;;;; What a handler for a parent reads as the message, and that it is still
;;;; there after the unwind. The clause below runs after the signalling frames
;;;; are gone, and [scribble] reuses their stack before the message is
;;;; printed, so a message left pointing into them prints as garbage.

(defstruct IoError :parent Error)
(defstruct Empty :parent Error [])
(defstruct MyErr :parent Error [code i32  why str])

(defonce zero i64)

;; Deep enough, and wide enough, to overwrite what the signal left below.
(defn scribble [n i32] i64
  (let [junk (array-fill [64] (i64 n))]
    (if (= n 0) (at junk 3) (+ (at junk 5) (scribble (- n 1))))))

(defn caught [thunk (Fn [] i32)] i32
  (handler-case (thunk)
    [(Error [e]
       (scribble 40)
       (println (.name e))
       (println (.message e))
       -1)]))

(defn main [] i32
  ;; A category the program filled in is its own name and message.
  (caught (fn [] (do (error (IoError {.name "io" .message "disk gone"})) 0)))
  ;; An empty field vector is the same category.
  (caught (fn [] (do (error (Empty {.name "empty" .message "nothing"})) 0)))
  ;; A program's own condition is printed with its values.
  (caught (fn [] (do (error (MyErr {.code 3 .why "bad"})) 0)))
  ;; A runtime condition carries the sentence with its values.
  (caught (fn [] (i32 (/ 7 zero))))
  0)