Parsing with Traversy

parse.clj

(defn integer [s] (java.lang.Integer/parseInt s))
(defn parse [from-str]
  (lens (fn [x] (-> x from-str list))
        (fn [f x] (-> x from-str f str))))

(fact "The 'parse' lens focuses on a value in a string."
      (-> "2015" (view (parse integer))) => [2015]
      (-> "2015" (update (parse integer) inc)) => "2016")

(defn catching [thunk x] (try (thunk) (catch Exception e x)))
(defn safely [l]
  (lens (fn [x] (catching #(view x l) []))
        (fn [f x] (catching #(update x l f) x)))) 

(fact "The 'safely' lens tolerates exceptions."
      (-> "foo" (view (safely (parse integer)))) => []
      (-> "foo" (update (safely (parse integer)) inc)) => "foo"
      (-> "2015" (view (safely (parse integer)))) => [2015]
      (-> "2015" (update (safely (parse integer)) inc)) => "2016")