Get Rid of That Code Smell – Primitive Obsession

This is a post from the Get Rid of That Code Smell series.

Primitive Obsession is another popular code smell in Ruby land. It’s very easy, tempting and just feels convenient to use primitive objects to represent various concepts in our code. Here are some primitive classes in Ruby that we like to be obsessed about:

  • Array

  • Hash

  • String

  • Fixnum

  • Float

Whenever you use one of these classes in a context where they don’t actually fit being semantically incorrect, that’s when you introduce Primitive Obsession code smell. Nothing explains it better than a few simple examples:

  • a string representing a URI

  • a float number representing money

  • a hash representing a set of objects

Think about it this way: would you use a string to represent a date? You could, right? Just create a string, let’s say "2012-06-25" and you’ve got a date! Well, no, not really - it’s a string. It doesn’t have semantics of a date, it’s missing a lot of useful methods that are available in an instance of Date class. You should definitely use Date class and that’s probably obvious for everybody. This is exactly what Primitive Obsession smell is about.

Detecting Primitive Obsession Smell

Unfortunately I’m not aware of any tools that would know how to detect this smell so you need to rely on your own analysis. If you have a good understanding of the domain you’re dealing with it should be fairly easy to detect the smell. Simply look for cases where primitive ruby objects are being used intensively in domain specific contexts. Monkey patching primitive classes could also be considered as a smell indicator.

Here’s an example of the smell from Virtus project, a hash with attributes:

module Virtus
  module ClassMethods

    # Returns all the attributes defined on a Class.
    # @return [Hash]
    #   an attributes hash indexed by attribute names
    # @api public
    def attributes
      @attributes ||= {}


What happens here is that we use Hash to represent a set of attributes. It just feels convenient to use a hash here, we want to access attribute objects via #[] method and hash gives us that for free. What we forget about is that we don’t really need a hash, we need something with set semantics and methods to add and merge attributes. Building such API on top of Hash is a mistake.

Removing Primitive Obsession Smell

It’s easy, just come up with a rich object that has correct semantics and exposes API that you need. The attributes hash in Virtus was replaced with an instance of Virtus::AttributeSet.

Here’s its piece (full source is here):

module Virtus

  # A set of Attribute objects
  class AttributeSet
    include Enumerable

    def initialize(parent = nil, attributes = [])
      @parent       = parent
      @attributes   = attributes.dup
      @index        = {}

    def each
      return to_enum unless block_given?
      @index.values.uniq.each { |attribute| yield attribute }

    def <<(attribute)
      self[] = attribute

    def [](name)

    def []=(name, attribute)
      @attributes << attribute
      update_index(name, attribute)

The class exposes smaller API than Hash and at the same time has additional responsibility that we needed (dealing with attributes from a parent). With a primitive hash object we would have to put this responsibility in a place where it wouldn’t really belong. win-win.

Summing Up

In OOP it’s important to use rich objects representing various concepts from your domain. Implement Money class if you need to deal with money, it’s much better than using floats all over the place. Don’t use Hash for configuration objects, use a custom Configuration class instead. Have fun with GeoLocation class instances rather than having pairs of latitude and longitude values embedded inside other objects. This way you will achieve better encapsulation and slim API.

Next stop: we’re going to take a look at Feature Envy.