From: James Hunt <filefrog@...>
Date: 2007-09-09T16:37:25+09:00
Subject: Re: module_function :func vs. MyModule.func?

On 9/9/07, 7stud -- <dolgun@excite.com> wrote:
> 7stud -- wrote:
> >
> > module MyModule
> >     def greet
> >         puts "hello"
> >     end
> > end
> >
> > module OtherModule
> >     include MyModule
> >
> >     module_function :greet
> > end
> >
> > OtherModule.greet
>
>
> On p. 355 of "Programming Ruby (2nd ed)", there is the equivalent of
> this example:
>
> module MyModule
>     def greet
>         puts "hello"
>     end
>
>     module_function :greet
> end
>
> MyModule.greet
>
> As far as I can tell, in that example the module_function has no purpose
> since I could just as easily make this change:
>
> module MyModule
>     def MyModule.greet
>         puts "hello"
>     end
> end
>
> MyModule.greet
>
> Is there any reason to use module_function in that example?
>
>
> --
> Posted via http://www.ruby-forum.com/.
>
>

I believe the primary different shows up when the module is included
in another class.  Consider this little snippet:

module MyModule
  def greet
    puts "hello"
  end

  module_function :greet
end

class MyClass
  include MyModule

  def hello
    greet
  end
end

MyModule.greet
MyClass.new.hello

When this gets run, the output will be:
hello
hello

The kicker is that greet is mixed into MyClass as a private method
(try calling MyClass.new.greet and watch the access error).

Using your explicit definition as such:

module MyExplicitModule
  def MyExplicitModule.greet
    puts "hello, explicitly!"
  end
end

class MyExplicitClass
  include MyExplicitModule

  def hello
    greet
  end
end

MyExplicitModule.greet
MyExplicitClass.new.greet

The output of this code when run is
hello
module_function.rb:31:in `hello': undefined local variable or method
`greet' for #<MyExplicitClass:0xb7cc7eac> (NameError)
        from module_function.rb:36

Essentially, by defining explicitly as MyExplicitModule.greet, the
method becomes a member of the module's singleton class, and does not
get mixed into classes that include it.  The module_function call
makes the method passed private and creates a singleton method.

You may also want to check out:
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/17670

The poster gives a good example of the usage (Math.sqrt)

HTH
-- 
James