gapmap function for Clojure

gapmap.clj

(defn gapmap
  "Returns a lazy sequence consisting of the result of applying f to all
  adjacent values of coll. f should be a function of 2 arguments. If coll
  contains less than 2 items, it is returned and f is not called."
  {:added "1.6"}
  ([f coll]
   (if-let [r (next coll)] ; at least two items?
     (lazy-seq (gapmap-internal f (first coll) r))
     coll)))

(defn- gapmap-internal
  [f x coll]
  (if-let [y (first coll)]
    (cons (f x y) (gapmap-internal f y (next coll)))))

;;------------------------------------------------------------------------------
;; Tests

(use 'clojure.test)

(deftest gapmap-tests
  (is (= (gapmap + []) []) "Ignore empty lists")
  (is (= (gapmap + [0]) [0]) "Ignore lists with only one item")
  (is (= (gapmap + [0 0]) [0]))
  (is (= (gapmap + [0 1 0]) [1 1]))
  (is (= (gapmap + [0 1 1 0]) [1 2 1]))
  (is (= (gapmap + [0 1 2 1 0]) [1 3 3 1]))
  (is (false? (realized? (gapmap + (range))))) "Assert lazy sequence")

(gapmap-tests)

;;------------------------------------------------------------------------------
;; Example

=> (take 5 (iterate #(gapmap + (concat [0] % [0])) [1]))
((1) (1 1) (1 2 1) (1 3 3 1) (1 4 6 4 1))