(defstruct Cursor
  [src [u8]      ; a non-owning slice
   pos i32])     ; no initialiser means zeroed

(defn peek [c (Ptr Cursor)] u8
  (if (< (.pos c) (len (.src c)))
    (at (.src c) (.pos c))
    0))

(defn advance [c (Ptr Cursor)] ()
  (set (.pos c) (+ (.pos c) 1)))   ; field access derefs one level

(defn main [] ()
  (let [c (Cursor {.src (bytes "hi")})]   ; pos omitted, so pos is 0
    (print (peek (addr c))) (println "")
    (advance (addr c))
    (print (peek (addr c))) (println "")))