Last exercise of the Read and Eval Chapter of Brave Clojre

infix.clj

(ns infix)

(defn index-of
  "Clojure doesn't have an index-of function. The Java .indexOf method
  works reliably for vectors and strings, but not for lists. This solution
  works for all three.
  https://stackoverflow.com/a/37858115/344501
  "
  [item coll]
  (let [v (if
            (or (vector? coll) (string? coll))
            coll
            (apply vector coll))]
    (.indexOf coll item)))

(defn in?
  "true if coll contains elm"
  [coll elm]
  (some #(= elm %) coll))

(def precedences [['* '/] ['+ '-]])

(def operators (flatten precedences))

(defn extract-operator [l]
  (filter #(in? operators %) l))

(defn operator-grade [op]
  (first (keep-indexed #(when (in? %2 op) [%1 op]) precedences)))

(defn max-op-of [l]
  (last (apply max-key first (map operator-grade (extract-operator l)))))

(defn left-right-of [op l]
  (let [op-pos (index-of op l)
        [ll rl] (split-at op-pos l)]
    [ll (rest rl)]))

(defn infix [l]
  (if (= (count l) 1)
    (first l)
    (let
        [max-op (max-op-of l)
          [left-l right-l] (left-right-of max-op l)]
      (list max-op (infix left-l) (infix right-l)))))