task-precedence.clj

(ns schejule.core
  (:require [clojure.core.logic :as lg]
            [clojure.core.logic.pldb :as pldb]))

;; Declare database relations
(pldb/db-rel tasko ^:index id)
(pldb/db-rel precedo ^:index id other-id bool)


;; Make example db
(def mini-db
  (-> (pldb/db)
      (pldb/db-fact tasko 1)
      (pldb/db-fact tasko 2)
      (pldb/db-fact tasko 3)
      (pldb/db-fact tasko 4)
      (pldb/db-fact precedo 1 2 0)
      (pldb/db-fact precedo 1 3 0)
      (pldb/db-fact precedo 1 4 0)
      (pldb/db-fact precedo 2 1 1)
      ;; (pldb/db-fact precedo 2 3 0)  <--- precedence missing here
      (pldb/db-fact precedo 2 4 0)
      (pldb/db-fact precedo 3 1 1)
      (pldb/db-fact precedo 3 2 1)
      (pldb/db-fact precedo 3 4 0)
      (pldb/db-fact precedo 4 1 1)
      (pldb/db-fact precedo 4 2 1)
      (pldb/db-fact precedo 4 3 1)))

;; Try to bind q to task with missing precedence
(pldb/with-db mini-db
  (lg/run* [q]
    (lg/fresh [x y z]
      (tasko q)
      (precedo x y z)
      (lg/!= q x))))

;; This fails because whatever x unifies to,
;; there are 3 other values that q can take.
;; Thus, q unifies with the full range of tasks.