From: dblack@...
Date: 2006-10-12T04:06:18+09:00
Subject: Re: Singleton Class Constants

Hi --

On Thu, 12 Oct 2006, jdance@gmail.com wrote:

> David Black wrote in Ruby For Rails (page 341):
>
>> (There's also a subtle difference between these two approaches to
>> defining a singleton method, involving the scope of constants, but that's an
>> arcane point. For the most part, you can treat them as equivalent.)

For the answer to your last question:

> What is the "arcane point" about singleton methods and constants?

see the recent thread on '"def self.method" vs "class << self; def
method"'.  (It's the example with the top-level vs. class-specific
contants.)

> The subject of singleton methods and constants has recently gotten me
> into trouble. Here is a simplified example:
>
> module ExtendMeFirst
>  BAR = 3
> end
>
> module ExtendMeSecond
>  def print_bar
>    puts BAR
>  end
> end
>
> class A
>  extend ExtendMeFirst
>  extend ExtendMeSecond
> end
>
> A.print_bar #=> NameError: uninitialized constant ExtendMeSecond::BAR
>
> Now, I somewhat understand why this doesn't work - the constants are in
> different scopes. The challenge is to get around this problem and use
> the constant BAR from ExtendMeFirst in module ExtendMeSecond. We
> discovered that what we needed to was access the singleton class's
> constants, but there is basically no easy way to do this, since there
> is no "singleton_class" method in Object/Kernel (which has been
> discussed a long time ago on this list). We eventually added one:
>
> class Object
>  def singleton_class
>    class << self; self; end
>  end
> end
>
> And then doing this allowed us to get to the constant:
>
> module ExtendMeSecond
>  def print_bar
>    puts singleton_class::BAR
>  end
> end
>
> Is there a better way to do this?

I don't think there's any way that doesn't involve some kind of
coupling, since you're basically printing one module's constant from
another module (using "module" as class-or-module).  The way you've
done it presupposes that ExtendMeSecond will indeed be mixed in to a
singleton class subsequent to the mixing in of ExtendMeFirst.  You
could, instead, cut out one leg of the journey and just say:

   def print_bar
     puts ExtendMeFirst::BAR
   end

which is a different kind of coupling, but really isn't inherently
worse than calling Math::PI or any other constant from another module.


David

-- 
                   David A. Black | dblack@wobblini.net
Author of "Ruby for Rails"   [1] | Ruby/Rails training & consultancy [3]
DABlog (DAB's Weblog)        [2] | Co-director, Ruby Central, Inc.   [4]
[1] http://www.manning.com/black | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com    | [4] http://www.rubycentral.org