Copyright | (c) 2007 Yitzak Gale Eric Kidd |
---|---|

License | BSD-style (see the file LICENSE) |

Maintainer | R.Paterson@city.ac.uk |

Stability | experimental |

Portability | portable |

Safe Haskell | Safe |

Language | Haskell98 |

The `MaybeT`

monad transformer extends a monad with the ability to exit
the computation without returning a value.

A sequence of actions produces a value only if all the actions in the sequence do. If one exits, the rest of the sequence is skipped and the composite action exits.

For a variant allowing a range of exception values, see Control.Monad.Trans.Except.

## Synopsis

- newtype MaybeT m a = MaybeT {}
- mapMaybeT :: (m (Maybe a) -> n (Maybe b)) -> MaybeT m a -> MaybeT n b
- maybeToExceptT :: Functor m => e -> MaybeT m a -> ExceptT e m a
- exceptToMaybeT :: Functor m => ExceptT e m a -> MaybeT m a
- liftCallCC :: CallCC m (Maybe a) (Maybe b) -> CallCC (MaybeT m) a b
- liftCatch :: Catch e m (Maybe a) -> Catch e (MaybeT m) a
- liftListen :: Monad m => Listen w m (Maybe a) -> Listen w (MaybeT m) a
- liftPass :: Monad m => Pass w m (Maybe a) -> Pass 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 exit.

The `return`

function yields a computation that produces that
value, while `>>=`

sequences two subcomputations, exiting if either
computation does.

#### Instances

# Monad transformations

maybeToExceptT :: Functor m => e -> MaybeT m a -> ExceptT e m a #

exceptToMaybeT :: Functor m => ExceptT e m a -> MaybeT m a #

# Lifting other operations

liftCallCC :: CallCC m (Maybe a) (Maybe b) -> CallCC (MaybeT m) a b #

Lift a `callCC`

operation to the new monad.