methods

embed :: Monad n => (forall a. m a -> MaybeT n a) -> MaybeT m b -> MaybeT n b

MonadReader r m => MonadReader r (MaybeT m)

Defined in Control.Monad.Reader.Class

Methods

ask :: MaybeT m r

local :: (r -> r) -> MaybeT m a -> MaybeT m a

reader :: (r -> a) -> MaybeT m a

MonadState s m => MonadState s (MaybeT m)

Defined in Control.Monad.State.Class

Methods

get :: MaybeT m s

put :: s -> MaybeT m ()

state :: (s -> (a, s)) -> MaybeT m a

(Functor f, MonadFree f m) => MonadFree f (MaybeT m)

Defined in Control.Monad.Free.Class

Methods

wrap :: f (MaybeT m a) -> MaybeT m a

MonadError e m => MonadError e (MaybeT m)

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> MaybeT m a

catchError :: MaybeT m a -> (e -> MaybeT m a) -> MaybeT m a

MonadWriter w m => MonadWriter w (MaybeT m)

Defined in Control.Monad.Writer.Class

Methods

writer :: (a, w) -> MaybeT m a

tell :: w -> MaybeT m ()

listen :: MaybeT m a -> MaybeT m (a, w)

pass :: MaybeT m (a, w -> w) -> MaybeT m a

MonadBase b m => MonadBase b (MaybeT m)

Defined in Control.Monad.Base

Methods

liftBase :: b α -> MaybeT m α

Monad m => Monad (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

(>>=) :: MaybeT m a -> (a -> MaybeT m b) -> MaybeT m b

(>>) :: MaybeT m a -> MaybeT m b -> MaybeT m b

return :: a -> MaybeT m a

fail :: String -> MaybeT m a

Functor m => Functor (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

fmap :: (a -> b) -> MaybeT m a -> MaybeT m b

(<$) :: a -> MaybeT m b -> MaybeT m a

MonadFix m => MonadFix (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

mfix :: (a -> MaybeT m a) -> MaybeT m a

Monad m => MonadFail (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

fail :: String -> MaybeT m a

(Functor m, Monad m) => Applicative (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

pure :: a -> MaybeT m a

(<*>) :: MaybeT m (a -> b) -> MaybeT m a -> MaybeT m b

liftA2 :: (a -> b -> c) -> MaybeT m a -> MaybeT m b -> MaybeT m c

(*>) :: MaybeT m a -> MaybeT m b -> MaybeT m b

(<*) :: MaybeT m a -> MaybeT m b -> MaybeT m a

Foldable f => Foldable (MaybeT f)

Defined in Control.Monad.Trans.Maybe

Methods

fold :: Monoid m => MaybeT f m -> m

foldMap :: Monoid m => (a -> m) -> MaybeT f a -> m

foldr :: (a -> b -> b) -> b -> MaybeT f a -> b

foldr' :: (a -> b -> b) -> b -> MaybeT f a -> b

foldl :: (b -> a -> b) -> b -> MaybeT f a -> b

foldl' :: (b -> a -> b) -> b -> MaybeT f a -> b

foldr1 :: (a -> a -> a) -> MaybeT f a -> a

foldl1 :: (a -> a -> a) -> MaybeT f a -> a

toList :: MaybeT f a -> [a]

null :: MaybeT f a -> Bool

length :: MaybeT f a -> Int

elem :: Eq a => a -> MaybeT f a -> Bool

maximum :: Ord a => MaybeT f a -> a

minimum :: Ord a => MaybeT f a -> a

sum :: Num a => MaybeT f a -> a

product :: Num a => MaybeT f a -> a

Traversable f => Traversable (MaybeT f)

Defined in Control.Monad.Trans.Maybe

Methods

traverse :: Applicative f0 => (a -> f0 b) -> MaybeT f a -> f0 (MaybeT f b)

sequenceA :: Applicative f0 => MaybeT f (f0 a) -> f0 (MaybeT f a)

mapM :: Monad m => (a -> m b) -> MaybeT f a -> m (MaybeT f b)

sequence :: Monad m => MaybeT f (m a) -> m (MaybeT f a)

