[ruby-talk:16786] Re: Simulating call-by-reference in Ruby

On Fri, 22 Jun 2001, Tobin Baker wrote:
> It occurred to me that return-by-reference (e.g. IDL out parameters)
> could be simulated using the fact that parameters to a block bind to
> variables of the same name in the enclosing scope.  So we could have:
> #Ruby
> baz = nil
> myFoo.bar {|baz|}
> p baz #=> 10

This parallels something I've been elaborating about RubyX11 (and finally
coded last night)

In X11 you don't have inout parameters; you have a "in" list and a "out"
list. An "inout" parameter simply appears in both. (But that possibility
is rarely used, in comparison of references to remote objects)

A X11 call is a Send/Receive pair. In between the two you you have a loop
accepting Events, Errors, and Replies, the three kinds of input. When a
matching Reply is found, the call returns a "out" Object. (a decoded
reply)

Obviously this is very wasteful as each call is a round-trip in the
network. So half of the calls have no Receive half, and so they just
return nil. so you can send many at once, quickly.

Now I've added this feature (which afaik is unique in X11 client
libraries): a Send/Receive pair can be split in two like this:

a_remote_object.a_method(in1,in2,in3) {|out1,out2,out3|

	some code that is called whenever \
	the reply is accepted
}

And those can be resynced by wrapping them in a Display#wait_for call:

my_display.wait_for {
	foo.bar(in1,in2) {|out1,out2| blah }
	foo.baz(in1,in2) {|out1,out2| blah }
	foo.qux(in1,in2) {|out1,out2| blah }
}

performs three Sends, then waits until the three Receives that go with
it have been completed (code blocks executed, etc)

So to get back on topic, if you want a particular in/out pair to be
considered really as in/out, you do, for example:

foo.bar(a,b,c) {|b,d,e| ... }

where a,c are in, b is inout, and d,e are out.

but if you want b to keep the old b, then you only have to rename your
params:

foo.bar(a,b,c) {|z,d,e| ... }

so now your b is in only, but you have a new out called z.

The control is in the hands of the user.

matju