From: Jim Weirich <jweirich@...>
Date: 2003-04-24T21:21:45+09:00
Subject: Re: Subject: Re: [ANN] Ruby.shop

On Wed, 2003-04-23 at 23:10, Hal E. Fulton wrote:

> What's your take on the circularity at the top? Chris says
> that Object doesn't take anything from Class. I thought it
> did. I'll have to think about that tomorrow.
> 
> Maybe I could use magenta arrows with heads at both ends. 

I had to fire up IRB to investigate this.  Remember, there are different
kinds of dependencies/relationships.  There is the releationship between
a specific instance and its class (Fido is a Great Dane).  There is also
the relationship between a class and a more general class (Great Dane
are a subclass of Dog).  In Ruby, the first (object=>class) relationship
is represented by the "class" method.  The second (subclass=>superclass)
by the "superclass" method.

Since Classes *are* Objects in Ruby, every class will have an
(object=>class) as well as a (subclass=>superclass) relationship.

For example ...

    class Dog; end
    class GreatDane < Dog; end
    fido = GreatDane.new

    fido.class           => GreatDane
    GreatDane.class      => Class
    GreatDane.superclass => Dog

Regarding the Object/Class/Module diagram, look at
http://w3.one.net/~jweirich/images/rubycore.jpg.

   NOTE: I used a cheap UML drawing tool that rendered <<class>> into
         ?class? when generating a JPEG file.  Yuck.

The open head arrows (eg. from Class to Module) represent the
(subclass=>superclass) relationship.  The arrows marked ?class? are the
(object=>class) relationship.  As you can see, *every* class object in
the diagram has a class pointer to Class (even Class itself).   All of
the white boxes in your diagram should have a class pointer to Class. 
Since you are using color (White boxes for classse, blue for modules),
including the class arrow in your diagram would be redundant, I think I
would omit it.  That would remove the apparent circularity from Object
to Class.  If you decided to leave it in, then I would consider adding
*all* the class relationships, otherwise you leave the impression that
there is something unique about Object.

I also put Kernel into the diagram.  Since the Kernel object is not a
Class, but a Module, its ?class? arrow points appropriately at Module. 
I also show an ?includes? arrow from Object to Kernel.  There you get an
interesting circularity where each link in the circle is a different
kind of link.

Does this help?

-- 
-- Jim Weirich     jweirich@one.net    http://w3.one.net/~jweirich
---------------------------------------------------------------------
"Beware of bugs in the above code; I have only proved it correct, 
not tried it." -- Donald Knuth (in a memo to Peter van Emde Boas)