err.clj

(ns yeller.infrastructure.err)

(deftype Failure [f])

(defn unwrap-fail [^Failure fail]
  (.f fail))

(defn fail? [potential-fail]
  (isa? (class potential-fail) Failure))

(defmacro err->>
  "Threads like ->>, but if
  an expr returns a Failure,
  run the function embedded in
  the failure instead of continuing"
  ([x form]
   (if (seq? form)
     (with-meta `(~(first form) ~@(next form)  ~x) (meta form))
     (list form x)))

  ([x form & more]
   `(let [result# (->> ~x ~form)]
      (if (fail? result#)
        (unwrap-fail result#)
        (err->> result# ~@more)))))

(defmacro fail [& body]
  `(do
     ~@(butlast body)
     (Failure.
       ~(last body))))