| Copyright | (c) 2007 Yitzak Gale, Eric Kidd |
|---|---|
| License | BSD-style (see the file LICENSE) |
| Maintainer | ross@soi.city.ac.uk |
| Stability | experimental |
| Portability | portable |
| Safe Haskell | Safe-Inferred |
| Language | Haskell98 |
Control.Monad.Trans.Maybe
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.
- newtype MaybeT m a = MaybeT {}
- 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
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.
Instances
| MonadTrans MaybeT | |
| (Functor m, Monad m) => Alternative (MaybeT m) | |
| 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) | |
| MonadIO m => MonadIO (MaybeT m) |
Lifting other operations
liftCallCC :: (((Maybe a -> m (Maybe b)) -> m (Maybe a)) -> m (Maybe a)) -> ((a -> MaybeT m b) -> MaybeT m a) -> MaybeT m a Source
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 a Source
Lift a catchError operation to the new monad.