forkIO package:base

Creates a new thread to run the IO computation passed as the first argument, and returns the ThreadId of the newly created thread. The new thread will be a lightweight, unbound thread. Foreign calls made by this thread are not guaranteed to be made by any particular OS thread; if you need foreign calls to be made by a particular OS thread, then use forkOS instead. The new thread inherits the masked state of the parent (see mask). The newly created thread has an exception handler that discards the exceptions BlockedIndefinitelyOnMVar, BlockedIndefinitelyOnSTM, and ThreadKilled, and passes all other exceptions to the uncaught exception handler. WARNING: Exceptions in the new thread will not be rethrown in the thread that created it. This means that you might be completely unaware of the problem if/when this happens. You may want to use the async library instead.

Like forkIO, but the child thread is passed a function that can be used to unmask asynchronous exceptions. This function is typically used in the following way

... mask_ $ forkIOWithUnmask $ \unmask ->
catch (unmask ...) handler

so that the exception handler in the child thread is established with asynchronous exceptions masked, meanwhile the main body of the child thread is executed in the unmasked state. Note that the unmask function passed to the child thread should only be used in that thread; the behaviour is undefined if it is invoked in a different thread.