Mixins: Not Very Encapsulatey.

gistfile1.md

(In reply to https://twitter.com/gregmcintyre/status/432012568966619137)

Mixins (included Modules) all share one object's state and method set; you need to know everything about every module a class includes or extends before touching it, or risk breaking your code unintentionally. This is practically antithetical to Encapsulation, which hides state behind defined interfaces.

A good quote from a good article:

Using mixins like this is akin to “cleaning” a messy room by dumping the clutter into six separate junk drawers and slamming them shut. Sure, it looks cleaner at the surface, but the junk drawers actually make it harder to identify and implement the decompositions and extractions necessary to clarify the domain model.

same_instance_var.rb

module Searching
  def search(value)
    @cache ||= ExpensiveSearchOperation.do(value)
    # ...
  end
end

module Formatting
  def format(value)
    @cache ||= ExpensiveFormattingOperation.do(value)
    # ...
  end
end

class Post
  include Searching
  include Formatting
end

same_method.rb

module Titling
  attr_reader :title
  def title=(value)
    @title = normalize(value)
  end

  private
    def normalize(title)
      title.split(' ').select {|w| w.capitalize! || w }.join(' ');
    end
end

module Slugging
  attr_reader :slug
  def slug=(value)
    @slug = normalize(value)
  end

  private
    def normalize(slug)
      slug.downcase.gsub /[^a-z0-9]/, '-'
    end
end

class Post
  include Titling
  include Slugging
end

p = Post.new
p.title = "Foo Bar"
p.slug = "Foo Bar"

puts p.title # => "foo-bar"
puts p.slug  # => "foo-bar"

unexpected_state_munging.rb

module Slugging
  def self.included(klass)
    klass.instance_eval do
      validates :slug, :presence => true, :uniqueness => true
      before_create do
        self.slug = calculate_slug(self.title) # Permanent, set once at creation
      end
    end
  end
  def calculate_slug(title)
    title.downcase.gsub /[^a-z0-9]/, '-'
  end
end

module SKU
  def generate_sku
    save! unless persisted? # Needs an ID first
    "#{id}#{title.upcase.gsub(/[^A-Z]/, '')[0..10]}"
  end
end

class Product < ActiveRecord::Base
  include Slugging
  include SKU
end

# In a controller somewhere...

p = Product.new
p.generate_sku       # Saves, calls before_create from Slugging, explodes with validation error.
p.title = "Red Shoe" # ... Never makes it this far.


# What happened? Something in a module messed with the state of the object in
# a way you didn't expect; the Product got saved earlier than expected, which
# started a chain of actions that resulted in an invalid Product and an exception.
#
# "Just don't do that, then! Show some discipline!"
# Sure, but then why expose yourself to the possibility of it happening at all?
# Hive off the slugging and the SKU building into their own classes, keeping them
# isolated so they can't mess the object itself.