Added the Either monad as per Haskell with left and right values.

lib/monads/either.rb

@@ -0,0 +1,48 @@
+require 'monads/monad'
+
+module Monads
+  class Either
+    include Monad
+
+    attr_reader :value
+
+    def initialize(right, value)
+      @right = right
+      @value = value
+    end
+
+    def self.right(value)
+      new(true, value)
+    end
+
+    def self.left(value)
+      new(false, value)
+    end
+
+    def right?
+      @right
+    end
+
+    def left?
+      !right?
+    end
+
+    def and_then(&block)
+      block = ensure_monadic_result(&block)
+
+      if right?
+        block.call(value)
+      else
+        self
+      end
+    end
+
+    def respond_to_missing?(method_name, include_private = false)
+      super || right? || value.respond_to?(method_name, include_private)
+    end
+
+    def self.from_value(value)
+      Either.right(value)
+    end
+  end
+end

spec/monads/either_spec.rb

@@ -0,0 +1,79 @@
+# encoding: utf-8
+
+require 'monads/either'
+
+RSpec.describe Monads::Either do
+  describe '::right' do
+    subject { described_class.right(0) }
+    it { is_expected.to be_right }
+  end
+
+  describe '::left' do
+    subject { described_class.left(0) }
+    it { is_expected.to be_left }
+  end
+
+  describe '#value' do
+    subject { described_class.right(value) }
+    let(:value) { 42 }
+
+    it 'retrieves the value from an Either' do
+      expect(subject.value).to eq value
+    end
+  end
+
+  describe '#right?' do
+    context 'given a right value' do
+      subject { described_class.right(0) }
+      it { is_expected.to be_right }
+    end
+
+    context 'given a left value' do
+      subject { described_class.left(0) }
+      it { is_expected.not_to be_right }
+    end
+  end
+
+  describe '#left?' do
+    context 'given a right value' do
+      subject { described_class.right(0) }
+      it { is_expected.not_to be_left }
+    end
+
+    context 'given a left value' do
+      subject { described_class.left(0) }
+      it { is_expected.to be_left }
+    end
+  end
+
+  describe '#and_then' do
+    context 'when the value is left' do
+      subject { described_class.left(value) }
+      let(:value) { "error" }
+
+      it 'doesn’t call the block' do
+        expect { |block| subject.and_then(&block) }.not_to yield_control
+      end
+    end
+
+    context 'when the value is right' do
+      subject { described_class.right(value) }
+      let(:value) { 42 }
+
+      it 'calls the block with the value' do
+        @value = nil
+        subject.and_then { |value| @value = value; described_class.right(double) }
+        expect(@value).to eq value
+      end
+
+      it 'returns the block’s result' do
+        result = double
+        expect(subject.and_then { |value| described_class.right(result) }.value).to eq result
+      end
+
+      it 'raises an error if the block doesn’t return another Either' do
+        expect { subject.and_then { double } }.to raise_error(TypeError)
+      end
+    end
+  end
+end