Implementation of an expert system using Clara rules in Clojure

Following the previous article, I will describe an implementation using the Clara Rule, an expert system in Clojure, based on the code in “The world of clara-rules” written by iku00088 in 2017. The simplest working code is as follows.

(defrecord Num  [number])

(defrule is-two
  [Num (= number 2)] => (println "Two!"))

(-> (mk-session)
    (insert (->Num 2))
    (fire-rules))
;;-> "Two!"

The following is a sample code for a more specific use case (determining whether or not to change a certain product (Item) with money (MyMoney). The final price of the item is determined by the discount rate and tax (rule)).

(def discounted-items #{:fish :pants})

(def tax-rates {:food 0.1 :cloth 0.2 :alcohol 0.3})

(defrecord MyMoney [amt])

(defrecord Tax [category rate])

(defrecord Item [id price category])

(defrecord DiscountedItem [discounted-price category])

(defrecord TaxedItem [final-price])

(defrule DiscountItem
  [Item (= ?id id) (= ?category category) (= ?price price)]
  =>
  (println "Item is" ?id)
  (println "Original Price is" ?price)
  (println "Discounted item?" (some? (discounted-items ?id)))
  (insert! (->DiscountedItem (* ?price 0.9) ?category)))

(defrule TaxItem
  [DiscountedItem (= ?category category) (= ?discounted-price discounted-price)]
  =>
  (println "Tax rate is " (?category tax-rates))
  (insert! (->TaxedItem (+ ?discounted-price (* ?discounted-price (?category tax-rates))))))

(defrule EnoughMoney
  [MyMoney (= ?amt amt)]
  [TaxedItem (= ?final-price final-price) (> ?amt final-price)]
  =>
  (println "Final price:" ?final-price)
  (println "You have enough money!"))

(defrule NotEnoughMoney
  [MyMoney (= ?amt amt)]
  [TaxedItem (= ?final-price final-price) (not (> ?amt final-price))]
  =>
  (println "Final price:" ?final-price)
  (println "You do not have enough money!"))

(-> (mk-session)
    (insert (->MyMoney 199) (->Item :fish 200 :food))
    (fire-rules))
;;Item is :fish
;;Original Price is 200
;;Discounted item? true
;;Discounted Price 180.0
;;Tax rate is  0.1
;;Final price: 198.0
;;You have enough money!

Next time, we will take a more flexible approach. We will discuss the case of automatic rule generation from natural text.

Clara Rule’s expert system (simple usecase)

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