From: dblack@...
Date: 2003-08-12T07:54:37+09:00
Subject: Re: newbie mixin questions

Hi --

On Tue, 12 Aug 2003, Michael Garriss wrote:

> Gennady wrote:
>
> >----- Original Message -----
> >From: "Michael Garriss" <mgarriss@earthlink.net>
> >To: "ruby-talk ML" <ruby-talk@ruby-lang.org>
> >Sent: Monday, August 11, 2003 3:16 PM
> >Subject: newbie mixin questions
> >
> >
> >
> >
> >>Hi,
> >>    Just 2 questions:
> >>
> >>1) How can I mixin a class method (not an instance method)?
> >>
> >>
> >
> >class A
> >  class << self
> >    include SomeModule
> > end
> >end
> >
> >
> Very cool.  Will this require me to have two seperate modules though?
> One for including class methods and the other of including the instance
> methods.

No, modules are very modular and can fit almost anywhere :-)

Here's a little illustration of how and why this works.

Step 1: create a module

  module SomeModule
    def talk
      puts "hi"
    end
  end

and include it in a class in the usual way:

  class A
    include SomeModule
  end

Now all instances of A can talk:

  A.new.talk         # hi

Now, by way of parallel example: instead of class A, do the same thing
with class Class:

  class Class
    include SomeModule
  end

Now all instances of Class can talk:

  A.talk              # hi
  String.talk         # hi
  Class.new.talk      # hi

That, by the way, is the heart of class methods: essentially, adding
instance methods to objects which are instances of Class.

The only problem is that you probably don't want all Class objects to
talk; you only want A to talk.  So actually, instead of the above, you
can do what Gennady did, which restricts the addition of 'talk' to
just the one Class object (A):

  class A
    class << self         # operating just on A's singleton class,
      include SomeModule  # so only A will gain the ability to talk
    end
  end

  A.talk         # hi
  String.talk    # error!  only added 'talk' capability to A


David

-- 
David Alan Black
home: dblack@superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav