From: Florian Frank <flori@...>
Date: 2006-11-27T08:05:11+09:00
Subject: Re: Extremely Noobish Documentation Question

Patrick McNally schrieb:
>> open methed is a method defined in kernel module
>>     
>
> Thank you for the quick response.
This answer was not correct. You cannot find the File.open method, 
because there is none. It is inherited from the IO class, because File < 
IO == true. If you want to find out where a method is located you can 
type this into irb:
 >> File.method(:open)
# => #<Method: File(IO).open>

This shows the inheritance relation. Ruby's rdoc/ri is a bit stupid, and 
can't locate inherited methods so well.

There is also a Kernel#open method, that is mixed into the Object class 
as a private method. This makes it possible to open a file by calling
open(filename) do |file|
  #...
end
from everywhere. If you call File.open this isn't the method called, 
because IO#open is found first:

 >> require 'pp'; pp File.ancestors.map { |klass| [ klass, 
klass.method(:open) ] }
[[File, #<Method: File(IO).open>],
 [IO, #<Method: IO.open>],
 [File::Constants, #<Method: Module(Kernel)#open>],
 [Enumerable, #<Method: Module(Kernel)#open>],
 [Object, #<Method: Class(Kernel)#open>],
 [PP::ObjectMixin, #<Method: Module(Kernel)#open>],
 [Kernel, #<Method: Kernel.open>]]

You can call Kernel#open only without an explicit receiver, because it 
is private. That's why calling, e. g., Array.open would fail with a 
NoMethodError.

-- 
Florian Frank