First I will pre-emptively apologize for the somewhat confusing "object" term I use, and admit I don't know much about Perl's real object system.

Ok, so I ended up with the following thing, which is basically a simple object system (message passing):

# an object is simply a closure with a hash of closures
sub make_object {
    my %msg;
    $msg{"add-msg"} = sub {
        my $m = shift();
        $msg{$m} = shift();
    };
    return sub {
        my $m = shift();
        return $msg{$m} ? 
            $msg{$m}->(@_) : print "unknown message\n";
    }
}
[download]

Basically, this creates an object that only has one "method" initially, which allows for the addition of other methods or slots ("member values"). Methods added are internally stored in a hash which is local to the object's closure.

Here is some example code using this thing:

# build an object
my $object = make_object();

# add functions to our object
&$object("add-msg", "status", sub {print "up and running\n"});
&$object("add-msg", "square", sub {return shift()**2});


# test the functions
&$object("status"); # ==> displays "up and running"
print &$object("square", 25.80697580112788) . "\n"; # ==> "666"


# here is something fun...
&$object("add-msg", "build-object", \&make_object);


# ...objects can generate objects
my $other_object = &$object("build-object");
&$other_object("status"); # ==> error, this is a distinct object
&$other_object("add-msg", "status", sub {print "new object up\n"});
&$other_object("status"); # ==> display "new object up"


# an object could manipulate itself
&$object("add-msg", "get-self", sub {return $object});
&$object("get-self")->("add-msg", "test", sub {print "test ok!\n"});
&$object("test"); # ==> display "test ok!"
[download]

So what's the value of all of this? Well, I don't think this code could be of some use in practice, since it is more like a toy example and the syntax just doesn't cut it. But at the very least I had some fun writing it, and hopefully it can be seen as a demonstration of what closures allows you to do.

Comments, thoughts and/or insults would be appreciated! :-)

Schemer