right? and righto

l1.clj

(ns l1
  (:use [clojure.core.logic
         :only [fresh conde firsto resto run]]))

(defn right?
  [a b l]
  (or (and (== a (first l))
           (== b (second l)))
      (when-let [t (next l)]
        (recur a b t))))

(comment
  (right? 1 2 [1 2 3])
  ;;=> true
  (right? 2 3 [1 2 3])
  ;;=> true
  (right? 1 3 [1 2 3])
  ;;=> nil
  )

(defn righto
  [X Y L]
  (conde [(fresh [t]
                 (firsto L X)
                 (resto L t)
                 (firsto t Y))]
         [(fresh [t]
                 (resto L t)
                 (righto X Y t))]))

(comment
  (run 1 [q]
       (righto 1 2 [1 2 3]))
  ;;=> (_.0)
  (run 1 [q]
       (righto 2 3 [1 2 3]))
  ;;=> (_.0)
  (run 1 [q]
       (righto 1 3 [1 2 3]))
  ;;=> ()

  ;; and for free you have
  (run 1 [q]
       (righto 1 q [1 2 3]))
  ;;=> (2)
  (run 1 [q]
       (righto 2 3 [1 2 q]))
  ;;=> (3)
  )