Minimal Ruby Maybe monad

maybe.rb

require 'singleton'

module Maybe
  class Just < BasicObject
    attr_reader :value

    def initialize(value)
      @value = value
    end

    def present?
      true
    end
  end

  class Nothing < BasicObject
    include ::Singleton

    def present?
      false
    end
  end

  def self.just(value)
    Just.new(value)
  end

  def self.nothing
    Nothing.instance
  end
end

using_maybe.rb

puts Maybe.just('foo').value # "foo"
puts Maybe.nothing.value rescue puts "No value on nothing"

puts Maybe.just('foo').to_s rescue puts "Can't call random stuff on Maybe - unwrap using #value"


def use_maybe(maybe)
  if maybe.present?
    puts maybe.value
  else
    puts "no, no, cannot use it"
  end
end

use_maybe Maybe.just('foo')
use_maybe Maybe.nothing