forkProcess comes with a giant warning: since any other running threads

  are not copied into the child process, it's easy to go wrong: e.g. by
  accessing some shared resource that was held by another thread in the
  parent.

In my experience, forkProcess often behaves incomprehensibly (to me)
with -threaded, typically resulting in a forked process hanging, and
quite often only some small percentage of the time, which makes it
really hard to track down and try to diagnose what thunk might not be
getting forced until after the fork, or whatever.