What’s he building in there?

Defining methods, Javascript-style

The knowledgeable amongst you must have known that Ruby supports so called singleton methods which basically means methods can be defined on the instances of the class, rather than class itself. So, for example, you could redefine a method on a particular object and do all sorts of fun stuff. This is how it's usually done:

obj = "something"

def obj.my_new_method
  "does nothing"
end

obj.my_new_method # => "does nothing"

However, I like the javascript approach to this better. In javascript, function is treated as a variable you simply assign to the property of the object.

var obj = new String("something")
obj.myNewMethod = function() { return "does nothing" };
obj.myNewMethod(); // => "does nothing"
 

Fear not! With some meta programming you can achieve the same in Ruby. Let's see how.

module DynamicMethods
  def method_missing(method, *args)
    raise NoMethodError unless method.to_s[-1..-1] == "="
 
    (class << self; self; end).class_eval do
      define_method(method.to_s[0..-2].to_sym, args.first)
    end
  end
end

obj = "hello, friend!" 
obj.extend DynamicMethods
obj.piratize = proc { self.gsub("hello", "ahoy").gsub("friend", "matey") }
obj.piratize # => "ahoy, matey!"

As you have guessed, the trick is in our DynamicMethods module. This module creates a method on the fly, by invoking a non-existent method ending with "=" and a single proc argument. Note that the proc method is evaluated in the context of the "hello, friend!" object, not it's surroundings - which is the expected behaviour when defining methods.

So there you have it - instant pirate talk and method definitions javascript-style.