transformers- Concrete functor and monad transformers

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Safe HaskellSafe-Inferred




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.


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.




runMaybeT :: m (Maybe a)


MonadTrans MaybeT 
Monad m => Monad (MaybeT m) 
Functor m => Functor (MaybeT m) 
MonadFix m => MonadFix (MaybeT m) 
Monad m => MonadPlus (MaybeT m) 
(Functor m, Monad m) => Applicative (MaybeT m) 
Foldable f => Foldable (MaybeT f) 
Traversable f => Traversable (MaybeT f) 
(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.