[JAVA] The world of clara-rules (3)

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I learned from the video of the lecture that clara-rules can be used for join operations between different entities, so I implemented a simple example myself.



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

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

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

(defrecord TypeC [attr-a attr-b join-key])

(defrule Join
  [TypeA (= ?jk-a join-key) (= ?a attr-a)]
  [TypeB (= join-key ?jk-a) (= ?b attr-b)]
  (insert! (->TypeC ?a ?b ?jk-a)))

(defrule Print
  [?result <- TypeC];; Will always match
  (println ?result))

(-> (mk-session)
    (insert (->TypeA "a1" :foo)
            (->TypeA "a2" :bar)
            (->TypeA "a3" :baz)
            (->TypeB "b1" :foo)
            (->TypeB "b2" :bar)
            (->TypeB "b3" :baz)
            (->TypeB "b4" :qux))
;;#clara_rules.join.TypeC{:attr-a a1, :attr-b b1, :join-key :foo}
;;#clara_rules.join.TypeC{:attr-a a2, :attr-b b2, :join-key :bar}
;;#clara_rules.join.TypeC{:attr-a a3, :attr-b b3, :join-key :baz}

--Type C is printed for Type A and Type B pairs that have a common : join-key as intended. You did it! --(-> TypeB" b4 ": qux) does not appear in the results because there is no TypeA fact with the same : join-key: qux. I see.

Explanation of <- on the left side of defrule

Bind the fact itself to a variable, not the attribute of the fact. Taking [? result <-TypeC] as an example, the entire matched TypeC instance is bound to? result.



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