Control.Concurrent.Async.Either

These functions are partially inspired by the 'Catching All Exceptions' blog post that explains a useful way of pushing off a IO a of a risky or smelly nature, and having it provide a value indicating success or failure. This library does not go any further than taking this result to a value, be it an exception or the desired result. You get an either, that's it.

It can be used where normally you might have to use catch or similar mechanisms and you've already built a ExceptT having monad transformer stack in your application:

catch dangerous handleError

or if you've found yourself in the yucky position of having a throw buried in your otherwise pure code:

toMyError :: SomeException -> MyError
toMyError e = fromMaybe (throw e) | toMyError1 e | toMyError2 e
  where
    toMyError1 e' = Err1 $ fromException e'
    toMyError2 e' = Err2 $ fromException e'

Documentation

runAsyncForE :: MonadIO m => IO a -> m (Either SomeException a)

The workhorse of this package, executes the IO a in a separate thread using STM. It should catch any and all exceptions in the process, even asynchronous ones!! The result is a nice friendly Either that you can then use however you desire. Instead of things lurking around to break your nice Haskell code when you least expect it, BECAUSE IT WASN'T IN THE TYPE.

• mumble mumble*

runAsyncToE :: MonadIO m => IO a -> (SomeException -> b) -> m (Either b a)

As per runAsyncForE except you can attempt to turn the SomeException into an Error type of your own. Useful if there are instance of Exception that you would like to annotate differently, or you have an error type of your own that provides more context regarding what was going on at the time of the fire.

handleRetryE :: MonadIO m => RetryPolicyM IO -> (RetryStatus -> IO a) -> m (Either SomeException a)

Leverage the retry package to run your action in a separate thread with a specific plan for retrying failures, yet capturing any exceptions and returning the final result.

handleRetryToE :: MonadIO m => RetryPolicyM IO -> (RetryStatus -> IO a) -> (SomeException -> e) -> m (Either e a)

As per handleRetryE but allows you to annotate or extract different error information from the SomeException.