Bertram Felgenhauer wrote:
Now in fact, IO actions are indistinguishable from pure computations by the RTS, so this mechanism also makes IO actions resumable, in principle, if you can access the corresponding thunk somehow. Normally you can't - there is no reference to that thunk - but unsafePerformIO gives you that reference.
I wonder if other things break in the presence of resumable IO computations... the first thing that comes to mind is, inside a "block" or "unblock" (which have to initiate, take down and deal with all that infrastructure -- luckily you only use them inside a separate thread, a forkIO within the unsafePerformIO... Which admittedly makes things horribly confusing in its own way. Also does forkIO copy any state? it copies blocked status now, which the unamb-calling thread might have...). The state of the thread when entering the computation again, could be different than it was when the computation was first suspended, I'm guessing. (At least you need to be careful about what invariants you assume; I'm not yet sure if it's possible to be careful enough.) -Isaac