From: Rick DeNatale <rick.denatale@...>
Date: 2008-05-18T23:09:22+09:00
Subject: Re: Class methods in modules

On Sat, May 17, 2008 at 6:39 PM, Christoph Schiessl <c.schiessl@gmx.net> wrote:
> Interesting - so class methods in modules don't make any sense at all?
> Calling the modules class methods directly and special class methods such as
> included(base) and extended(base) are the exception of course.
>
> Would you agree with that statement (question to everyone)?

Well, since Modules aren't classes, they can't have class methods per
se.  They can have module methods:

module Mod
    def self.i_am_a_module_method
    end
end

Modules are objects themselves so can have methods.

For an example of module methods in the standard library, look at the
Math module.  For example to get the natural log of 10 you can write:

  Math.log(10)

If you simply write:

  log(10)

you'll get a method not found error.  But you could also include Math
into a class or module and then any methods of that class or module,
could use the simpler call.

Ruby has a method in Module#module_function which takes one or more
method names of module instance methods and copies them as module
methods.  The Math module effectively does just that to make it's
methods available either as methods of the Math object, or as instance
methods when the Math module is included.

As an interesting sidenote, although Class is a subclass of Module,
you can't use module_function in a class to copy instance methods to
class methods, the MRI implementation undefines the method in Class.

-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/