transformers-0.5.5.0: Concrete functor and monad transformers

Copyright(c) 2007 Yitzak Gale Eric Kidd
LicenseBSD-style (see the file LICENSE)
MaintainerR.Paterson@city.ac.uk
Stabilityexperimental
Portabilityportable
Safe HaskellSafe
LanguageHaskell98

Control.Monad.Trans.Maybe

Contents

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.

Synopsis

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

MonadTrans MaybeT Source # 

Methods

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

Monad m => Monad (MaybeT m) Source # 

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) Source # 

Methods

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

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

MonadFix m => MonadFix (MaybeT m) Source # 

Methods

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

Monad m => MonadFail (MaybeT m) Source # 

Methods

fail :: String -> MaybeT m a #

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

Methods

pure :: a -> MaybeT m a #

(<*>) :: 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 #

Foldable f => Foldable (MaybeT f) Source # 

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) Source # 

Methods

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

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

Eq1 m => Eq1 (MaybeT m) Source # 

Methods

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

Ord1 m => Ord1 (MaybeT m) Source # 

Methods

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

Read1 m => Read1 (MaybeT m) Source # 

Methods

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

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

Show1 m => Show1 (MaybeT m) Source # 

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) Source # 

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) Source # 

Methods

liftIO :: IO a -> MaybeT m a #

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

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] #

Monad m => MonadPlus (MaybeT m) Source # 

Methods

mzero :: MaybeT m a #

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

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

Methods

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

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

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

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

Methods

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

show :: MaybeT m a -> String #

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

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.