(defn my-count [coll]
  (reduce (fn [n x] (inc n)) 0 coll))

(my-count [1 2 3 3]) ;; produces 4

(defn my-reverse [coll]
  (reduce (fn [xs x] (into [x] xs)) [] coll))

(my-reverse [1 2 3]) ;; => [3 2 1]

(defn my-map [f coll]
  (reduce (fn [xs x] (conj xs (f x))) [] coll))

(my-map inc [1 2 3]) ;; => [2 3 4]

(defn my-filter [pred coll]
  (reduce (fn [xs x] (if (pred x)
                       (conj xs x)
                       xs))
          []
          coll))

(my-filter even? [1 2 3]) ;; => [2]