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...

Any help would be appreciated. Thank you.

Answer 1

define_method is a 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.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

Answer 2

self.class.define_method(:screech) doesn't work,because define_method is private method you can do that class << self public :define_method end def method_missing(m) puts "No #{m}, so I'll make one..." Monkey.define_method(:screech) do puts "This is the new screech." end

Answer 3

def method_missing(m) self.class.class_eval do define_method(:screech) {puts "This is the new screech."} end end

screech method will be available for all Monkey objects.