Simple Ruby implementations of some monads (just for fun)

monads.rb

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

    def initialize(value)
      @value = value
    end

    def >>
      self.class.new yield @value
    end

    def <=
      block_given? ? yield(@value) : @value
    end
  end

  class Maybe < Box
    def >>
      self.class.new(@value.nil? ? nil : block_given? ? yield(@value) : @value)
    end

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

  class Try
    def self.to
      Right[yield]
    rescue StandardError => e
      Left[e]
    end

    class Right < Box
      def >>
        self.class[yield @value]
      rescue StandardError => e
        Left[e]
      end

      def <=
        yield nil, @value
      end
    end

    class Left < Box
      def >>
        self
      end

      def <=
        yield @value, nil
      end
    end
  end
end

monads_examples.rb

###############################
# Box
#----------before--------------
def filtered_sales
  filter_by_tags \
    filter_by_category \
      filter_by_client \
        filter_by_seller \
          sales_by_dates
end
#----------after--------------
def filtered_sales
  Monads::Box[sales_by_dates].
    >> {|v| filter_by_seller v}.
    >> {|v| filter_by_client v}.
    >> {|v| filter_by_category v}.
    <= {|v| filter_by_tags v}
end
###############################

###############################
# Maybe
#----------before--------------
def email_service 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
#----------after--------------
def email_service user_id
  Monads::Maybe[user_id].
    >> {|v| User.find_by_id(v)}.
    >> (&:email).
    >> {|v| v.split '@'}.
    >> (&:last).
    <= (&:upcase)
end

###############################
# Try aka Either
#----------before--------------
def get_service_port default_port
  begin
    file = File.open('config.json')
    config = JSON.parse(file)
    config['port'] || default_port
  rescue StandardError => e
    default_port
  end
end

def get_service_port default_port
  Monads::Try.to { File.open('config.json') }.
    >> {|v| JSON.parse v }.
    >> {|v| v['port'] || default_port }.
    <= {|err,v| err ? default_port : v }
end