meta_programming.md

Meta-Programming Pattern

Problem

We want to gain more flexibility when defining new classes and create custom tailored objects on the fly.

Solution

As we already know, Ruby is a very dynamic language, but that doesn't apply only to typing: we can even define new methods in our classes at runtime. Thanks to singleton methods we can add new methods to existing instances. If instead of adding one method we want to add a group of them, we can also use the extend method, that has the same effect as including a module. Last but no least, with the class_eval method we can evaluate a string in the context of a class, which combined with string interpolations is a really great asset to create new methods at runtime.

Example

Coming back to the Factory Pattern where we created flora and fauna habitats, we might think we need more flexibility. Back then, we had multiple factories that provided us with a fixed list of combinations to create different organisms. If we wanted to have full flexibility for creating any combination we come up with, but avoiding a ton of classes that support every single combination, we can make use of singleton methods:

def new_plant(stem_type, leaf_type)
  plant = Object.new
  if stem_type == :fleshy
    def plant.stem
      'fleshy'
    end
  else
    def plant.stem
      'woody'
    end
  end
  if leaf_type == :broad
    def plant.leaf
      'broad'
    end
  else
    def plant.leaf
      'needle'
    end
  end
  plant
end

plant1 = new_plant(:fleshy, :broad)
plant2 = new_plant(:woody, :needle)
puts "Plant 1's stem: #{plant1.stem} leaf: #{plant1.leaf}"
puts "Plant 2's stem: #{plant2.stem} leaf: #{plant2.leaf}"

We start with a plain Ruby Object and tailor it adding new methods. Instead of adding them one by one, we can add a group of them:

def new_animal(diet, awake)
  animal = Object.new
  if diet == :meat
    animal.extend(Carnivore)
  else
    animal.extend(Herbivore)
  end
  if awake == :day
    animal.extend(Diurnal)
  else
    animal.extend(Nocturnal)
  end
  animal
end

In this example, the extend method does the same as including a module.

Now, let's imagine that we want to group together animals and trees that share a section of the jungle, and also keep track of their biological classifications. Although they look like two different programming problems, they are quite similar: both of them aim to group a set of objects. Ideally, we could stablish different kind of relationships between objects on the fly like this:

class Tiger < CompositeBase
  member_of(:population)
  member_of(:classification)
end

class Tree < CompositeBase
  member_of(:population)
  member_of(:classification)
end

class Jungle < CompositeBase
  composite_of(:population)
end

class Species < CompositeBase
  composite_of(:classification)
end

tony_tiger = Tiger.new('tony')
se_jungle = Jungle.new('southeastern jungle tigers')
se_jungle.add_sub_population(tony_tiger)

The method composite_of(:group) would provide a method for including members to any :group we could think of by adding dynamic add_sub_:group methods to the class instances. The same way, the method member_of(:group) would a method parent_:group to leaf nodes so that they can know what group they are member of. It happens that all this is absolutely feasible with some meta programming:

class CompositeBase
  attr_reader :name

  def initialize(name)
    @name = name
  end

  def self.member_of(composite_name)
    code = %Q{
      attr_accessor :parent_#{composite_name}
    }
    class_eval(code)
  end

  def self.composite_of(composite_name)
    member_of composite_name
    code = %Q{
      def sub_#{composite_name}s
        @sub_#{composite_name}s = [] unless @sub_#{composite_name}s
        @sub_#{composite_name}s
      end
      def add_sub_#{composite_name}(child)
        return if sub_#{composite_name}s.include?(child)
        sub_#{composite_name}s << child
        child.parent_#{composite_name} = self
      end
      def delete_sub_#{composite_name}(child)
        return unless sub_#{composite_name}s.include?(child)
        sub_#{composite_name}s.delete(child)
        child.parent_#{composite_name} = nil
      end
    }
    class_eval(code)
  end
end 

CompositeBase is the base class of the rest of components and implements the member_of and composite_of methods. Simply passing them the name of the group, they set up all the methods we need by constructing a string that defines them. The call to class_eval interprets the string in the context of the class, making them available.