(Functor m, Monad m) => Alternative (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

empty :: MaybeT m a

(<|>) :: MaybeT m a -> MaybeT m a -> MaybeT m a

some :: MaybeT m a -> MaybeT m [a]

many :: MaybeT m a -> MaybeT m [a]

Contravariant m => Contravariant (MaybeT m)

Defined in Data.Functor.Contravariant

Methods

contramap :: (a -> b) -> MaybeT m b -> MaybeT m a

(>$) :: b -> MaybeT m b -> MaybeT m a

Monad m => MonadPlus (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

mzero :: MaybeT m a

mplus :: MaybeT m a -> MaybeT m a -> MaybeT m a

Eq1 m => Eq1 (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

liftEq :: (a -> b -> Bool) -> MaybeT m a -> MaybeT m b -> Bool

Ord1 m => Ord1 (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

liftCompare :: (a -> b -> Ordering) -> MaybeT m a -> MaybeT m b -> Ordering

Read1 m => Read1 (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (MaybeT m a)

liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [MaybeT m a]

liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (MaybeT m a)

liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [MaybeT m a]

Show1 m => Show1 (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> MaybeT m a -> ShowS

liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [MaybeT m a] -> ShowS

MonadZip m => MonadZip (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

mzip :: MaybeT m a -> MaybeT m b -> MaybeT m (a, b)

mzipWith :: (a -> b -> c) -> MaybeT m a -> MaybeT m b -> MaybeT m c

munzip :: MaybeT m (a, b) -> (MaybeT m a, MaybeT m b)

MonadIO m => MonadIO (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

liftIO :: IO a -> MaybeT m a

Divisible m => Divisible (MaybeT m)

Defined in Data.Functor.Contravariant.Divisible

Methods

divide :: (a -> (b, c)) -> MaybeT m b -> MaybeT m c -> MaybeT m a

conquer :: MaybeT m a

Divisible m => Decidable (MaybeT m)

Defined in Data.Functor.Contravariant.Divisible

Methods

lose :: (a -> Void) -> MaybeT m a

choose :: (a -> Either b c) -> MaybeT m b -> MaybeT m c -> MaybeT m a

MonadThrow m => MonadThrow (MaybeT m)

Throws exceptions into the base monad.

Defined in Control.Monad.Catch

Methods

throwM :: Exception e => e -> MaybeT m a

MonadCatch m => MonadCatch (MaybeT m)

Catches exceptions from the base monad.

Defined in Control.Monad.Catch

Methods

catch :: Exception e => MaybeT m a -> (e -> MaybeT m a) -> MaybeT m a

MonadMask m => MonadMask (MaybeT m)

Since: exceptions-0.10.0

Defined in Control.Monad.Catch

Methods

mask :: ((forall a. MaybeT m a -> MaybeT m a) -> MaybeT m b) -> MaybeT m b

uninterruptibleMask :: ((forall a. MaybeT m a -> MaybeT m a) -> MaybeT m b) -> MaybeT m b

generalBracket :: MaybeT m a -> (a -> ExitCase b -> MaybeT m c) -> (a -> MaybeT m b) -> MaybeT m (b, c)

PrimMonad m => PrimMonad (MaybeT m)

Defined in Control.Monad.Primitive

Associated Types

type PrimState (MaybeT m) :: *

Methods

primitive :: (State# (PrimState (MaybeT m)) -> (#State# (PrimState (MaybeT m)), a#)) -> MaybeT m a

(Functor m, Monad m) => Apply (MaybeT m)

Defined in Data.Functor.Bind.Class

Methods

(<.>) :: MaybeT m (a -> b) -> MaybeT m a -> MaybeT m b

(.>) :: MaybeT m a -> MaybeT m b -> MaybeT m b

(<.) :: MaybeT m a -> MaybeT m b -> MaybeT m a

liftF2 :: (a -> b -> c) -> MaybeT m a -> MaybeT m b -> MaybeT m c

MonadManaged m => MonadManaged (MaybeT m)

Defined in Control.Monad.Managed

Methods

using :: Managed a -> MaybeT m a

MonadCont m => MonadCont (MaybeT m)

