catch.cljs

(ns blog.errors.core
  (:require-macros
    [cljs.core.async.macros :refer [go]]
    [blog.utils.macros :refer [<?]])
  (:require
    [cljs.core.async :refer [>! <! chan close!]]))

;; convert Node.js async function into a something
;; that returns a value or error on a channel
(defn run-task [f & args]
  (let [out (chan)
        cb  (fn [err & results]
              (go (if err
                    (>! out err)
                    (>! out results))
                (close! out)))]
    (apply f (concat args [cb]))
    out))

(def fs (js/require "fs"))

;; BOOM!!! we can convert async errors into exceptions
(go (try
      (let [x (<? (run-task (.-readFile fs) "foo.txt" "utf8"))]
        (.log js/console "Success" x))
      (catch js/Error e
        (.log js/console "Oops" e))))

core.cljs

(ns blog.errors.core
  (:require-macros
    [cljs.core.async.macros :refer [go]])
  (:require
    [cljs.core.async :refer [>! <! chan close!]]))

(defn error? [x]
  (instance? js/Error x))

(defn run-task [f & args]
  (let [out (chan)
         cb  (fn [err & results]
               (go (if err
                     (>! out err)
                     (>! out results))
                 (close! out)))]
    (apply f (concat args [cb]))
    out))

;; wrap a task to run in a thunk so a supervisor can call it
(defn task [& args]
  (fn [] (apply run-task args)))

;; a policy so a supervisor knows to retry a task
(defprotocol IPolicy
  (-retry? [this err attempts]))

;; retry a task up to N times policy
(defn maxtries [n]
  (reify
    IPolicy
    (-retry? [_ err attempts]
      (< attempts n))))

;; take a task to run and a policy for retries
(defn supervisor [task policy]
  (go (loop [attempts 0 err nil]
        (if-not (-retry? policy err attempts)
          err
          (let [v (<! (task))]
            (if (error? v)
              (let [attempts (inc attempts)]
                (do (.log js/console "Attempt" attempts)
                  (recur attempts v)))
              v))))))

(def fs (js/require "fs"))

;; go macro lets us write straight line code even though supervisor
;; implements sophisticated async behavior
(go (let [result (<! (supervisor
                       (task (.-readFile fs) "foo.txt" "utf8")
                       (maxtries 3)))]
      (if-not (error? result)
        (.log js/console (.split (first result) "\n"))
        (.log js/console result))))