Add something like Control.Exception.Safe.withException to Effectful.Exception.

[torgeirsh]

The documentation says:

If you want to catch all exceptions, run a cleanup action and rethrow, use one of the functions from the cleanup section.

While several of those functions let you specify a an action to be performed when an exception is thrown, they don't give you access to the actual exception. This problem is shared with the Control.Exception module in the base package, but the safe-exceptions package contains this useful function that otherwise behaves like onException:

withException :: Exception e => IO a -> (e -> IO b) -> IO a

It would be great if effectful could provide a similar function, since it's easy to get the edge cases wrong when dealing with asynchronous exceptions.

[arybczak]

it's easy to get the edge cases wrong when dealing with asynchronous exceptions.

What do you mean by this? onException action cleanup runs cleanup when action throws any exception, including the asynchronous one. This means that withException should give you access to the asynchronous exception, right?

[torgeirsh]

I meant that it's easy to get the edge cases wrong for users who implement something like withException manually, which is why it would be great to have a known good implementation in Effectful. And yes, withException gives you access to asynchronous exceptions, without the option of intercepting them. Since they're always rethrown, there are no sharp edges to worry about.