monad.rb

Optional = Struct.new(:value) do
  def and_then(&block)
    if value.nil?
      Optional.new(nil)
    else
      block.call(value)
    end
  end

  def method_missing(*args, &block)
    and_then do |value|
      Optional.new(value.public_send(*args, &block))
    end
  end
end

Many = Struct.new(:values) do
  def and_then(&block)
    Many.new(values.map(&block).flat_map(&:values))
  end

  def method_missing(*args, &block)
    and_then do |value|
      Many.new(value.public_send(*args, &block))
    end
  end
end

Eventually = Struct.new(:block) do
  def initialize(&block)
    super(block)
  end

  def run(&success)
    block.call(success)
  end

  def and_then(&next_block)
    Eventually.new do |success|
      run do |value|
        next_block.call(value).run(&success)
      end
    end
  end
end