Fibers

E.fork(effect) starts an effect concurrently and returns an E.Fiber. fiber.join() is the fiber's value, failing with the fiber's own cause; fiber.wait() is its Exit and never fails; fiber.poll() peeks without waiting; fiber.interrupt() cancels it, lets its finalizers run and returns the Exit; E.yield_now() lets other fibers take a turn. Fibers are asyncio tasks underneath, so fork is async only and a forked run starts with a fresh environment: provide what it needs.

@E.gen
def program() -> E.EffectGen[int, FetchError]:
    fiber
 = yield from E.fork(fetch_total())  # runs concurrently
    other = yield from do_other_work()
    total = yield from fiber.join()  # Effect[int, FetchError]
    return total + other

More examples: test_fiber.py.