From: Woody Peterson <woody.peterson@...>
Date: 2010-09-18T05:39:31+09:00
Subject: composition vs 'leaf-class mixins' (vs class inheritance)

I have a design question inspired by Rails 3 internals. In Design
Patterns in Ruby, the Strategy pattern is cited as a way to vary an
algorithm following the oft-spoken "prefer composition over inheritance"
advice. For a concrete example, let's say we wanted to fight in the
matrix:

    class Person
      attr_accessor :skills

      def initialize(skills)
        @skills = skills
      end

      def fight
        @skills.each { |skill| skill.invoke }
      end
    end

    class KungFu
      def invoke
        puts "5-fisted punch"
      end
    end

    class Dodge
      def invoke
        puts "bullet-time"
      end
    end

    class SeeTheMatrix
      def invoke
        puts "woah"
      end
    end

    me = Person.new([])
    me.fight # i've got nothing

    neo = Person.new([KungFu.new, Dodge.new, SeeTheMatrix.new])
    neo.fight # "5-fisted punch"
              # "bullet-time"
              # "woah"

Rails 3 might fight in the matrix like this:

    class Person
      def fight
      end
    end

    module KungFu
      def fight
        puts "5-fisted punch"
        super
      end
    end

    module Dodge
      def fight
        puts "bullet-time"
        super
      end
    end

    class Neo < Person
      include KungFu
      include Dodge

      def fight
        puts "woah"
        super
      end
    end

    Neo.new.fight # "woah"
                  # "bullet-time"
                  # "5-fisted punch"

Maybe a class-based inheritance implementation would be:

    class Person
      def fight
      end
    end

    class PersonKnowsKungFu < Person
      def fight
        "5-fisted punch"
        super
      end
    end

    class PersonMovesLikeThem < PersonKnowsKungFu
      def fight
        "bullet-time"
        super
      end
    end

    class Neo < PersonMovesLikeThem
      def fight
        "woah"
        super
      end
    end

The goal of all of these would be to easily add new skills and
characters. Obviously the class-based inheritance example is brittle
because it'd take a refactoring to give someone bullet-time without
kung-fu. I'm mostly interested in the differences between the
"leaf-class-mixins" strategy and the strategy pattern. "Prefer
composition over inheritance" would suggest you should use the strategy
pattern, but mixing in modules sure seems like a simple and ruby-ish way
of achieving the same thing. The only difference I can see is that the
strategy pattern allows for run-time skill manipulation, which I don't
find myself needing often. What other advantages would I gain from
preferring composition here?
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