import multiprocessing as mp
from joy import (
    default_defs,
    initialize,
    joy,
    repl,
    get_n_items,
    isnt_stack,
    inscribe,
    )


class ForkException(Exception): pass


def fork_joy(send, stack, expr, dictionary):
    try:
        stack, dictionary = joy(stack, expr, dictionary)
        result, _ = get_n_items(1, stack)
    except Exception as err:
        send.send((True, repr(err)))
    else:
        send.send((False, result))


@inscribe
def fork(stack, expr, dictionary):
    '''
    Take two quoted programs from the stack and
    run them in parallel.

        fork ≡ [i] app2

    '''
    q, p, stack = get_n_items(2, stack)
    isnt_stack(q)
    isnt_stack(p)
    q_pipe_recv, q_pipe_send = mp.Pipe(False)
    p_pipe_recv, p_pipe_send = mp.Pipe(False)

    P = mp.Process(
        target=fork_joy,
        args=(p_pipe_send, stack, p, dictionary),
        )
    Q = mp.Process(
        target=fork_joy,
        args=(q_pipe_send, stack, q, dictionary),
        )
    P.start()
    Q.start()
    P.join()
    Q.join()
    p_err, p_result = p_pipe_recv.recv()
    q_err, q_result = q_pipe_recv.recv()
    if p_err:
        raise ForkException(p_result)
    if q_err:
        raise ForkException(q_result)

    stack = (q_result, (p_result, stack))

    return stack, expr, dictionary


if __name__ == '__main__':
    mp.set_start_method('fork')

    dictionary = initialize()
    default_defs(dictionary)
    try:
        stack = repl(dictionary=dictionary)
    except SystemExit:
        pass