Decorate all methods in a module - don't try this at home

decorate.rb

module Foo
  def foo
    puts bar
    return 42
  end

  def what(a, b)
    puts "#{bar} = #{a} + #{b}"
    return 42
  end

  def how(a, b)
    result = yield a, b
    puts "#{bar}: #{result}"
    return 42
  end
end

module Decorator
  class << self
    def decorate(klass, mod, &block)
      klass.class_eval do
        include mod
        mod.instance_methods.each do |name|
          class_eval "alias :#{name}_undecorated :#{name}"
          define_method("#{name}_decorated") do |*args, &internal_block|
            result = nil
            internal_method = -> {
              result = instance_exec do
                send("#{name}_undecorated", *args, &internal_block)
              end
            }
            yield internal_method
            result
          end
          class_eval "alias :#{name} :#{name}_decorated"
        end
      end
    end
  end
end

class Bar
  Decorator.decorate(self, Foo) do |undecorated|
    puts '.' * 50
    puts 'Before'
    undecorated.call
    puts 'After'
  end

  def bar
    'bar'
  end
end

b = Bar.new
b.foo_undecorated
x = b.foo
y = b.what(3, 5)
z = b.how(3, 5) { |a, b| a ** b }
puts "return values: #{x}, #{y}, #{z}"

output.txt

$ ruby test.rb 
bar
..................................................
Before
bar
After
..................................................
Before
bar = 3 + 5
After
..................................................
Before
bar: 243
After
42, 42, 42