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
    def new(*args)
      super.extend(Baz)
    end
  end
  
  type_to_patch.class_eval do
    extend patcher
  end
end

extend_every(Foo, Baz)

assert_equal "weebar", Foo.new.bar

# Great, but we've had to hard-code Baz onto line 38. How can we make this method generic?

My attempt at solving this is in my fork above; basically, using define_method gives us the closure-style behaviour we need.