Split on pred implemented by reducing with a state machine

split.clj

(defn split-after
  [pred coll]
  (let [post-accum (fn [accum x]
                     (update accum :after conj x))
        pre-accum (fn [accum x]
                    (cond-> (update accum :before conj x)
                      (pred x) (assoc :f post-accum)))]
    ((juxt :before :after)
     (reduce (fn [{:keys [f] :as accum} x] (f accum x))
             {:before [] :after [] :f pre-accum}
             coll))))

(defn split-before
  [pred coll]
  (let [post-accum (fn [accum x]
                     (update accum :after conj x))
        pre-accum (fn [accum x]
                    (if (pred x)
                      (-> (update accum :after conj x)
                          (assoc :f post-accum))
                      (update accum :before conj x)))]
    ((juxt :before :after)
     (reduce (fn [{:keys [f] :as accum} x] (f accum x))
             {:before [] :after [] :f pre-accum}
             coll))))