From: Trans <transfire@...>
Date: 2005-10-20T11:16:58+09:00
Subject: Re: Cut-based AOP

|Thanks. I do feel pretty strongly about my first point (classes are
|instantiable) and I think it really comes down to that on the Module
side
|versus cuts being part of the class inheritance heirarchy on the
other. And
|the nature of cut inheritance does separate it from the class
heirarchy.

On the other hand cuts are in essence classes --transparent
*subclasses* to be exact. While you do not instantiate them directly,
they get instantiated as part of the class they cut.

I think the real incongruency comes more from the fact that Class
itself is a subclass of Module. It works okay, but conceptualy it is
odd. And on occasion you have undef a method in Class that's been
defined in Module. I think Matz even metions something liek this in his
recent presentation. It think a better solution would come from having
a "ClassKernel" module which is included in Module and Class, so they
can stand on their own.

Finally, I think cuts are simliar to singleton_classes in that
instantiating them is restricted.

 o = Object.new
=> #<Object:0xb7aa96a0>
irb(main):002:0> eigenclass = class << o; self ; end
=> #<Class:#<Object:0xb7aa96a0>>
irb(main):003:0> k = eigenclass.new
TypeError: can't create instance of virtual class
        from (irb):3:in `new'
        from (irb):3

But I don't think there's any techincal reason they could not be. It's
purposefully disallowed, I guess for conceptual reasons:

VALUE
rb_class_new(super)
    VALUE super;
{
    Check_Type(super, T_CLASS);
    if (super == rb_cClass) {
	rb_raise(rb_eTypeError, "can't make subclass of Class");
    }
    if (FL_TEST(super, FL_SINGLETON)) {
	rb_raise(rb_eTypeError, "can't make subclass of virtual class");
    }
    return rb_class_boot(super);
}

Hey, this still says "virtual class"; _why doesn't his say eigenclass!?
;)

T.