datanaive.clj

(ns datanaive.core)

;; https://courses.cs.washington.edu/courses/cse544/16au/lectures/lecture05-datalog.pdf


;; slide 7
;; Actor(344759,‘Douglas’, ‘ Fowley ’). 
;; Casts(344759, 29851). 
;; Casts(355713, 29000). 
;; Movie(7909, ‘A Night in Armour ’, 1910). 
;; Movie(29000, ‘Arizona’, 1940). 
;; Movie(29445, ‘Ave Maria’, 1940). 


(def actor {344759 {"Douglas" "Fowley"}})

(def casts {344759 29851
            355713 29000})

(def movie {7909 {"A Night in Armour" 1910}
            29000 {"Arizona" 1940}
            29445 {"Ave Maria" 1940}})

(def q1 [[movie :x :y :z]
         [= :z 1940]])

;; S elect- P roject- J oin- U nion query 

(defn select [r]
  (let [[relation v1 v2 v3] r]
    (if-not (map? relation)
      r ;; do nothing for intensional clauses
      (for [[e m1] relation
            [a v] m1]
        (merge {v1 e v2 a}
               (when v {v3 v}))))))


(defn extensional? [r]
  (seq? r))

(defn intensional? [r]
  (not (extensional? r)))

(defn join [r1 r2]
  (when-not (or (extensional? r1)
                (extensional? r2))
    (throw (ex-info "Cannot join two intensional rules."
                    {:r1 r1 :r2 r2})))
  (if-not (extensional? r1)
    (recur r2 r1)
    (if (extensional? r2)
      :todo
      (let [[pred a b] r2
            [a b] (if (keyword? a) [a b] [b a])]
        (assert (keyword? a))
        (filter #(pred (a %) b) r1)))))


(join (select [movie :x :y :z])
      [= :z 1940])

(defn project [vars frames]
  (map #(select-keys % vars) frames))


(defn query [vars q]
  ;; TODO cover union over OR clauses
  (->> q
       (map select)
       (apply join)
       (project vars)))

(query #{:x :z} q1)