Copyright | (C) 2017 Tim McGilchrist |
---|---|

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

Maintainer | timmcgil@gmail.com |

Stability | experimental |

Portability | portable |

Safe Haskell | Safe-Inferred |

Language | Haskell98 |

This monad transformer extends Control.Monad.Trans.Except with a few more conveniences.

## Synopsis

- newExceptT :: m (Either x a) -> ExceptT x m a
- runExceptT :: ExceptT e m a -> m (Either e a)
- exceptT :: Monad m => (x -> m b) -> (a -> m b) -> ExceptT x m a -> m b
- left :: Monad m => x -> ExceptT x m a
- right :: Monad m => a -> ExceptT x m a
- mapExceptT :: (m (Either e a) -> n (Either e' b)) -> ExceptT e m a -> ExceptT e' n b
- hoistEither :: Monad m => Either x a -> ExceptT x m a
- bimapExceptT :: Functor m => (x -> y) -> (a -> b) -> ExceptT x m a -> ExceptT y m b
- firstExceptT :: Functor m => (x -> y) -> ExceptT x m a -> ExceptT y m a
- secondExceptT :: Functor m => (a -> b) -> ExceptT x m a -> ExceptT x m b
- hoistMaybe :: Monad m => x -> Maybe a -> ExceptT x m a
- hoistExceptT :: (forall b. m b -> n b) -> ExceptT x m a -> ExceptT x n a
- handleIOExceptT :: MonadIO m => (IOException -> x) -> IO a -> ExceptT x m a
- handleExceptT :: (MonadCatch m, Exception e) => (e -> x) -> m a -> ExceptT x m a
- handlesExceptT :: (Foldable f, MonadCatch m) => f (Handler m x) -> m a -> ExceptT x m a
- handleLeftT :: Monad m => (e -> ExceptT e m a) -> ExceptT e m a -> ExceptT e m a
- catchIOExceptT :: MonadIO m => IO a -> (IOException -> x) -> ExceptT x m a
- catchExceptT :: (MonadCatch m, Exception e) => m a -> (e -> x) -> ExceptT x m a
- catchesExceptT :: (Foldable f, MonadCatch m) => m a -> f (Handler m x) -> ExceptT x m a
- catchLeftT :: Monad m => ExceptT e m a -> (e -> ExceptT e m a) -> ExceptT e m a
- bracketExceptT :: Monad m => ExceptT e m a -> (a -> ExceptT e m b) -> (a -> ExceptT e m c) -> ExceptT e m c
- bracketExceptionT :: MonadMask m => ExceptT e m a -> (a -> ExceptT e m c) -> (a -> ExceptT e m b) -> ExceptT e m b
- hushM :: Monad m => Either e a -> (e -> m ()) -> m (Maybe a)
- onLeft :: forall e x m a. Monad m => (e -> ExceptT x m a) -> ExceptT x m (Either e a) -> ExceptT x m a
- onNothing :: forall x m a. Monad m => ExceptT x m a -> ExceptT x m (Maybe a) -> ExceptT x m a

# Control.Monad.Trans.Except.Extra

newExceptT :: m (Either x a) -> ExceptT x m a Source #

Constructor for computations in the ExceptT monad.
(The inverse of `runExceptT`

).

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

The inverse of `ExceptT`

.

exceptT :: Monad m => (x -> m b) -> (a -> m b) -> ExceptT x m a -> m b Source #

Map over both arguments at the same time.

Specialised version of `bimap`

for `ExceptT`

.

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

Map the unwrapped computation using the given function.

`runExceptT`

(`mapExceptT`

f m) = f (`runExceptT`

m)

bimapExceptT :: Functor m => (x -> y) -> (a -> b) -> ExceptT x m a -> ExceptT y m b Source #

Map the unwrapped computation using the given function.

# Extensions

firstExceptT :: Functor m => (x -> y) -> ExceptT x m a -> ExceptT y m a Source #

Map the `Left`

unwrapped computation using the given function.

secondExceptT :: Functor m => (a -> b) -> ExceptT x m a -> ExceptT x m b Source #

Map the `Right`

unwrapped computation using the given function.

hoistExceptT :: (forall b. m b -> n b) -> ExceptT x m a -> ExceptT x n a Source #

handleIOExceptT :: MonadIO m => (IOException -> x) -> IO a -> ExceptT x m a Source #

Try an `IO`

action inside an `ExceptT`

. If the `IO`

action throws an
`IOException`

, catch it and wrap it with the provided handler to convert it
to the error type of the `ExceptT`

transformer. Exceptions other than
`IOException`

will escape the ExceptT transformer.

Note: `IOError`

is a type synonym for `IOException`

.

handleExceptT :: (MonadCatch m, Exception e) => (e -> x) -> m a -> ExceptT x m a Source #

Try any monad action and catch the specified exception, wrapping it to
convert it to the error type of the `ExceptT`

transformer. Exceptions other
that the specified exception type will escape the `ExceptT`

transformer.

- Warning*: This function should be used with caution!
In particular, it is bad practice to catch
`SomeException`

because that includes asynchronous exceptions like stack/heap overflow, thread killed and user interrupt. Trying to handle`StackOverflow`

,`HeapOverflow`

and`ThreadKilled`

exceptions could cause your program to crash or behave in unexpected ways.

handlesExceptT :: (Foldable f, MonadCatch m) => f (Handler m x) -> m a -> ExceptT x m a Source #

handleLeftT :: Monad m => (e -> ExceptT e m a) -> ExceptT e m a -> ExceptT e m a Source #

Handle an error. Equivalent to `handleError`

in mtl package.

catchIOExceptT :: MonadIO m => IO a -> (IOException -> x) -> ExceptT x m a Source #

Flipped `handleIOExceptT`

.

catchExceptT :: (MonadCatch m, Exception e) => m a -> (e -> x) -> ExceptT x m a Source #

Flipped `handleExceptT`

.

catchesExceptT :: (Foldable f, MonadCatch m) => m a -> f (Handler m x) -> ExceptT x m a Source #

Flipped `handlesExceptT`

.

catchLeftT :: Monad m => ExceptT e m a -> (e -> ExceptT e m a) -> ExceptT e m a Source #

Flipped `handleLeftT`

.

bracketExceptT :: Monad m => ExceptT e m a -> (a -> ExceptT e m b) -> (a -> ExceptT e m c) -> ExceptT e m c Source #

Acquire a resource in `ExceptT`

and then perform an action with
it, cleaning up afterwards regardless of `left`

.

This function does not clean up in the event of an exception.
Prefer `bracketExceptionT`

in any impure setting.

bracketExceptionT :: MonadMask m => ExceptT e m a -> (a -> ExceptT e m c) -> (a -> ExceptT e m b) -> ExceptT e m b Source #

Acquire a resource in ExceptT and then perform an action with it,
cleaning up afterwards regardless of `left`

or exception.

Like `bracketExceptT`

, but the cleanup is called even when the bracketed
function throws an exception. Exceptions in the bracketed function are caught
to allow the cleanup to run and then rethrown.

hushM :: Monad m => Either e a -> (e -> m ()) -> m (Maybe a) Source #

Convert an Either to a Maybe and execute the supplied handler in the Left case.