coupling.rb

class Dog
  def speak
    puts 'Ruff! Grr! Roof! Rawr!'
  end
end

class Cat
  def speak
    puts 'Meeeow...'
  end
end

class Bird
  def speak
    puts 'Tweety, tweet, tweet'
  end
end

class AnimalShelter
  attr_accessor :animals

  def initialize
    @animals = Array.new
  end

  def add(animal)
    @animals << animal
  end

  def open_doors
    animals.each do |animal|
      animal.speak
    end
  end
end

animal_shelter = AnimalShelter.new
3.times { animal_shelter.add Cat.new }
2.times { animal_shelter.add Dog.new }
4.times { animal_shelter.add Bird.new }
animal_shelter.open_doors

# Explorations:

# 1. How might we sort/get data on animals without adding coupling to
#    AnimalShelter? (interweave, specify order, 1 of each, count each)

# 2. How could we change the mechanism for output without changing all the
#    animal classes?

# 3. How would the impl be different if we used Ruby's Enumerable for
#    AnimalShelter?

2. How could we change the mechanism for output without changing all the animal classes?

Here's a way to generalize speech. Move speak from each animal and place it in Animal superclass, then define speech for each animal with just a string.

class Animal
  # ...
  def speak(&block)
    block ||= ->(str) { puts str }
    block.call(speech)
  end
end

class Dog < Animal
  # ...
  def speech
    'Ruff! Grr! Roof! Rawr!'
  end
end

# same with Cat and Bird

The interesting change here is the speak method implementation. First we've objectified the block (if any) in the argument list - the & keyword turns a block into an object (specifically a proc), or turns a proc into a block.

Then we've basically said "block is equal to block unless I didn't give you anything, in which case it's equal to a proc that contains a puts statement".

Dog.new.speak
  "Ruff! Grr! Roof! Rawr!"
  => nil
# runs #puts on speech

Dog.new.speak do |s|
  file = File.open('spoken.txt', ?w)
  file << s
  file.close
end
# opens a new file called spoken.txt and writes the speech to it

Basically you can give the speak method any writer you want in its block during runtime. You can definitely get fancier, but that's a basic start.