extend_self-vs-module_function-vs-class<<self.rb

# Defining private methods is less clean using module_function

module ExtendSelf
  extend self

  def bar
    foo
  end

  private

  def foo
    42
  end
end

module ModuleFunction
  module_function

  def bar
    foo
  end

  def foo
    42
  end
  private_class_method :foo
end

def fails
  yield
  fail Exception, "should have failed"
rescue => e
  p e
end

p ExtendSelf.bar
fails { ExtendSelf.foo }

p ModuleFunction.bar
fails { ModuleFunction.foo }

puts '='*80

# module_function copies methods, and as such can break metaprogramming

module Bigger
  def bigger(name)
    method = method(name)
    define_method(name) { method.call + 1 }
  end
end

module ExtendSelf
  extend Bigger
  bigger :foo
end
module ModuleFunction
  extend Bigger
  bigger :foo
end

p ExtendSelf.bar
p ModuleFunction.bar

puts '='*80

# class << self changes constant lookup

module ConstantDefiningModule
  CONSTANT = 42
  PRIVATE_CONSTANT = 42
  private_constant :PRIVATE_CONSTANT
end

module ExtendSelf
  include ConstantDefiningModule

  def constant_lookup
    CONSTANT
  end

  def private_constant_lookup
    PRIVATE_CONSTANT
  end
end

module ModuleFunction
  include ConstantDefiningModule

  module_function

  def constant_lookup
    CONSTANT
  end

  def private_constant_lookup
    PRIVATE_CONSTANT
  end
end

module ClassSelfChild
  include ConstantDefiningModule

  class << self
    def constant_lookup
      CONSTANT
    end

    def private_constant_lookup
      PRIVATE_CONSTANT
    end

    def constant_lookup_on_self
      self::CONSTANT
    end

    def private_constant_lookup_on_self
      self::PRIVATE_CONSTANT
    end
  end

  def self.def_self_constant_lookup
    CONSTANT
  end

  def self.private_def_self_constant_lookup
    PRIVATE_CONSTANT
  end
end

p ExtendSelf.constant_lookup
p ModuleFunction.constant_lookup

fails { ClassSelfChild.constant_lookup }
p ClassSelfChild.def_self_constant_lookup
p ClassSelfChild.constant_lookup_on_self

p ClassSelfChild.private_def_self_constant_lookup
fails { ClassSelfChild.private_constant_lookup }
fails { ClassSelfChild.private_constant_lookup_on_self }