Portability | portable |
---|---|

Stability | experimental |

Maintainer | ross@soi.city.ac.uk |

Safe Haskell | Safe-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.

- 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

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.

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) |

# 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.