Ruby: define_method vs. def

Question

As a programming exercise, I've written a Ruby snippet that creates a class, instantiates two objects from that class, monkeypatches one object, and relies on method_missing to monkeypatch the other one.

Here's the deal. This works as intended:

class Monkey    def chatter     puts "I am a chattering monkey!"   end    def method_missing(m)     puts "No #{m}, so I'll make one..."     def screech       puts "This is the new screech."     end   end end  m1 = Monkey.new m2 = Monkey.new  m1.chatter m2.chatter  def m1.screech   puts "Aaaaaargh!" end  m1.screech m2.screech m2.screech m1.screech m2.screech 

You'll notice that I have a parameter for method_missing. I did this because I was hoping to use define_method to dynamically create missing methods with the appropriate name. However, it doesn't work. In fact, even using define_method with a static name like so:

def method_missing(m)   puts "No #{m}, so I'll make one..."   define_method(:screech) do     puts "This is the new screech."   end end 

Ends with the following result:

ArgumentError: wrong number of arguments (2 for 1)  method method_missing   in untitled document at line 9 method method_missing   in untitled document at line 9 at top level    in untitled document at line 26 Program exited. 

What makes the error message more bewildering is that I only have one argument for method_missing...

Answer 1

define_method is a (private) method of the object Class. You are calling it from an instance. There is no instance method called define_method, so it recurses to your method_missing, this time with :define_method (the name of the missing method), and :screech (the sole argument you passed to define_method).

Try this instead (to define the new method on all Monkey objects):

def method_missing(m)     puts "No #{m}, so I'll make one..."     self.class.send(:define_method, :screech) do       puts "This is the new screech."     end end 

Or this (to define it only on the object it is called upon, using the object's "eigenclass"):

def method_missing(m)     puts "No #{m}, so I'll make one..."     class << self       define_method(:screech) do         puts "This is the new screech."       end     end end