Core logic example

core.clj

(ns logical-db.core
  (:require [clojure.core.logic :as l]
            [clojure.core.logic.protocols :refer [walk]]
            [clojure.core.logic.pldb :as db]))

; [org.clojure/clojure "1.8.0"]
; [org.clojure/core.logic "0.8.10"]


(def db { :city->zip {"City1" [1 2 3 4] "City2" [5 6]}
          :zip->city { 1 "City1" 2 "City1" 3 "City1" 4 "City1"
                      5 "City2" 6 "City2"}})

(defn testo [city zip-code]
  (fn [a]
    (let [zip (walk a zip-code)
          city (walk a city)]
      (condp = [(not (l/lvar? zip))
                (-> city l/lvar? not)]
         [true false] (l/unify a city (-> db :zip->city (get zip-code)))
         [false true] (let [zips-for-city (-> db :city->zip (get city))]
                          (->> zips-for-city
                               (map #(l/unify a zip %))
                               (l/to-stream)))
         [true true] (when (= city (-> db :zip->city (get zip)))
                        a)))))

(l/run* [q]
  (testo "City2" q))


(l/run* [q]
  (l/fresh [x]
      (l/membero x ["City1" "City2"])
      (l/== q 1)
      (testo x q)))