From: gwtmp01@...
Date: 2006-02-03T09:01:51+09:00
Subject: Singleton Method Questions

I've been thinking about singleton methods some more.  The current  
implementation
tucks away singleton methods in a class object.  Method lookup is  
subsequently
altered to consult the singleton class of the receiver (if it exists).

Is there a particular reason why a class object is used for this purpose
rather than a module?

My motivation for this question was the observation that the  
semantics of a
singleton class seem closer to a module than a class:

  - method lookup is altered like a module mixin, but
    the inheritance structure is not modified nor is the change
    visible via ancestors (which makes sense because ancestors is
    only defined for a class not for an object but which suggests that
    ancestors could be defined in Kernel and return an array which
    includes the singleton classes for the object)

  - if B is a subclass of A then method lookup for B.x consults
    the singleton class of B and the singleton class of A *as if*
    the singleton class of B included the singleton class of A
    even though the singleton class of B is *not* a subclass of the
    singleton class of A. (i.e., method lookup is like included modules
    not like subclasses)

  - the singleton class doesn't have an instance (in the sense that  
there is
    no object x such that x.class references the singleton class).   
Calling
    new on a singleton class generates an exception. This is like a  
module.

  - singleton classes don't really participate in the inheritance  
hierarchy
    or perhaps it is more accurate to say that they participate in an  
extremely
    constrained way (superclass returns the singleton class of Class,  
yet I
    can't come up with an example that would cause a method to be  
looked up
    in the singleton class of Class other than a direct message send  
to Class).

I think I've got the details correct but I welcome any corrections/ 
clarifications.

Gary Wright