Why won't Ruby allow me to specify self as a receiver inside a private method?

Question

Ruby as an Object Oriented Language. What that means is whatever message I send, I strictly send it on some object/instance of class.

Example:

 class Test
   def test1
    puts "I am in test1. A public method"
    self.test2
   end

   def test2
    puts "I am in test2. A public Method"
   end
 end

makes sense I call method test2 on self object

But I cannot do this

  class Test
   def test1
    puts "I am in test1. A public method"
    self.test2 # Don't work
    test2 # works. (where is the object that I am calling this method on?)
   end

   private
   def test2
    puts "I am in test2. A private Method"
   end
 end

When test2 is public method I can call it on self (fair enough, a method sent to self object). But when test2 is private method I cannot call it on self. So where is the object that I am sending method on?

Answer 1

The Problem

In Ruby, private methods can't be called directly with an explicit receiver; self doesn't get any special treatment here. By definition, when you call self.some_method you are specifying self as the explicit receiver, so Ruby says "No!"

The Solution

Ruby has rules for its method lookups. There may be a more canonical source for the rules (other than going to the Ruby source), but this blog post lays out the rules right at the top:

1) Methods defined in the object’s singleton class (i.e. the object itself)
2) Modules mixed into the singleton class in reverse order of inclusion
3) Methods defined by the object’s class
4) Modules included into the object’s class in reverse order of inclusion
5) Methods defined by the object’s superclass, i.e. inherited methods

In other words, private methods are first looked up in self without requiring (or allowing) an explicit receiver.

Answer 2

where is the object that I am sending method on

It's self. Whenenver you don't specify a receiver, the receiver is self.

The definition of private in Ruby is that private methods can only be called without a receiver, i.e. with an implicit receiver of self. Interestingly, it didn't bother you at all with the puts method which is also a private instance method ;-)

Note: there's an exception to this rule. Private setters can be called with an explicit receiver, as long as the receiver is self. In fact, they must be called with an explicit receiver, because otherwise there would be an ambiguity with local variable assignments:

foo = :fortytwo      # local variable
self.foo = :fortytwo # setter

Answer 3

self means the current instance of the object you are in.

class Test
  def test1
    self
  end
end

Calling Test.new.test1 will return something like #<Test:0x007fca9a8d7928>.
This is the instance of the Test object you are currently using.

Defining a method as private means it can only be used inside the current object.
When using self.test2, you are going outside of the current object (you get the instance) and you call the method.
So you are calling a private methods as if you were not in the object, which is why you can't.

When you don't specify self, you remain inside the current object.
So you can just call the method. Ruby is smart enough to know that test2 is a method and not a variable and to call it.