my_prelude.rb

module Prelude
  class Functor
    def initialize(value)
      @value = value
    end

    # All implementers must satisfy
    # (a -> b) -> f a -> f b
    def fmap
      self.class.new(yield @value)
    end
  end

  # This is technically Either, but we're gonna use it as Result
  module Result
    class Success < Functor; end

    class Failure < Functor
      def fmap
        # TODO: explore how we're gonna implement identity (id a -> a)
        super { |v| v }
      end
    end
  end
end