(* An example of agent hopping. *)

(* First execute this line at another site: 
   net_exportEngine("E1", "", "at site1"); *)

let shout =
   proc(str) sys_printText(str&"\n"); sys_printFlush() end;

let state = {nextHop => "E1"};

let hop = 
  proc(engine, agent, state)
    engine(
      proc(arg) 
        thread_fork(
          proc () agent(copy(state), arg) end, 
          0);
	    ok
      end);
  end;

let rec agent = 
  proc(state, arg) 
    shout(arg);
    if state.nextHop is "" then
	  thread_pause(3.0); (* pretend we are working *)
	  shout("all done");
    else
      let atNextSite = net_importEngine(state.nextHop, "");
	  shout("hopping to "&state.nextHop);
	  state.nextHop := "";
      hop(atNextSite, agent, state);
	  shout("disengaged");
    end;
  end;

agent(state, "at site 0");

(* We want site 0 to disengage from the computation that carries on
   at site 1; site 0 should not wait for site 1 to complete: this would not
   be real agent migration.
   This disengagement is achieved in the hop procedure by forking at the 'next' 
   site and returning. We are implementing GOTO by RPC and fork!
   The "ok" after forking is necessary because thread_fork returns a thread,
   and threads cannot be returned across address spaces. *)

(* For some reason having to do with sys_print I/O (which is used only for
   demonstation purposes) we must type ^D at the interpreter at site 0, to 
   get it back to the top level. Note that this does not mean that site 0 is 
   still waiting for site 1 to return: the "disengaged" message appears at the
   right time (and commenting out the sys_prints makes the problem disappear). 
*)