It is often useful to distinguish between synchronous and asynchronous exceptions. The common idiom is to run a user-supplied computation catching any synchronous exceptions but allowing asynchronous exceptions (such as user interrupt) pass through.

There's no way to know how — synchronously or asynchronously — an exception was thrown, so we have to work around it by relying on the exception type itself.

This module provides an extensible type for asynchronous exceptions — SomeAsyncException — as well as functions for catching synchronous exceptions.

To mark an exception as asynchronous:

instance Exception MyException where
  fromException = asyncFromException
  toException = asyncToException

Note that until base 4.7 (GHC 7.8) AsyncException was not a subclass of SomeAsyncException. Use isAsynchronous to recognize both AsyncException and SomeAsyncException.

The re-exported documentation may say «Since: 4.7.0.0» — ignor that. For older versions this package provides its own compatible definitions.