Implementing value object semantics as an RSpec shared_example.

dollar.rb

class Dollar
  attr_reader :cents

  def initialize(cents:)
    @cents = cents
  end

  def hash
    [self.class, cents].hash
  end

  def ==(other)
    cents == other.cents
  end
  alias_method :eql?, :==
end

dollar_spec.rb

require_relative "dollar"
require_relative "value_object_shared_examples"

RSpec.describe Dollar do
  it_behaves_like "value object" do
    let(:item) { Dollar.new(cents: 5) }
    let(:same) { Dollar.new(cents: 5) }
    let(:different) { Dollar.new(cents: 10) }
  end
end

duration.rb

class Duration
  attr_reader :milliseconds

  def initialize(milliseconds:)
    @milliseconds = milliseconds
  end

  def hash
    [self.class, milliseconds].hash
  end

  def ==(other)
    milliseconds == other.milliseconds
  end
  alias_method :eql?, :==
end

duration_spec.rb

require_relative "duration"
require_relative "value_object_shared_examples"

RSpec.describe Duration do
  it_behaves_like "value object" do
    let(:item) { Duration.new(milliseconds: 1000) }
    let(:same) { Duration.new(milliseconds: 1000) }
    let(:different) { Duration.new(milliseconds: 1234) }
  end
end

shell_output.log

 % rspec duration_spec.rb dollar_spec.rb --format documentation

Randomized with seed 9567

Dollar
  behaves like value object
    #==
      two instances are not == if they have a different value
      two instances are == if they have the same value
    #hash
      is the same as another instance of the same value
      is the different from another instance of a different value
    #equal?
      is equal if two object share the same identity
      is not equal if two different objects share the same value
    #eql?
      two instances are not eql if they have a different value
      two instances are eql if they have the same value

Duration
  behaves like value object
    #==
      two instances are == if they have the same value
      two instances are not == if they have a different value
    #hash
      is the same as another instance of the same value
      is the different from another instance of a different value
    #eql?
      two instances are eql if they have the same value
      two instances are not eql if they have a different value
    #equal?
      is not equal if two different objects share the same value
      is equal if two object share the same identity

Finished in 0.00453 seconds (files took 0.11736 seconds to load)
16 examples, 0 failures

Randomized with seed 9567

value_object_shared_examples.rb

RSpec.shared_examples_for "value object" do
  describe "#hash" do
    it "is the same as another instance of the same value" do
      expect(item.hash).to eq same.hash
    end

    it "is the different from another instance of a different value" do
      expect(item.hash).not_to eq different.hash
    end
  end

  describe "#eql?" do
    it "two instances are eql if they have the same value" do
      expect(item).to be_eql same
    end

    it "two instances are not eql if they have a different value" do
      expect(item).not_to be_eql different
    end
  end

  describe "#==" do
    it "two instances are == if they have the same value" do
      expect(item).to eq same
    end

    it "two instances are not == if they have a different value" do
      expect(item).not_to eq different
    end
  end

  describe "#equal?" do
    it "is equal if two object share the same identity" do
      expect(item).to be_equal item
    end

    it "is not equal if two different objects share the same value" do
      expect(item).not_to be_equal same
    end
  end
end