transformers-0.2.1.0: Concrete functor and monad transformersSource codeContentsIndex
Control.Monad.Trans.Maybe
Portabilityportable
Stabilityexperimental
Maintainerlibraries@haskell.org
Contents
The MaybeT monad transformer
Lifting other operations
Description

The MaybeT monad transformer adds the ability to fail to a monad.

A sequence of actions succeeds, producing a value, only if all the actions in the sequence are successful. If one fails, the rest of the sequence is skipped and the composite action fails.

For a variant allowing a range of error values, see Control.Monad.Trans.Error.

Synopsis
newtype MaybeT m a = MaybeT {
runMaybeT :: m (Maybe a)
}
mapMaybeT :: (m (Maybe a) -> n (Maybe b)) -> MaybeT m a -> MaybeT n b
liftCallCC :: (((Maybe a -> m (Maybe b)) -> m (Maybe a)) -> m (Maybe a)) -> ((a -> MaybeT m b) -> MaybeT m a) -> MaybeT m a
liftCatch :: (m (Maybe a) -> (e -> m (Maybe a)) -> m (Maybe a)) -> MaybeT m a -> (e -> MaybeT m a) -> MaybeT m a
liftListen :: Monad m => (m (Maybe a) -> m (Maybe a, w)) -> MaybeT m a -> MaybeT m (a, w)
liftPass :: Monad m => (m (Maybe a, w -> w) -> m (Maybe a)) -> MaybeT m (a, w -> 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 fail.

The return function yields a successful computation, while >>= sequences two subcomputations, failing on the first error.

Constructors
MaybeT
runMaybeT :: m (Maybe a)
mapMaybeT :: (m (Maybe a) -> n (Maybe b)) -> MaybeT m a -> MaybeT n bSource
Transform the computation inside a MaybeT.
Lifting other operations
liftCallCC :: (((Maybe a -> m (Maybe b)) -> m (Maybe a)) -> m (Maybe a)) -> ((a -> MaybeT m b) -> MaybeT m a) -> MaybeT m aSource
Lift a callCC operation to the new monad.
liftCatch :: (m (Maybe a) -> (e -> m (Maybe a)) -> m (Maybe a)) -> MaybeT m a -> (e -> MaybeT m a) -> MaybeT m aSource
Lift a catchError operation to the new monad.
liftListen :: Monad m => (m (Maybe a) -> m (Maybe a, w)) -> MaybeT m a -> MaybeT m (a, w)Source
Lift a listen operation to the new monad.
liftPass :: Monad m => (m (Maybe a, w -> w) -> m (Maybe a)) -> MaybeT m (a, w -> w) -> MaybeT m aSource
Lift a pass operation to the new monad.
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