Copyright | (C) 2013 Ross Paterson |
---|---|

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

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

Stability | experimental |

Portability | portable |

Safe Haskell | Safe |

Language | Haskell98 |

This monad transformer extends a monad with the ability throw exceptions.

A sequence of actions terminates normally, producing a value, only if none of the actions in the sequence throws an exception. If one throws an exception, the rest of the sequence is skipped and the composite action exits with that exception.

If the value of the exception is not required, the variant in Control.Monad.Trans.Maybe may be used instead.

- type Except e = ExceptT e Identity
- except :: Either e a -> Except e a
- runExcept :: Except e a -> Either e a
- mapExcept :: (Either e a -> Either e' b) -> Except e a -> Except e' b
- withExcept :: (e -> e') -> Except e a -> Except e' a
- newtype ExceptT e m a = ExceptT (m (Either e a))
- runExceptT :: ExceptT e m a -> m (Either e a)
- mapExceptT :: (m (Either e a) -> n (Either e' b)) -> ExceptT e m a -> ExceptT e' n b
- withExceptT :: Functor m => (e -> e') -> ExceptT e m a -> ExceptT e' m a
- throwE :: Monad m => e -> ExceptT e m a
- catchE :: Monad m => ExceptT e m a -> (e -> ExceptT e' m a) -> ExceptT e' m a
- liftCallCC :: CallCC m (Either e a) (Either e b) -> CallCC (ExceptT e m) a b
- liftListen :: Monad m => Listen w m (Either e a) -> Listen w (ExceptT e m) a
- liftPass :: Monad m => Pass w m (Either e a) -> Pass w (ExceptT e m) a

# The Except monad

except :: Either e a -> Except e a Source

Constructor for computations in the exception monad.
(The inverse of `runExcept`

).

runExcept :: Except e a -> Either e a Source

Extractor for computations in the exception monad.
(The inverse of `except`

).

withExcept :: (e -> e') -> Except e a -> Except e' a Source

Transform any exceptions thrown by the computation using the given
function (a specialization of `withExceptT`

).

# The ExceptT monad transformer

A monad transformer that adds exceptions to other monads.

`ExceptT`

constructs a monad parameterized over two things:

- e - The exception type.
- m - The inner monad.

The `return`

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

sequences two subcomputations, exiting on the
first exception.

MonadTrans (ExceptT e) Source | |

Monad m => Monad (ExceptT e m) Source | |

Functor m => Functor (ExceptT e m) Source | |

MonadFix m => MonadFix (ExceptT e m) Source | |

(Functor m, Monad m) => Applicative (ExceptT e m) Source | |

Foldable f => Foldable (ExceptT e f) Source | |

Traversable f => Traversable (ExceptT e f) Source | |

(Functor m, Monad m, Monoid e) => Alternative (ExceptT e m) Source | |

(Monad m, Monoid e) => MonadPlus (ExceptT e m) Source | |

MonadIO m => MonadIO (ExceptT e m) Source | |

(Show e, Show1 m) => Show1 (ExceptT e m) Source | |

(Read e, Read1 m) => Read1 (ExceptT e m) Source | |

(Ord e, Ord1 m) => Ord1 (ExceptT e m) Source | |

(Eq e, Eq1 m) => Eq1 (ExceptT e m) Source | |

(Eq e, Eq1 m, Eq a) => Eq (ExceptT e m a) Source | |

(Ord e, Ord1 m, Ord a) => Ord (ExceptT e m a) Source | |

(Read e, Read1 m, Read a) => Read (ExceptT e m a) Source | |

(Show e, Show1 m, Show a) => Show (ExceptT e m a) Source |

runExceptT :: ExceptT e m a -> m (Either e a) Source

The inverse of `ExceptT`

.

mapExceptT :: (m (Either e a) -> n (Either e' b)) -> ExceptT e m a -> ExceptT e' n b Source

Map the unwrapped computation using the given function.

`runExceptT`

(`mapExceptT`

f m) = f (`runExceptT`

m)

withExceptT :: Functor m => (e -> e') -> ExceptT e m a -> ExceptT e' m a Source

Transform any exceptions thrown by the computation using the given function.

# Exception operations

# Lifting other operations

liftCallCC :: CallCC m (Either e a) (Either e b) -> CallCC (ExceptT e m) a b Source

Lift a `callCC`

operation to the new monad.