Ruby Domain Specific Languages - The Basics (Part 1)

I have been working on a prototype of a Ruby domain specific language for one of my clients, a very large financial services company. I have learned a whole bunch of really interesting lessons, which I will share over a short series of posts as I make more progress.

The first thing I tried to do was create a basic bit of DSL tastiness like this:

class Dog < Animal
 number_of_legs 4
end

class Bird < Animal
 number_of_legs 2
end

class Snail < Animal
 number_of_legs 0
end

My first implementation of the Animal class looked like this:

class Animal
 attr_accessor :number_of_legs

 def self.number_of_legs(number_of_legs)
  @number_of_legs = number_of_legs
 end
end

Just looking at this bit of code, it seems like it should work. However, when trying this, it doesn't work as expected.

pet = Dog.new
p pet.number_of_legs # prints nil

In Ruby EVERYTHING is an object. This includes the class objects themselves, not just the instances of class objects! This means that when the number_of_legs method is called, it is actually being called before the actual instance object itself has been created. We need to get to our instance variable, and one way is to copy the class-level instance variable to the instance itself when the actual instance is being initialized like this:

class Animal
 attr_accessor :number_of_legs

 def self.number_of_legs(number_of_legs)
  @number_of_legs = number_of_legs
 end

 def initialize
  instance_variable_set("@number_of_legs", self.class.instance_variable_get("@number_of_legs"))
 end
end

Now the class works as expected:

pet = Dog.new
p pet.number_of_legs # prints 4

As you add new things to your DSL, having to copy each variable manually is boring. The final change is to copy every class instance variable automatically at initialization, as follows:

class Animal
 attr_accessor :number_of_legs
 
 def self.number_of_legs(number_of_legs)
  @number_of_legs = number_of_legs
 end
 
 def initialize
  self.class.instance_variables.each do var
   instance_variable_set(var, self.class.instance_variable_get(var))
  end
 end 
end

This is a lot cleaner, and is much more maintainable code.