forkProcess type changed?

For the development snapshot 6.3.20031201, System.Posix.forkProcess has the type IO () -> IO System.Posix.Types.ProcessID. In 6.0.1 it has type IO () -> IO (Maybe System.Posix.Types.ProcessID). Is this change intentional, and if so how are you supposed to test after the fork if this is the parent or child process?