Перевод примера из книги "Seven Languages in Seven Weeks" с Prolog на clojure.core.logic

likes.clj

(ns logic-in-clojure.core
  (:refer-clojure :exclude [==])
  (:use clojure.core.logic))

;; prolog example:
;;
;; likes(wallace, cheese).
;; likes(grommit, cheese).
;; likes(wendolene, sheep).
;; friend(X, Y) :- \+(X = Y), likes(X, Z), likes(Y, Z).
;;
;; likes(wallace, sheep).       -> no
;; likes(wallace, cheese).      -> yes
;; friend(wallace, wallace).    -> no
;; friend(wallace, grommit).    -> yes
;; friend(wendolene, grommit).  -> no

(defrel likes c i)
(fact likes 'wallace 'cheese)
(fact likes 'grommit 'cheese)
(fact likes 'wendolene 'sheep)

(defn friend? [x y]
  (not (empty? (run* [q]
    (fresh [z]
      (likes x z)
      (likes y z)
      (if (= x y) fail succeed))))))

(friend? 'wallace 'grommit)         ;; -> true
(friend? 'grommit 'wallace)         ;; -> true
(friend? 'wallace 'wallace)         ;; -> false
(friend? 'wallace 'wendolene)       ;; -> false