Control.Monad.Trans.MSF.Maybe

The Maybe monad is very versatile. It can stand for default arguments, for absent values, and for (nondescript) exceptions. The latter viewpoint is most natural in the context of MSFs.

Documentation

exit :: Monad m => MSF (MaybeT m) a b

Throw the exception immediately.

exitWhen :: Monad m => (a -> Bool) -> MSF (MaybeT m) a a

Throw the exception when the condition becomes true on the input.

exitIf :: Monad m => MSF (MaybeT m) Bool ()

Exit when the incoming value is True.

maybeExit :: Monad m => MSF (MaybeT m) (Maybe a) a

Just a is passed along, Nothing causes the whole MSF to exit.

inMaybeT :: Monad m => MSF (MaybeT m) (Maybe a) a

Embed a Maybe value in the MaybeT layer. Identical to maybeExit.

untilMaybe :: Monad m => MSF m a b -> MSF m b Bool -> MSF (MaybeT m) a b

Run the first msf until the second one produces True from the output of the first.

catchMaybe :: (Functor m, Monad m) => MSF (MaybeT m) a b -> MSF m a b -> MSF m a b

When an exception occurs in the first msf, the second msf is executed from there.

exceptToMaybeS :: (Functor m, Monad m) => MSF (ExceptT e m) a b -> MSF (MaybeT m) a b

Convert exceptions into Nothing, discarding the exception value.

listToMaybeS :: (Functor m, Monad m) => [b] -> MSF (MaybeT m) a b

Converts a list to an MSF in MaybeT, which outputs an element of the list at each step, throwing Nothing when the list ends.

runMaybeS :: (Functor m, Monad m) => MSF (MaybeT m) a b -> MSF m a (Maybe b)

Remove the MaybeT layer by outputting Nothing when the exception occurs. The continuation in which the exception occurred is then tested on the next input.

reactimateMaybe :: (Functor m, Monad m) => MSF (MaybeT m) () () -> m ()

Reactimates an MSF in the MaybeT monad until it throws Nothing.

embed_ :: (Functor m, Monad m) => MSF m a () -> [a] -> m ()

Run an MSF fed from a list, discarding results. Useful when one needs to combine effects and streams (i.e., for testing purposes).

newtype MaybeT (m :: Type -> Type) 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

MonadSplit g m => MonadSplit g (MaybeT m)

Defined in Control.Monad.Random.Class

Methods

getSplit :: MaybeT m g

Functor m => Generic1 (MaybeT m :: Type -> Type)

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

MonadBase b m => MonadBase b (MaybeT m)

Defined in Control.Monad.Base

Methods

liftBase :: b α -> MaybeT m α

MonadInterleave m => MonadInterleave (MaybeT m)

Defined in Control.Monad.Random.Class

Methods

interleave :: MaybeT m a -> MaybeT m a

MonadRandom m => MonadRandom (MaybeT m)

Defined in Control.Monad.Random.Class

Methods

getRandomR :: Random a => (a, a) -> MaybeT m a

getRandom :: Random a => MaybeT m a

getRandomRs :: Random a => (a, a) -> MaybeT m [a]

getRandoms :: Random a => MaybeT m [a]

Monad m => MonadFail (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

fail :: String -> MaybeT m a

MonadFix m => MonadFix (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

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

MonadIO m => MonadIO (MaybeT m)

Defined in Control.Monad.Trans.Maybe

Methods

liftIO :: IO a -> MaybeT m a

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)

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

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)

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

Contravariant m => Contravariant (MaybeT m)

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)

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]

(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

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

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

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

Generic (MaybeT m a)

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)

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

(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

type Rep1 (MaybeT m :: Type -> Type)

Defined in Control.Monad.Trans.Maybe

type Rep1 (MaybeT m :: Type -> Type) = D1 ('MetaData "MaybeT" "Control.Monad.Trans.Maybe" "transformers-0.6.1.0" 'True) (C1 ('MetaCons "MaybeT" 'PrefixI 'True) (S1 ('MetaSel ('Just "runMaybeT") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (m :.: Rec1 Maybe)))

type Rep (MaybeT m a)

Defined in Control.Monad.Trans.Maybe

type Rep (MaybeT m a) = D1 ('MetaData "MaybeT" "Control.Monad.Trans.Maybe" "transformers-0.6.1.0" '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

Transform the computation inside a MaybeT.

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

hoistMaybe :: forall (m :: Type -> Type) b. Applicative m => Maybe b -> MaybeT m b

Convert a Maybe computation to MaybeT.

maybeToExceptT :: forall (m :: Type -> Type) e a. Functor m => e -> MaybeT m a -> ExceptT e m a

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

exceptToMaybeT :: forall (m :: Type -> Type) e a. Functor m => ExceptT e m a -> MaybeT m a

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

maybeToExceptS :: (Functor m, Monad m) => MSF (MaybeT m) a b -> MSF (ExceptT () m) a b

Converts an MSF in MaybeT to an MSF in ExceptT. Whenever Nothing is thrown, throw () instead.