Question

I ruby 1.8.7, Why I can use require in main, but can’t use self.require?

require('date') # ok
self.require('date') 
NoMethodError: private method `require' called for main:Object
from (irb):22
from /usr/lib/ruby/1.8/date.rb:437

Well known that main is Object class:
irb(main):045:0> self
=> main

irb(main):043:0> self.class
=> Object

But I discovered that it have Kernel mixin:

irb(main):042:0> self.class.included_modules
=> [Kernel]

Moreover, I found that require is private method of self:

irb(main):037:0> self.private_methods
=> [... "require", ...]

Same way, I can’t use self.attr_accessor:

irb(main):051:0> class X
irb(main):052:1> self.attr_accessor(:ssss)
irb(main):053:1> end
NoMethodError: private method `attr_accessor' called for X:Class
from (irb):52
from /usr/lib/ruby/1.8/date.rb:437

How does it happend? Can anybody clarify that questions?

Answer 1

Check the following simple example:

class Person
  def initialize(age)
    @age = age
  end

  def compare_to(other)
    # we're calling a protected method on the other instance of the current class
    age <=> other.age
  end

  # it will not work if we use 'private' here
  protected

  def age
    @age
  end
end

In ruby we have implicit and explicit methods receiver, check the next code snippet:

class Foo
  def a; end

  # call 'a' with explicit 'self' as receiver
  def b; self.a; end

  # call 'a' with implicit 'self' as receiver
  def c; a; end
end

Basically in ruby if a method is private it can be called only on implicit receiver (without self keyword). In your example require is a private method defined the Kernel module and it can be called only on the implicit subject.