From: lasitha <lasitha.ranatunga@...>
Date: 2009-02-22T15:14:05+09:00
Subject: Re: How to call a module method dynamically

On Sun, Feb 22, 2009 at 1:58 AM, 7stud -- <bbxx789_05ss@yahoo.com> wrote:
> Can anyone explain why I can do this:
>
> num = 1
> puts Object.module_eval("num")   #=>1
>
> def f
>  "xyz"
> end
>
> puts Object.module_eval("f")   #=>xyz
>
> but I can't do this:
>
> module Foo
>  def mymethod
>    "xyz"
>  end
> end
>
> puts Foo.module_eval("mymethod")
>
> --output:--
> `module_eval': undefined local variable or method `mymethod' for
> Foo:Module (NameError)
>
> That result and the result from my previous example seem to imply that
> free standing def's are added to Object's singleton class, i.e. they are
> class methods.

No, they are private instance methods of Object:

$: irb
01> def free_standing; end
--> nil
02> Object.private_instance_methods.grep /free_standing/
--> [:free_standing]

I suppose the confusion comes about because of the line:
> puts Object.module_eval("f")       #=>xyz

The reason this works is not that f was defined as a class method.
Its because the context in which f is resolved inherits from Object.

03> Object.module_eval do
04>   puts "self: #{self}"
05>   puts "ancestors:", self.class.ancestors
06> end
self: Object
ancestors:
Class
Module
Object
PP::ObjectMixin
Kernel
BasicObject

The current object inherits from Object (just like all objects) and
the method f was defined as an instance method of Object, so it is
available in this context.

It is of course confusing that the current object is Object and also
inherits from Object.  How ruby keeps all that straight is beyond my
knowledge :), but it does make sense intuitively.

Cheers,
lasitha