class_methods.rb

# These two approaches are not the same:

class Person2
  def self.first_name
    full_name.split(' ').first
  end

  private

  def self.full_name
    'Ben Franklin'
  end
end

puts Person2.first_name # => Ben
puts Person2.full_name # => Ben Franklin (no exception raised)

class Person1
  class << self
    def first_name
      full_name.split(' ').first
    end

    private

    def full_name
      'Ben Franklin'
    end
  end
end

puts Person1.first_name # => Ben
puts Person1.full_name # Raises exception


# Recommended reading:
#
# http://yehudakatz.com/2009/11/15/metaprogramming-in-ruby-its-all-about-the-self/
# http://viewsourcecode.org/why/hacking/seeingMetaclassesClearly.html
# http://www.amazon.com/Metaprogramming-Ruby-Program-Like-Pros/dp/1934356476
#

# There is one hack to get around this if you INSIST on using the `self.` approach:

class Person3
  def self.first_name
    full_name.split(' ').first
  end

  def self.full_name
    'Ben Franklin'
  end
  private_class_method :full_name # See http://ruby-doc.org/core-1.9.3/Module.html#method-i-private_class_method
end

puts Person3.first_name # => Ben
puts Person3.full_name # Raises exception