Copyright	(c) 2007 Yitzak Gale Eric Kidd
License	BSD-style (see the file LICENSE)
Maintainer	R.Paterson@city.ac.uk
Stability	experimental
Portability	portable
Safe Haskell	Safe
Language	Haskell2010

Control.Monad.Trans.Maybe

Contents

The MaybeT monad transformer
Monad transformations
Lifting other operations

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

newtype MaybeT m a = MaybeT {
- runMaybeT :: m (Maybe a)
}
mapMaybeT :: (m (Maybe a) -> n (Maybe b)) -> MaybeT m a -> MaybeT n b
hoistMaybe :: Applicative m => Maybe b -> MaybeT m b
maybeToExceptT :: Functor m => e -> MaybeT m a -> ExceptT e m a
exceptToMaybeT :: Functor m => ExceptT e m a -> MaybeT m a
liftCallCC :: CallCC m (Maybe a) (Maybe b) -> CallCC (MaybeT m) a b
liftCatch :: Catch e m (Maybe a) -> Catch e (MaybeT m) a
liftListen :: Monad m => Listen w m (Maybe a) -> Listen w (MaybeT m) a
liftPass :: Monad m => Pass w m (Maybe a) -> Pass w (MaybeT m) a

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 runMaybeT :: m (Maybe a)

Instances

Instances details

MonadTrans MaybeT Source #
Instance details Defined in Control.Monad.Trans.Maybe Methods lift :: Monad m => m a -> MaybeT m a Source #
Functor m => Generic1 (MaybeT m :: Type -> Type) Source #
Instance details Defined in Control.Monad.Trans.Maybe Associated Types type Rep1 (MaybeT m) :: k -> Type # Methods from1 :: forall (a :: k). MaybeT m a -> Rep1 (MaybeT m) a # to1 :: forall (a :: k). Rep1 (MaybeT m) a -> MaybeT m a #
Monad m => MonadFail (MaybeT m) Source #
Instance details Defined in Control.Monad.Trans.Maybe Methods fail :: String -> MaybeT m a #
MonadFix m => MonadFix (MaybeT m) Source #
Instance details Defined in Control.Monad.Trans.Maybe Methods mfix :: (a -> MaybeT m a) -> MaybeT m a #
MonadIO m => MonadIO (MaybeT m) Source #
Instance details Defined in Control.Monad.Trans.Maybe Methods liftIO :: IO a -> MaybeT m a #
MonadZip m => MonadZip (MaybeT m) Source #
Instance details 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) #
Foldable f => Foldable (MaybeT f) Source #
Instance details Defined in Control.Monad.Trans.Maybe Methods fold :: Monoid m => MaybeT f m -> m # foldMap :: Monoid m => (a -> m) -> MaybeT f a -> 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 #
Eq1 m => Eq1 (MaybeT m) Source #
Instance details Defined in Control.Monad.Trans.Maybe Methods liftEq :: (a -> b -> Bool) -> MaybeT m a -> MaybeT m b -> Bool #
Ord1 m => Ord1 (MaybeT m) Source #
Instance details Defined in Control.Monad.Trans.Maybe Methods liftCompare :: (a -> b -> Ordering) -> MaybeT m a -> MaybeT m b -> Ordering #
Read1 m => Read1 (MaybeT m) Source #
Instance details 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) Source #
Instance details 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 #
Contravariant m => Contravariant (MaybeT m) Source #
Instance details Defined in Control.Monad.Trans.Maybe Methods contramap :: (a' -> a) -> MaybeT m a -> MaybeT m a' # (>$) :: b -> MaybeT m b -> MaybeT m a #
Traversable f => Traversable (MaybeT f) Source #
Instance details Defined in Control.Monad.Trans.Maybe Methods traverse :: Applicative f0 => (a -> f0 b) -> MaybeT f a -> f0 (MaybeT f b) # sequenceA :: Applicative f0 => MaybeT f (f0 a) -> f0 (MaybeT f a) # mapM :: Monad m => (a -> m b) -> MaybeT f a -> m (MaybeT f b) # sequence :: Monad m => MaybeT f (m a) -> m (MaybeT f a) #
(Functor m, Monad m) => Alternative (MaybeT m) Source #
Instance details Defined in Control.Monad.Trans.Maybe Methods empty :: MaybeT m a # (<\|>) :: MaybeT m a -> MaybeT m a -> MaybeT m a # some :: MaybeT m a -> MaybeT m [a] # many :: MaybeT m a -> MaybeT m [a] #
(Functor m, Monad m) => Applicative (MaybeT m) Source #
Instance details Defined in Control.Monad.Trans.Maybe Methods pure :: a -> MaybeT m a # (<>) :: MaybeT m (a -> b) -> MaybeT m a -> MaybeT m b # liftA2 :: (a -> b -> c) -> MaybeT m a -> MaybeT m b -> MaybeT m c # (>) :: MaybeT m a -> MaybeT m b -> MaybeT m b # (<*) :: MaybeT m a -> MaybeT m b -> MaybeT m a #
Functor m => Functor (MaybeT m) Source #
Instance details Defined in Control.Monad.Trans.Maybe Methods fmap :: (a -> b) -> MaybeT m a -> MaybeT m b # (<$) :: a -> MaybeT m b -> MaybeT m a #
Monad m => Monad (MaybeT m) Source #
Instance details 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 #
Monad m => MonadPlus (MaybeT m) Source #
Instance details Defined in Control.Monad.Trans.Maybe Methods mzero :: MaybeT m a # mplus :: MaybeT m a -> MaybeT m a -> MaybeT m a #
Generic (MaybeT m a) Source #
Instance details Defined in Control.Monad.Trans.Maybe Associated Types type Rep (MaybeT m a) :: Type -> Type # Methods from :: MaybeT m a -> Rep (MaybeT m a) x # to :: Rep (MaybeT m a) x -> MaybeT m a #
(Read1 m, Read a) => Read (MaybeT m a) Source #
Instance details 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) Source #
Instance details Defined in Control.Monad.Trans.Maybe Methods showsPrec :: Int -> MaybeT m a -> ShowS # show :: MaybeT m a -> String # showList :: [MaybeT m a] -> ShowS #
(Eq1 m, Eq a) => Eq (MaybeT m a) Source #
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) Source #
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 #
type Rep1 (MaybeT m :: Type -> Type) Source #
Instance details Defined in Control.Monad.Trans.Maybe type Rep1 (MaybeT m :: Type -> Type) = D1 ('MetaData "MaybeT" "Control.Monad.Trans.Maybe" "transformers-0.6.1.2-KquO3qiMMr21gbsKwWKH4A" 'True) (C1 ('MetaCons "MaybeT" 'PrefixI 'True) (S1 ('MetaSel ('Just "runMaybeT") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (m :.: Rec1 Maybe)))
type Rep (MaybeT m a) Source #
Instance details Defined in Control.Monad.Trans.Maybe type Rep (MaybeT m a) = D1 ('MetaData "MaybeT" "Control.Monad.Trans.Maybe" "transformers-0.6.1.2-KquO3qiMMr21gbsKwWKH4A" 'True) (C1 ('MetaCons "MaybeT" 'PrefixI 'True) (S1 ('MetaSel ('Just "runMaybeT") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 (m (Maybe a)))))

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

Transform the computation inside a MaybeT.

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

Monad transformations

hoistMaybe :: Applicative m => Maybe b -> MaybeT m b Source #

Convert a Maybe computation to MaybeT.

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.