The world of clara-rules (4)

Last time

There is a query function for checking the contents of the clara-rules session from the outside, so I tried to find out how to use it.

--Basically, only the left side of the rule is defined.

Difference in role with rules

--Rules are used to derive new facts from existing facts added to the session --Query is used purely to look inside the session --Debug --Intermediate step to extract information from the calculated session and pass it to another process

Code example


(ns  clara-rules.join
  (:require [clara.rules :refer [defrule fire-rules insert
                                 mk-session query defquery]]))

(defrecord TypeA [attr-a join-key])

(defrecord TypeB [attr-b join-key])

;;Queries
(defquery find-all-As
  [];;parameters
  [?a <- TypeA];;Matching condition
  )

(defquery find-As-by-attr
  [:?value];; Parameter needs to be a keyword
  [?a <- TypeA (= attr-a ?value)])

(defquery find-join-key-of-As
  []
  [TypeA (= ?jk join-key)])

(defquery find-A-and-B-pair-by-join-key
  [:?value]
  [?a <- TypeA (= join-key ?value)]
  [?b <- TypeB (= join-key ?value)])

(def sess (-> (mk-session)
              (insert (->TypeA "a1" :foo)
                      (->TypeA "a2" :bar)
                      (->TypeA "a3" :baz)
                      (->TypeB "b1" :foo)
                      (->TypeB "b2" :bar)
                      (->TypeB "b3" :baz)
                      (->TypeB "b4" :qux))
              (fire-rules)))

(query sess find-all-As)
;; ({:?a {:attr-a "a1", :join-key :foo}}
;;  {:?a {:attr-a "a2", :join-key :bar}}
;;  {:?a {:attr-a "a3", :join-key :baz}})

(query sess find-As-by-attr :?value "a1")
;; => ({:?a #clara_rules.join.TypeA{:attr-a "a1", :join-key :foo}, :?value "a1"})

(query sess find-join-key-of-As)
;; => ({:?jk :foo} {:?jk :bar} {:?jk :baz})

(query sess find-A-and-B-pair-by-join-key :?value :foo)
;; => ({:?a {:attr-a "a1", :join-key :foo},
;;      :?value :foo,
;;      :?b {:attr-b "b1", :join-key :foo}})

Points of interest in the above code

--defquery can be written in the same way as the left side of defrule, except that parameters can be received externally. --Variable binding etc --query returns the variable bound in defquery as the key of the resulting map --query returns the match as a sequence of maps

Impressions

In the previous examples, the side effect (println) was only used on the right side of the rule to check the result. I'm glad that I was able to solve one of the questions I had since I touched on whether there was a way to check the results without using side effects.

Next time