From: Hal Fulton <hal9000@...>
Date: 2005-05-07T02:56:57+09:00
Subject: Re: [RCR] Object#inside_metaclass?

Ilias Lazaridis wrote:

> A "singleton class" is instantiated to 0..1 object

You're thinking of the Singleton pattern, which deals
with a class which can only be instantiated once,
resulting in a single object.

The term "singleton" simply means "something which is
unique or occurs only once."

The term "singleton" is used in the Ruby community to
mean several things:

   - an object of a class which can only be instantiated
     once (Singleton Pattern)
   - a method that is unique to its object, rather than
     originating in the class of the object
   - an object containing methods that are unique to that
     object, i.e., containing singleton methods

When we say "singleton class," we are not really saying
"a class which is a singleton."

In English, the adjective does not always modify the noun
in the same way. "Human Services" means services FOR
humans; a "Reptile Zoo" is a zoo OF reptiles (not one
that we intend reptiles to visit and pay admission).

We use the term "singleton class" not to signify "a class
that is a singleton," but rather to signify "a class where
we store the singleton methods."

To add to the confusion, a class in Ruby is an object. So
when is a class anything *but* a singleton? Is there more
than one occurrence of Fixnum in Ruby? I suppose we could
do Fixnum.dup and there would be.

In short, the Ruby community's usage is not 100% in line
with the rest of the CS community. But that is OK, because
1) even the CS community at large does use terms with 100%
consistency, and 2) we are using terms for concepts that do
not exist in most other OO languages.


Hal