dict_update   xtlang

(bind-func dict_update:[!v,List{Pair{String*,!v}*}*,String*,[!v,!v]*]*
  "set (or update) value for key in dict according to update_fn (returns the old value)"
  (lambda (dict key update_fn)
    (if (null? dict)
        (convert null)
        (if (equal (first (car dict)) key)
            (let ((oldval (second (car dict)))
                  (newval (update_fn oldval)))
              ;; if update_fn returns null, do nothing. otherwise
              ;; update the value for key
              (tset! (car dict) 1 newval)
              oldval)
            (if (null? (cdr dict))
                (begin (append_destructive dict (list (Pair key (update_fn (convert null)))))
                       (convert null))
                (dict_update (cdr dict) key update_fn))))))