I posted this on comp.lang.functional and someone suggested I try here. Apologies if it's not on-charter here - any redirections appreciated. I'm seeing unexpected behaviour from GHC 5. (unexpected by me, anyway :) The code which is producing the surprising results is at the bottom, but I'll breifly explain how it is meant to work here: ppm' spawns three threads, and hands them all an MVar. Each thread just puts a True into that MVar and exits. ppm' takes three values in turn from the MVar and makes sure they are all True - if it sees a False value, it stops looking. Once it has assessed whether they have all returned true, it kills all of its child threads before returning the result of the assessment. In this case, where each child returns True, the threads will have already exited, but my understanding is that they are not GCed while ppm' has a reference to their ThreadIds, so ppm' is still allowed to kill them. The ppm' function is "done" 11 times (but some slightly modified versions caused me to need up to a hundred invocations in order to see the problem, so if you don't see the problem on your GHC, try > 11) and each invocation is wrapped up in an unsafePerformIO. (I don't think the unsafePerformIO should be a problem, because there oughtn't be any interaction between the seperate ppm' instances.) Some of these 11 seem to return the expected result, but one of them doesn't. The output is this: --------------------------- (1,True) (2,True) (3,True) (4,True) (5,True) (6,True) (7,True) (8,True) (9,True) (10,True) PPM: no threads to run: infinite loop or deadlock? --------------------------- So there _does_ seem to be a strange interaction between seperate occurrences of ppm'. Can anyone see how that is happening? (of course the other possibility is that I've done something really silly in my code and not noticed, but I'm happy to take suggestions on that front, too. :) If noone here can see what is causing it, is anyone here brave enough to say "this is probably a GHC bug" and recommend how best I report it? Thanks for any insight you can offer me. -Greg Mildenhall