(use (only srfi-14 char-set:digit)
     comparse)

(define digit
  (in char-set:digit))

(define (as-number parser)
  (bind (as-string parser)
        (lambda (s)
          (result (string->number s)))))

(define digits
  (as-number (one-or-more digit)))

(define timestamp-hours
  (sequence* ((hours digits)
              (_ (is #\h)))
    (result hours)))

(define timestamp-minutes
  (sequence* ((minutes digits)
              (_ (is #\m)))
    (result minutes)))

(define timestamp-seconds
  (sequence* ((seconds digits)
              (_ (is #\s)))
    (result seconds)))

(define timestamp
  (sequence* ((hours (maybe timestamp-hours))
              (minutes (maybe timestamp-minutes))
              (seconds timestamp-seconds))
    (result (list (or hours 0) (or minutes 0) seconds))))

(define (parse-timestamp input)
  (parse (followed-by timestamp end-of-input) input))

(parse-timestamp "1s") ;=> (0 0 1)
(parse-timestamp "1m2s") ;=> (0 1 2)
(parse-timestamp "1h2m3s") ;=> (1 2 3)