RWExceptionT example

gistfile1.clj

  (defn stuff
    [base]
    (m/mlet rwx/rwexception-ctx
      [_ (writer/tell [:base base])

       {offs :config/offset} (reader/ask)
       _ (writer/tell [:config/offset offs])

       tot (m/return (+ base offs))
       _ (writer/tell [:total tot])]

      (m/return tot)))

  (defn handle-stuff
    [& args]
    (m/mlet rwx/rwexception-ctx
      (e/handle
       (apply stuff args)
       (fn [left right]
         (if (some? left)
           (m/mlet rwx/rwexception-ctx
             [_ (writer/tell [:ex :handled])]
             (m/return (ex-message left)))

           (m/mlet rwx/rwexception-ctx
             [_ (writer/tell [:ex :none])]
             (m/return right)))))))

  ;; writer log is preserved through errors,
  ;; and you can log in handle, catch and finally

  (r/run (handle-stuff 100)
    {:monad.reader/env {:config/offset 100}})

  =>

  {:monad.writer/output {:base [100],
                         :config/offset [100],
                         :total [200],
                         :ex [:none]},
   :monad/val 200}

  ;; here's how an error gets caught/handled

  (r/run (handle-stuff 100)
    {:monad.reader/env {:config/offset :foo}})

  {:monad.writer/output {:base [100],
                         :config/offset [:foo],
                         :ex [:handled]},
   :monad/val "class clojure.lang.Keyword cannot be cast to class java.lang.Number (clojure.lang.Keyword is in unnamed module of loader 'app'; java.lang.Number is in module java.base of loader 'bootstrap')"}