Simple implementation of the Either type for Ruby (except Left is called Failure and Right is Success)

either.rb

class Either
  attr_reader :value, :error

  def initialize(_)  ; raise(NotImplementedError.new) ; end
  def failure?       ; is_a?(Failure)                 ; end
  def success?       ; is_a?(Success)                 ; end
  def get_or_else(_) ; raise(NotImplementedError.new) ; end
  def map            ; raise(NotImplementedError.new) ; end

  def ==(other)
    [:class, :value, :error].all? { |s| public_send(s) == other.public_send(s) }
  end

  def hash
    [value, error].hash
  end

  alias :eql? :==
  alias :left? :failure?
  alias :right? :success?
end

class Failure < Either
  def initialize(error)
    @error = error
  end

  def get_or_else(value)
    value
  end

  def map
    Failure.new(error)
  end
end

class Success < Either
  def initialize(value)
    @value = value
  end

  def get_or_else(_)
    value
  end

  def map
    Success.new(yield(value))
  end
end

either_test.rb

require "minitest/autorun"
require_relative "either"

class TestFailure < Minitest::Test
  def setup
    @either = Failure.new("foo")
  end

  def test_failure?
    assert @either.failure?
  end

  def test_success?
    refute @either.success?
  end

  def test_map
    assert_equal Failure.new("foo"), @either.map { |e| "bar" }
  end

  def test_get_or_else
    assert_equal "bar", @either.get_or_else("bar")
  end

  def test_equality
    assert_equal @either, Failure.new("foo")
  end

  def test_inequality
    refute_equal @either, Failure.new("bar")
  end
end

class TestSuccess < Minitest::Test
  def setup
    @either = Success.new("foo")
  end

  def test_failure?
    refute @either.failure?
  end

  def test_success?
    assert @either.success?
  end

  def test_map
    assert_equal Success.new("bar"), @either.map { |e| "bar" }
  end

  def test_get_or_else
    assert_equal "foo", @either.get_or_else("bar")
  end

  def test_equality
    assert_equal @either, Success.new("foo")
  end

  def test_inequality
    refute_equal @either, Success.new("bar")
  end
end