a_ruby_metaprogramming_puzzle_for_polite_programmers.rb

require 'test/unit/assertions'
include Test::Unit::Assertions
assert_equal "1.9.2", RUBY_VERSION

# A Ruby meta-programming puzzle for polite programmers.

# This puzzle was created by Matt Wynne (@mattwynne) on 2011-04-10 inspired by Jim Weirich's
# talk at the Scottish Ruby Conference 2011.
#
# The challenge is: you have a class Foo which you want to monkey-patch, but politely.

# Here's the default behaviour of Foo
class Foo
  def bar
    "bar"
  end
end

assert_equal "bar", Foo.new.bar

module Baz
  def bar
    "wee" + super
  end
end

# # Unfortunately, if we try to monkey-patch Foo like this, it won't work
# class Foo
#   include Baz
# end
#
# # this fails:
# assert_equal "weebar", Foo.new.bar
# 
# This is because the include trick inserts the module Baz higher into the inheritance tree
# meaning Foo's own implementation of bar is hit first.

# So let's write a helper method that can do the patching for us, by extending each instance
# of Foo as it is created.
#
def extend_every(type_to_patch, module_to_apply)
  type_to_patch.define_singleton_method(:new) do |*args, &block|
    super(*args, &block).extend(module_to_apply)
  end
end

extend_every(Foo, Baz)

assert_equal "weebar", Foo.new.bar

The above solution will only apply the last one, but we could change the implementation into this for supporting multi levels:

require 'test/unit/assertions'
include Test::Unit::Assertions
assert_equal "1.9.2", RUBY_VERSION

# A Ruby meta-programming puzzle for polite programmers.

# This puzzle was created by Matt Wynne (@mattwynne) on 2011-04-10 inspired by Jim Weirich's
# talk at the Scottish Ruby Conference 2011.
#
# The challenge is: you have a class Foo which you want to monkey-patch, but politely.

# Here's the default behaviour of Foo
class Foo
  def bar
    "bar"
  end
end

assert_equal "bar", Foo.new.bar

module Baz
  def bar
    "wee" + super
  end
end

module Buzz
  def bar
    "buzz" + super
  end
end

# # Unfortunately, if we try to monkey-patch Foo like this, it won't work
# class Foo
#   include Baz
# end
#
# # this fails:
# assert_equal "weebar", Foo.new.bar
# 
# This is because the include trick inserts the module Baz higher into the inheritance tree
# meaning Foo's own implementation of bar is hit first.

# So let's write a helper method that can do the patching for us, by extending each instance
# of Foo as it is created.
#
def extend_every(type_to_patch, module_to_apply)
  type_to_patch.instance_eval do
    @__modules_to_apply ||= []
    @__modules_to_apply << module_to_apply

    def new(*args, &block)
      obj = super(*args, &block)
      @__modules_to_apply.each { |m| obj.extend(m) }

      obj
    end
  end
end

extend_every(Foo, Baz)
extend_every(Foo, Buzz)

assert_equal "buzzweebar", Foo.new.bar