clojure index of element matching predicate

index-of.clj

(defn index-of
  [pred coll]
  (first (keep-indexed #(when (pred %2) %1) coll)))

; or

(defn indices
  [pred coll]
  (keep-indexed #(when (pred %2) %1) coll))

(def index-of (comp first indices))
(def last-index-of (comp last indices))

; or

(defn index-of
  [pred coll]
  (loop [coll coll idx 0]
    (when (seq coll)
      (if (pred (first coll))
        idx
        (recur (rest coll) (inc idx))))))

(defn last-index-of
  [pred coll]
  (loop [coll coll idx 0 e nil]
    (if (seq coll)
      (recur (rest coll) 
             (inc idx)
             (if (pred (first coll)) idx e))
      e)))