It would be possible to livelock the system but deadlocks are easier to avoid: don't use locks [1] at all in the runtime and don't expose them in the language.

It would be possible to livelock the system but deadlocks are easier to avoid: don't use locks [1] at all in the runtime and don't expose them in the language. The longer answer of how the possibility of a deadlock in user code in Pony is prevented: * messages are handled FIFO so w/o selective receive or multiple channels, there is no way to skip or wait for a specific message w/o doing work (_progress must be made by receivers_) * combined with the last one, the sender can't wait for a response as it has to be sent as a FIFO message back (which can come interleaved with other messages; _progress must be made by senders_) * multiple actors cannot simultaneously modify anything (you can't create a lock out of shared memory + Lamport's bakery algorithm because memory can't be r+w on two threads and even then, your exclusion failure would require explicit looping, so the thread is technically livelocked/starved) What this results in is a system that always makes progress when more than one actor is involved. Now that doesn't stop hanging the system (imagine putting N actors in infinite loops, one for each scheduler thread in the pony runtime, which we should call a livelock rather than a deadlock) and of course you could call some kind of C code via the FFI that decides it wants to take a lock or similar exclusive operation. Or there could be a bug in the runtime [2] which is tightly integrated with the language design and its guarantees so it avoids using locks for common things and uses atomic operations which provide a wait-free properties. [1]: Pony uses atomic operations exclusively last time I looked. Relatively clever and there is room for bugs but theoretically this allows for wait-free progress in the runtime (there's a lot more to say here about theory vs practice but generally lock-free properties are enough here anyway). [2]: I believe there have been edge cases in Pony's GC which might fall into the bug category but it's been awhile since I've used Pony myself so I can't comment on the current status.