Overwrite Class/Instance methods (keyword: class_eval by itself, instance_eval & class_eval included)

does_not_overwrite.rb

# This structure will not overwrite bar class method.
# calling super will call the included module's bar.
# http://tech.tulentsev.com/2012/02/ruby-how-to-override-class-method-with-a-module/

module FooModule
  def self.included base
    base.extend ClassMethods
  end

  module ClassMethods
    def bar
      puts "module"
    end
  end
end

class Klass
  include FooModule

  def self.bar
    puts 'class'
  end
end


Klass.bar #=> class

open_classes.rb

puts [1, 2, 3].to_s #=> [1, 2, 3]

class Array
  def to_s
    "an array: #{join ', '}"
  end
end

puts [1, 2, 3].to_s #=> an array: 1, 2, 3

# Similar code using `class_eval`

puts [1, 2, 3].to_s #=> [1, 2, 3]

Array.class_eval do
  def to_s
    "an array: #{join ', '}"
  end
end

puts [1, 2, 3].to_s #=> an array: 1, 2, 3

overriding_class_methods.rb

class Klass
  def self.say
    puts 'class'
  end
end

module FooModule
  def self.included base
    base.instance_eval do
      def say
        puts "module"
      end
    end
  end
end


Klass.send(:include, FooModule)

Klass.say #=> module

overriding_instance_method.rb

class Klass
  def say
    puts 'class'
  end
end

module FooModule
  def self.included(base)
    base.class_eval do
      def say
        puts "module"
      end
    end
  end
end


Klass.send(:include, FooModule)

Klass.new.say #=> module

prepend.rb

# https://medium.com/@leo_hetsch/ruby-modules-include-vs-prepend-vs-extend-f09837a5b073
# will look into the `module` methods first before looking into the class
module ServiceDebugger
  def run(args)
    puts "Service run start: #{args.inspect}"
    result = super
    puts "Service run finished: #{result}" 
  end
end

class Service
  prepend ServiceDebugger
  
  # perform some real work
  def run(args)
    args.each do |arg|
      sleep 1
    end
    {result: "ok"}
  end
end