This is interesting, but not what I want. I want something to wait on a real, Posix, child, _process_!! Not a GHC thread.

Ah. Oops :) There's one other way that Marcin didn't mention: wait for SIGCHLD, which can be done without blocking the whole process. Unfortunately there's no easy way at the moment to tell *which* child generated the SIGCHLD. I guess we should really have a process wait as a threading primitive, like OCaml does. I've been looking at FreeBSD's `kevent' stuff recently - they have a really nice way to wait for several different types of event, including file descriptors, processes and even when a file is modified (eg. tail -f). Does Linux or Solaris have anything like this? I had a quick look around but didn't find anything. Cheers, Simon