Portability	portable
Stability	experimental
Maintainer	libraries@haskell.org

Control.Monad.Trans.Maybe

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

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
Fields runMaybeT :: m (Maybe a)

Instances

MonadTrans MaybeT
Monad m => Monad (MaybeT m)
Functor m => Functor (MaybeT m)
Monad m => MonadPlus (MaybeT m)
(Functor m, Monad m) => Applicative (MaybeT m)
(Functor m, Monad m) => Alternative (MaybeT m)
MonadIO m => MonadIO (MaybeT m)

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.