Don't do this in production, or do. I'm not your parent.

maybe.rb

class Maybe
  attr_reader :value

  # @returns [Maybe]
  def self.empty
    @empty ||= new(nil)
  end

  def initialize(value, &block)
    @value = value.is_a?(Maybe) ? value.value : value
    @value = MaybeBlock.maybe(self, &block) if block_given?
  end

  # Returns the inner value or a fallback.
  def value_or(fallback)
    value || fallback
  end

  def ==(other_value)
    if other_value.is_a?(Maybe)
      value == other_value.value
    else
      value == other_value
    end
  end

  # @returns [Maybe]
  def call(method_name, *args)
    if value && value.respond_to?(method_name)
      Maybe.new value.public_send(method_name, *args)
    else
      Maybe.empty
    end
  end
end

class MaybeBlock < BasicObject
  def self.maybe(value, &block)
    new(value).instance_exec(&block).instance_variable_get('@value')
  end

  def initialize(value)
    @value = value
  end

  def method_missing(*args)
    @value = @value.call(*args)
  end
end

if $0 == __FILE__
  require 'minitest/autorun'
  require 'minitest/spec'

  Class.new(Minitest::Spec) do
    it 'gracefully handles a nil object' do
      assert_nil Maybe.new(nil).(:to_s).(:reverse).(:upcase).value
    end

    it 'returns nil if the resulting expression would throw an exception' do
      assert_nil Maybe.new(:hello).(:rand).value
    end

    it 'is accepts other maybes' do
      assert_equal 'Hello', Maybe.new(Maybe.new('Hello')).value
    end

    it 'returns a value or a another value if the monad is empty' do
      assert_equal 'Yes', Maybe.empty.value_or('Yes')
    end

    it 'chains along the calls if they are correct' do
      assert_equal "OLLEH", Maybe.new(:hello).(:to_s).(:split, '').(:reverse).(:join, '').(:upcase).value
    end

    it 'allows for checking equality' do
      assert Maybe.empty == nil
      assert Maybe.new('A') == Maybe.new('A')
    end

    it 'allows for hilarious block syntaxes' do
      result = Maybe.new(:hello) do
        to_s
        split ''
        reverse
        join ''
        upcase
      end

      assert_equal 'OLLEH', result.value
    end
  end
end