gistfile1.clj

;; want this behavior 
(defn iof
  [val coll]
  (lazy-seq
   (loop [coll coll idx 0]
     (cond
      (empty? coll) '()
      (= val (first coll)) (cons idx (recur val (rest coll) (inc idx)))
      :else (recur val (rest coll) (inc idx))))))

;; works, but kind of ugly
(defn iof-2
  [val coll]
  ((fn f [coll idx]
     (lazy-seq (cond
                (empty? coll) '()
                (= val (first coll)) (cons idx (f (rest coll) (inc idx)))
                :else (f (rest coll) (inc idx))))) coll 0))


;; idiomatic?  (also could do arity overloading, but this is to keep from exposing the private API)
(defn- -indices-of
  [val coll idx]
  (lazy-seq
   (cond
    (empty? coll) '()
    (= val (first coll)) (cons idx (-indices-of val (rest coll) (inc idx)))
    :else (-indices-of val (rest coll) (inc idx)))))

(defn indices-of
  "Returns a lazy-seq of indices where 'val appears in 'coll."
  [val coll] (-indices-of val coll 0))

Ah, interesting. Similar to my iof-2, but with a clarifying let block.

Perhaps the below would be idiomatic? I'm averse to using more seq xs then necessary, if empty? works.

(defn iof-2
  [val coll]
  (letfn [(f [coll idx]
             (lazy-seq (cond
                (empty? coll) '()
                (= val (first coll)) (cons idx (f (rest coll) (inc idx)))
                :else (f (rest coll) (inc idx)))))
    (f col 0)))