From: Gary Wright <gwtmp01@...>
Date: 2007-12-22T16:51:14+09:00
Subject: Re: Singleton Method


On Dec 22, 2007, at 1:35 AM, Ryan Lewis wrote:

> So defining the 'second' method like so:
>    def Array.second
>       self[1]
>    end
>
> is the same is making the singleton class like below.
>    Class << Array
>       def second
>          self[1]
>       end
>    end

No.  Your examples are different than mine.
In your examples you are defining a singleton method
for 'Array', which is an object that just happens
to be a class.

In my examples I defined a singleton method for one
particular array, not all arrays, and not the object
known as Array.

Let's see if I can summarize things a bit:

All method definitions wind up being stored in a
module of one sort or another (which includes
classes since Class is a subclass of Module,
i.e. every class is also a module).

1) standard modules (instances of Module)
    -- the methods are called instance methods

2) standard classes (instances of Class)
    -- the methods are called instance methods

3) singleton classes of non-class/non-module objects
    -- the methods are called singleton methods

4) singleton classes of instances of Module
   -- the methods are called module methods or
      singleton methods of a (particular) module

5) singleton classes of instances of Class
   -- the methods are called class methods or
      singleton methods of a (particular) class.

Since singleton classes are also classes, you can
get an infinite regress of singleton methods of
a singleton class of a singleton class of a...
But just set that aside for the moment.  Here is
what the code looks like for the cases I listed:

# case 1
module Enumerable
   def foo; end
end

# case 2
class Array
   def foo; end
end

# case 3
a = Object.new
def a.foo; end

# also case 3
class <<a
   def foo; end
end

# case 4
def Enumerable.foo
end

# also case 4
module Enumerable
   def self.foo
   end
end

# also case 4
module Enumerable
   class <<self
     def foo; end
   end
end

# still case 4
class <<Enumerable
   def foo; end
end


# case 5
def Array.foo
end

# also case 5
class Array
   def self.foo
   end
end

# also case 5
class Array
   class <<self
     def foo; end
   end
end

# still case 5
class <<Array
   def foo; end
end



I hope that helps.

Gary Wright