(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
  (followed-by digits (is #\h)))

(define timestamp-minutes
  (followed-by digits (is #\m)))

(define timestamp-seconds
  (followed-by digits (is #\s)))

(define (maybe* parser)
  (any-of parser item))

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

;; expected: (0 0 1)
;; actual: (0 0 1)
(parse timestamp "1s")

;; expected: (0 1 2)
;; actual: #f
(parse timestamp "1m2s")

;; expected: (1 2 3)
;; actual: #f
(parse timestamp "1h2m3s")