Maybe monad in Ruby

maybe.rb

module Monads
  class Maybe
    def self.[](value)
      self.new(value)
    end

    def initialize(value)
      @value = value
    end

    def fmap 
      self.class.new(@value.nil? ? nil : block_given? ? yield(@value) : @value)
    end

    def fold
      @value.nil? ? nil : block_given? ? yield(@value) : @value
    end
  end
end

##########################################
def email_service_imperative user_id
  if user = User.find_by_id(user_id)
    if email = user.email
      if service = email.split('@').last
        service.upcase 
      end
    end
  end
end

def email_service_declarative user_id
  Monads::Maybe[user_id]
    .fmap{|v| User.find_by_id(v)}
    .fmap(&:email)
    .fmap{|v| v.split '@'}
    .fmap(&:last)
    .fold(&:upcase)
end