GHC.Types.ForeignCall

Might invoke Haskell GC, or do a call back, or switch threads, etc. So make sure things are tidy before the call. Additionally, in the threaded RTS we arrange for the external call to be executed by a separate OS thread, i.e., _concurrently_ to the execution of other Haskell threads.

Like PlaySafe, but additionally the worker thread running this foreign call may be unceremoniously killed, so it must be scheduled on an unbound thread.

None of the above can happen; the call will return without interacting with the runtime system at all. Specifically:

• No GC

• No call backs

• No blocking

• No precise exceptions