 | transformers-0.2.1.0: Concrete functor and monad transformers | Contents | Index |
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| Control.Monad.Trans.Maybe | | Portability | portable | | Stability | experimental | | Maintainer | libraries@haskell.org |
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| 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.
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| Synopsis |
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| The MaybeT monad transformer
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| newtype MaybeT m a |
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 | | Instances | |
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| mapMaybeT :: (m (Maybe a) -> n (Maybe b)) -> MaybeT m a -> MaybeT n b |
| Transform the computation inside a MaybeT.
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| Lifting other operations
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| liftCallCC :: (((Maybe a -> m (Maybe b)) -> m (Maybe a)) -> m (Maybe a)) -> ((a -> MaybeT m b) -> MaybeT m a) -> MaybeT m a |
| Lift a callCC operation to the new monad.
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| liftCatch :: (m (Maybe a) -> (e -> m (Maybe a)) -> m (Maybe a)) -> MaybeT m a -> (e -> MaybeT m a) -> MaybeT m a |
| Lift a catchError operation to the new monad.
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| liftListen :: Monad m => (m (Maybe a) -> m (Maybe a, w)) -> MaybeT m a -> MaybeT m (a, w) |
| Lift a listen operation to the new monad.
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| liftPass :: Monad m => (m (Maybe a, w -> w) -> m (Maybe a)) -> MaybeT m (a, w -> w) -> MaybeT m a |
| Lift a pass operation to the new monad.
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| Produced by Haddock version 2.7.2 |