Defined in Control.Monad.Cont.Class

Methods

callCC :: ((a -> MaybeT m b) -> MaybeT m a) -> MaybeT m a

Pointed m => Pointed (MaybeT m)

Defined in Data.Pointed

Methods

point :: a -> MaybeT m a

(Bind f, Monad f) => Plus (MaybeT f)

Defined in Data.Functor.Plus

Methods

zero :: MaybeT f a

(Bind f, Monad f) => Alt (MaybeT f)

Defined in Data.Functor.Alt

Methods

(<!>) :: MaybeT f a -> MaybeT f a -> MaybeT f a

some :: Applicative (MaybeT f) => MaybeT f a -> MaybeT f [a]

many :: Applicative (MaybeT f) => MaybeT f a -> MaybeT f [a]

(Functor m, Monad m) => Bind (MaybeT m)

Defined in Data.Functor.Bind.Class

Methods

(>>-) :: MaybeT m a -> (a -> MaybeT m b) -> MaybeT m b

join :: MaybeT m (MaybeT m a) -> MaybeT m a

MFunctor MaybeT

Defined in Control.Monad.Morph

Methods

hoist :: Monad m => (forall a. m a -> n a) -> MaybeT m b -> MaybeT n b

Zoom m n s t => Zoom (MaybeT m) (MaybeT n) s t

Defined in Control.Lens.Zoom

Methods

zoom :: LensLike' (Zoomed (MaybeT m) c) t s -> MaybeT m c -> MaybeT n c

(Eq1 m, Eq a) => Eq (MaybeT m a)

Defined in Control.Monad.Trans.Maybe

Methods

(==) :: MaybeT m a -> MaybeT m a -> Bool

(/=) :: MaybeT m a -> MaybeT m a -> Bool

(Ord1 m, Ord a) => Ord (MaybeT m a)

Defined in Control.Monad.Trans.Maybe

Methods

compare :: MaybeT m a -> MaybeT m a -> Ordering

(<) :: MaybeT m a -> MaybeT m a -> Bool

(<=) :: MaybeT m a -> MaybeT m a -> Bool

(>) :: MaybeT m a -> MaybeT m a -> Bool

(>=) :: MaybeT m a -> MaybeT m a -> Bool

max :: MaybeT m a -> MaybeT m a -> MaybeT m a

min :: MaybeT m a -> MaybeT m a -> MaybeT m a

(Read1 m, Read a) => Read (MaybeT m a)

Defined in Control.Monad.Trans.Maybe

Methods

readsPrec :: Int -> ReadS (MaybeT m a)

readList :: ReadS [MaybeT m a]

readPrec :: ReadPrec (MaybeT m a)

readListPrec :: ReadPrec [MaybeT m a]

(Show1 m, Show a) => Show (MaybeT m a)

Defined in Control.Monad.Trans.Maybe

Methods

showsPrec :: Int -> MaybeT m a -> ShowS

show :: MaybeT m a -> String

showList :: [MaybeT m a] -> ShowS

Wrapped (MaybeT m a)

Defined in Control.Lens.Wrapped

Associated Types

type Unwrapped (MaybeT m a) :: *

Methods

_Wrapped' :: Iso' (MaybeT m a) (Unwrapped (MaybeT m a))

t ~ MaybeT n b => Rewrapped (MaybeT m a) t

Defined in Control.Lens.Wrapped

type Zoomed (MaybeT m)

Defined in Control.Lens.Zoom

type Zoomed (MaybeT m) = FocusingMay (Zoomed m)

type PrimState (MaybeT m)

Defined in Control.Monad.Primitive

type PrimState (MaybeT m) = PrimState m

type Unwrapped (MaybeT m a)

Defined in Control.Lens.Wrapped

type Unwrapped (MaybeT m a) = m (Maybe a)

mapMaybeT :: (m (Maybe a) -> n (Maybe b)) -> MaybeT m a -> MaybeT n b

Transform the computation inside a MaybeT.

• runMaybeT (mapMaybeT f m) = f (runMaybeT m)