(define (foo txt)
  (flat
    (map
      (fn(s)
        (if (regex {^(-?\d+)-(-?\d+)} s)
          (sequence (int $1) (int $2))
          (int s)))
      (parse (trim txt) ","))))

(foo "-6,-3--1,3-5,7-11,14,15,17-20")

(-6 -3 -2 -1 3 4 5 7 8 9 10 11 14 15 17 18 19 20)


(foo "-6,-3-1,3-5,7-11,14,15,17-20")

(-6 -3 -2 -1 0 1 3 4 5 7 8 9 10 11 14 15 17 18 19 20)