Courteous Meta-Programming in Ruby

29 December 2011

Meta-programming is often kind of mean. It has a tendency to be hard to understand and it can be quite destructive - causing methods to be modified or overwritten in non-obvious ways. I decided to write a post about a more courteous form of meta-programming. I picked up the technique from a blog post about method_missing and respond_to?. I realized that although I am comfortable with meta-programming, other developers might not feel the same way. So, here's a simple pattern that can make your meta-programming more readable and less destructive.

I want to create a module named Greetings so that when I extend my classes with it I can declaratively add greetings to them.

class FrenchAndEnglish
  extend Greetings
  greeting "Bonjour!"
  greeting "Hello!"
end

irb(main):001:0> FrenchAndEnglish.new.greet
Bonjour!
Hello!
=> nil

There are a couple of ways to accomplish this with Ruby. But before we jump into code, let's add these constraints to the system. After all, we are trying to write courteous code.

Readability - the code should be easy to read and understand with minimal knowledge of meta-programming.
Super Support - the greet method should support the use of super to delegate to parent definitions.
Non Destructive - the extending class should be able to define it's own greet method.

For instance, consider the following use of the Greetings module. It supports the use of super to delegate to previous implementations of greet and it allows the base class to define it's own greet method.

class TriLingualWithEnglish
  extend Greetings
  greeting 'Hello!'
  greeting 'Howdy!'

  def initialize(message01, message02)
    @message01 = message01
    @message02 = message02
  end

  def greet
    super rescue nil
    puts "#{@message01} and #{@message02}!"
  end
end

irb(main):001:0> TriLingualWithEnglish.new('Bonjour', 'Kalimera').greet
Hello!
Howdy!
Bonjour and Kalimera!
=> nil

Let's look at the most common meta-programming approaches to solving this problem.

eval - using instance_eval or class_eval might be acceptable, but these options are generally destructive. They overwrite methods on the base class and this breaks our non-destructive requirement. If we were able to solve this problem by using alias method chains then we would definitely be entering the territory of un-readable code. Also method chains would not allow us to use super to call previous definitions of greet.
Tracking Class Variable - a variable defined on the class level that is responsible for keeping a list of greetings. This has lots of problems, for one it does not support super, but it also greatly restricts our flexibility. We can no longer define an alternate greet method on the base class.

Since these two approaches don't fit our courteous meta-programming requirements, here's the cool alternative that does.

module Greetings
  def greeting(phrase)
    m = Module.new do
      define_method(:greet) do
        super rescue nil
        puts(phrase)
      end
    end

    include m
  end
end

By using the anonymous module m, and including it we are actually creating a storage object in the class hierarcy for the method, instead of writing it on the original class. This approach is non-destructive and allows us to maintain a clean method chain that supports the use of super. The super rescue nil line in our TriLingualWithEnglish class now calls into the anonymous module and allows both greeting implementations to co-exist. The code is also very readable since the programmer does not need to understand class_eval and instance_eval. I would advocate this approach whenever possible since it preserves the class hierarchy and avoids some of the less readable meta-programming techniques.

Courteous Meta-Programming in Ruby

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