gistfile1.rb

class Object
  def chain_method(with_symbol, &with_block_returned_from)
    old_method = method with_symbol
    new_method = with_block_returned_from.call old_method
    eigenclass = class << self; self; end
    eigenclass.class_eval do
      define_method with_symbol, new_method
    end
  end
end

def foo(arg)
  print 'in foo: ', arg, "\n"
end

puts
foo 'first call'

chain_method :foo do |old_method|
  lambda do |arg|
    puts 'in first block_eval'
    old_method.call arg
  end
end

puts
foo 'second call'

chain_method :foo do |old_method|
  lambda do |arg|
    puts 'in second block_eval'
    old_method.call arg
  end
end

puts
foo 'third call'