Monkey Patch example (Ruby).

monkey_patching.rb

puts "--- Example 1:"

class Foo
  def bar; "bar" end
end
a = Foo.new
puts "a.bar: #{a.bar}"

puts "--- Example 2:"

class Foo
  def foobar; "foobar" end
end

b = Foo.new
puts "b.bar: #{b.bar}"
puts "b.foobar: #{b.foobar}"

puts "a.bar: #{a.bar}"
puts "a.foobar: #{a.foobar}"

puts "--- Example 3:"

c = Foo.new
class << c
  def foo; "foo" end
end

puts "c.bar: #{c.bar}"
puts "c.foobar: #{c.foobar}"
puts "c.foo: #{c.foo}"

puts "a.bar: #{a.bar}"
puts "a.foobar: #{a.foobar}"
puts "a.foo: #{a.foo}"