Capture and fwd exceptions in subforks.

(Should there be only one subtask?  Do the other quote in the main
proc?)

implementations/Python/joys.py

@@ -1,11 +1,73 @@
 import multiprocessing as mp
 from joy import (
-    initialize,
     default_defs,
-    text_to_expression,
+    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