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) 
  patcher = Module.new do
    # Use define method to get closure-type behaviour within the method "body"
    define_method(:new) do |*args|
      super(*args).extend(module_to_apply)
    end
  end
  
  type_to_patch.class_eval do
    extend patcher
  end
end

extend_every(Foo, Baz)

assert_equal "weebar", Foo.new.bar # => works.

I did not know you could do *args to a block like that. Ruby 1.9 FTW!

I think you'd want to do:

define_method(:new) do |*args, &block|
  super(*args, &block).extend(module_to_apply)
end

It doesn't matter in this case (since Foo.new does not expect a block), but if it did, this would be important.

Yep, that's a good point.

Here's my revision: https://gist.github.com/1087802

I make it into shorter by using define_singleton_method