Dynamic module lookup in Ruby

dynamic_module_lookup.rb

# What do you suppose the output of the examples are?
# Note: If you want to tinker executing these, don't just copy paste the entire file
# as defining everything in the same file acts differently (expected effects of reopening constants)

############## EXAMPLE 1 ##############
module NameHelper
  def full_name
    "#{first_name} #{last_name}"
  end
end

class Person
  include NameHelper

  def first_name
    "Abe"
  end

  def last_name
    "Lincoln"
  end
end

module NameHelper
  def full_name
    "George Washington"
  end
end

puts Person.new.full_name

############## EXAMPLE 2 ##############
module NameHelper
  def full_name
    "#{first_name} #{last_name}"
  end
end

class Person
  include NameHelper

  def first_name
    "Abe"
  end

  def last_name
    "Lincoln"
  end
end

module NameHelper
  prepend(Module.new {
    def full_name
      "George Washington"
    end
  })
end

puts Person.new.full_name

############## EXAMPLE 3 ##############
module NameHelper
  def full_name
    "#{first_name} #{last_name}"
  end
end

class Person
  include NameHelper

  def first_name
    "Abe"
  end

  def last_name
    "Lincoln"
  end
end

module NameHelper
  include(Module.new {
    def full_name
      "George Washington"
    end
  })
end

puts Person.new.full_name

# Don't open this until you've guessed the outputs!
# https://bugs.ruby-lang.org/issues/9112

The last one is the worst me

This gist doesn't demonstrate the same thing as the bug you referenced. This behaves as one would expect if you remember that including a module just inserts it into the module's/class' inheritance chain. So in example 1, you just reopen NameHelper and redefine full_name, but in 2 and 3 you include a module in NameHelper that implements full_name. Since you end up with Person < NameHelper < #<Module:0xdeadbeef>, and NameHelper overrides full_name, method lookup stops with the "#{first_name} #{last_name}" implementation.