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Control.Monad.Trans.Maybe

Copyright (c) 2007 Yitzak Gale Eric Kidd
License BSD-style (see the file LICENSE)
Maintainer [email protected]
Stability experimental
Portability portable
Safe Haskell Safe
Language Haskell98

Description

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.

The MaybeT monad transformer

newtype MaybeT m a Source

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

Instances
Instances details
MonadTrans MaybeT
Instance details

Defined in Control.Monad.Trans.Maybe

Methods

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

Monad m => Monad (MaybeT m)
Instance details

Defined in Control.Monad.Trans.Maybe

Methods

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

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

return :: a -> MaybeT m a Source

Functor m => Functor (MaybeT m)
Instance details

Defined in Control.Monad.Trans.Maybe

Methods

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

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

MonadFix m => MonadFix (MaybeT m)
Instance details

Defined in Control.Monad.Trans.Maybe

Methods

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

Monad m => MonadFail (MaybeT m)
Instance details

Defined in Control.Monad.Trans.Maybe

Methods

fail :: String -> MaybeT m a Source

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

Defined in Control.Monad.Trans.Maybe

Methods

pure :: a -> MaybeT m a Source

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

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

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

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

Foldable f => Foldable (MaybeT f)
Instance details

Defined in Control.Monad.Trans.Maybe

Methods

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

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

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

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

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

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

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

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

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

toList :: MaybeT f a -> [a] Source

null :: MaybeT f a -> Bool Source

length :: MaybeT f a -> Int Source

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

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

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

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

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

Traversable f => Traversable (MaybeT f)
Instance details

Defined in Control.Monad.Trans.Maybe

Methods

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

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

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

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

Contravariant m => Contravariant (MaybeT m)
Instance details

Defined in Control.Monad.Trans.Maybe

Methods

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

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

Eq1 m => Eq1 (MaybeT m)
Instance details

Defined in Control.Monad.Trans.Maybe

Methods

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

Ord1 m => Ord1 (MaybeT m)
Instance details

Defined in Control.Monad.Trans.Maybe

Methods

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

Read1 m => Read1 (MaybeT m)
Instance details

Defined in Control.Monad.Trans.Maybe

Methods

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

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

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

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

Show1 m => Show1 (MaybeT m)
Instance details

Defined in Control.Monad.Trans.Maybe

Methods

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

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

MonadZip m => MonadZip (MaybeT m)
Instance details

Defined in Control.Monad.Trans.Maybe

Methods

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

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

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

MonadIO m => MonadIO (MaybeT m)
Instance details

Defined in Control.Monad.Trans.Maybe

Methods

liftIO :: IO a -> MaybeT m a Source

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

Defined in Control.Monad.Trans.Maybe

Methods

empty :: MaybeT m a Source

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

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

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

Monad m => MonadPlus (MaybeT m)
Instance details

Defined in Control.Monad.Trans.Maybe

Methods

mzero :: MaybeT m a Source

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

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

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)
Instance details

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)
Instance details

Defined in Control.Monad.Trans.Maybe

Methods

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

readList :: ReadS [MaybeT m a] Source

readPrec :: ReadPrec (MaybeT m a) Source

readListPrec :: ReadPrec [MaybeT m a] Source

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

Defined in Control.Monad.Trans.Maybe

Methods

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

show :: MaybeT m a -> String Source

showList :: [MaybeT m a] -> ShowS Source

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

Transform the computation inside a MaybeT.

Monad transformations

maybeToExceptT :: Functor m => e -> MaybeT m a -> ExceptT e m a Source

Convert a MaybeT computation to ExceptT, with a default exception value.

exceptToMaybeT :: Functor m => ExceptT e m a -> MaybeT m a Source

Convert a ExceptT computation to MaybeT, discarding the value of any exception.

Lifting other operations

liftCallCC :: CallCC m (Maybe a) (Maybe b) -> CallCC (MaybeT m) a b Source

Lift a callCC operation to the new monad.

liftCatch :: Catch e m (Maybe a) -> Catch e (MaybeT m) a Source

Lift a catchE operation to the new monad.

liftListen :: Monad m => Listen w m (Maybe a) -> Listen w (MaybeT m) a Source

Lift a listen operation to the new monad.

liftPass :: Monad m => Pass w m (Maybe a) -> Pass w (MaybeT m) a Source

Lift a pass operation to the new monad.

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.8.3/docs/html/libraries/transformers-0.5.6.2/Control-Monad-Trans-Maybe.html