From: John Carter <john.carter@...>
Date: 2005-12-06T09:57:46+09:00
Subject: Subclassing Class.

Ok. This is a wild idea.

I don't like the factory pattern.

We don't need no stinkin Factories. We have the Class object.

All a Factory is really Class.

However, a traditional Factory object can have...
   * State.
   * Richer behaviour than the Class class.

Ok, this gets a little hairy so lets get concrete.

class Foo
end

fo = Foo.new

What is the class of fo? Foo

What is the class of Foo? Class

Now a FooFactory object is just an object with a bit more intelligence
and state that the traditional Foo class object for making Foo
instances in a better smarter way.


So the obvious thing to do is instead of making a new class FooFactory
< Object, it should inherit from Class.

Well, since Foo is already a Class, we really just want a smarter Class.

ie..

Instead of

class FooFactory
   def initialize( foo_state)
    @foo_state = foo_state
   end

   def foo_fu( bah)
     # Knowledge of Foo making
   end

   def create( needs_fu, stuff)
     Foo.new( foo_fu( needs_fu), @foo_state, stuff)
   end
end

factory = FooFactory.new( 'foo juice')
foo = factory.create( 'footile')

We really want

class Foo < Class
   def initialize( foo_state)
    @foo_state = foo_state
   end

   def foo_fu( bah)
     # Knowledge of Foo making
   end

   def new( needs_fu, stuff)
     super( foo_fu( needs_fu), @foo_state, stuff)
   end
end

foo = Foo.new( 'fooless', 'stuffed')


Except ruby doesn't like that
    class Foo < Class
bit.

It gets quite grumpy in fact.

Ruby says "can't make subclass of Class (TypeError)"

Rats.

We can however say...

FooClass = Class.new(Class)

Unfortunately we can't then say....

FooClass.new( Object)

Ruby says...
:in `new': wrong instance allocation (TypeError)

Ahh! I'm just going around in circles fighting Ruby.

We need a way of saying...

class FooClass < Class
   # Add state and behaviour to Class
end

And then instead of saying...
   class Foo < FooParent
     # Add state and behaviour to FooParent
   end
we need to be able to say...

FooClass Foo < FooParent
   # Add state and behaviour to FooParent
end

Aargh! I'm chasing my tail.

Is there anyway, (short of the traditional Factory Pattern) for me to 
catch my tail?


John Carter                             Phone : (64)(3) 358 6639
Tait Electronics                        Fax   : (64)(3) 359 4632
PO Box 1645 Christchurch                Email : john.carter@tait.co.nz
New Zealand

Carter's Clarification of Murphy's Law.

"Things only ever go right so that they may go more spectacularly wrong later."

From this principle, all of life and physics may be deduced.