Re: How to eat, how communicate.

[Marcus G. Daniels]

>>>>> "PJ" == Paul E Johnson <address@hidden> writes:

PJ> Suppose the outsider did have a "frob" method.  Would this program
PJ> call it?  I tested it, and it seems the answer is "yes."  The frob
PJ> method of the outsider does get used, even though the selector is
PJ> built from the method of the insider class.  What the heck is
PJ> going on?

Conceptually, a Selector is a message that has no association with a class.
Java reflection, however, requires such an association.

Now, any time you have the need for a dispatch on an agent type
(e.g. when calling actions across a set of heterogeneous agents), what
Swarm does is make an Objective C method call.  When such a method
call turns out to be for a foreign agent (say, Java) the message is
forwarded.  The process of forwarding involves taking apart the
Objective C message and reconstructing it in the right way for the
foreign target.  The reason the `outsider' can be sent `frob' is
because the process of forwarding a message implies knowing to whom
you are forwarding, and so Java reflection lookup requirements can
still be satisfied.

PJ> Is this Swarm runtime "magic" the sort of which
PJ> "createActionForEachHomogeneous" circumvents? 

Yes, since the set passed to this method must be homogeneous, there is
no point in doing dynamic method lookup over and over again.  The method
lookup is done once.  Homogeneous ForEach Actions are faster for this reason.

PJ> If the runtime can go shopping for methods, why do we have to use
PJ> the selector approach at all?

It's a question of context.  In Java, to know whether or not a message
is valid requires having a target type.

PJ> 2. I am curious to know about how this method gets called:
PJ> public void doesNotRecognize (Selector sel) { 
PJ>   System.out.println ("ouch! " + sel); }
PJ> }

The Objective C runtime makes doesNotRecognize calls when it
encounters a situation that can't be resolved by forwarding.  Swarm
then passes this on to the Java object involved...

PJ> Maybe I can't understand because my system gives a different piece
PJ> of output than yours, which seems to say "doesNotRecognize" is not
PJ> called.

As I indicated in the original message:

MD> $ javaswarm NoSelector --version
MD> NoSelector address@hidden (Swarm 2001-02-23)

(The glue code for passing on the doesNotRecognize message was not
there in Swarm for Java in previous versions.)

PJ> 3. Can you tell me how to write code to create Selectors when an
PJ> argument is needed for the java method? Or two arguments? Etc.

Here is an updated example:

import swarm.Globals;
import swarm.Selector;
import swarm.objectbase.SwarmObjectImpl;

class Agent extends SwarmObjectImpl {
}

class Insider extends Agent {
  public Object frob (Object with) {
    System.out.println ("frob with " + with);
    return this;
  }
}

class Outsider extends Agent {
  public Object frobX (Object with) {
    System.out.println ("frob outsider with " + with);
    return this;
  }
  public void doesNotRecognize (Selector sel) {
    System.out.println ("ouch! " + sel);
  }
}

public class NoSelector {
  static void main (String args[]) {
    Globals.env.initSwarm ("NoSelector", "address@hidden", "0.0", args);
  
    Agent insider = new Insider ();
    Agent outsider = new Outsider ();
    Selector sel = null;

    try {
      sel = new Selector (insider.getClass (), "frob", false);
    } catch (Exception e) {
      e.printStackTrace (System.err);
      System.exit (1);
    }
    Object with = new Object ();
    System.out.println ("passing with argument of " + with);
    insider.perform$with (sel, with);
    outsider.perform$with (sel, with);
  }
}

PJ> I put this example code in the repository,
PJ> 
http://lark.cc.ukans.edu/~pauljohn/SwarmFaq/WorkingExampleCode/java/RuntimeBinding.txt.

Hmm, I don't see it..  Maybe use the one above instead...


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