Courtesy Implementation

When you a write a class, you mostly strive to ensure that the features of that class make sense for that class. But there are occasions when it makes sense to add a feature to allow a class to conform to a richer interface that it naturally should.

The most common and obvious example of this is one that comes up when you use the composite pattern. Let's consider a simple example of containers. You have boxes which can contain other boxes and elephants (that's an advantage of virtual elephants.) You want to know how many elephants are in a box, considering that you need to count the elephants inside boxes inside boxes inside the box. The solution, of course, is a simple recursion.

# Ruby
class Node
end

class Box < Node
  def initialize 
    @children = []
  end
  def << aNode
    @children << aNode
  end
  def num_elephants
    result = 0
    @children.each do |c|
      if c.kind_of? Elephant
        result += 1
      else
        result += c.num_elephants
      end
    end
    return result
  end
end

class Elephant < Node
end

Now the kind_of? test in num_elephants is a smell, since we should be wary of any conditional that tests the type of an object. On the other hand is there an alternative? After all we are making the test because elephants can't contain boxes or elephants, so it doesn't make sense to ask them how many elephants are inside them. It doesn't fit our model of the world to ask elephants how many elephants they contain because they can't contain any. We might say it doesn't model the real world, but my example feels a touch too whimsical for that argument.

However when people use the composite pattern they often do provide a method to avoid the conditional - in other words they do this.

class Node
  #if this is a strongly typed language I define an abstract
  #num_elephants here
end

class Box < Node
  def initialize 
    @children = []
  end
  def << aNode
      @children << aNode
  end
  def num_elephants
    result = 0
    @children.each do |c|
      result += c.num_elephants
    end
    return result
  end
end

class Elephant < Node
  def num_elephants
    return 1
  end
end

Many people get very disturbed by this kind of thing, but it does a great deal to simplify the logic of code that sweeps through the composite structure. I think of it as getting the leaf class (elephant) to provide a simple implementation as a courtesy to its role as a node in the hierarchy.

The analogy I like to draw is the definition of raising a number to the power of 0 in mathematics. The definition is that any number raised to the power of 0 is 1. But intuitively I don't think it makes sense to say that any number multiplied by itself 0 times is 1 - why not zero? But the definition makes all the mathematics work out nicely - so we suspend our disbelief and follow the definition.

Whenever we build a model we are designing a model to suit how we want to perceive the world. Courtesy Implementations are worthwhile if they simplify our model.

reposted on 27 Aug 2014