(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 (number-unit suffix)
  (sequence* ((number digits)
              (_ (is suffix)))
    (result number)))

(define timestamp-hours
  (number-unit #\h))

(define timestamp-minutes
  (number-unit #\m))

(define timestamp-seconds
  (number-unit #\s))

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

(define (timestamp-to-seconds timestamp)
  (if timestamp
      (+ (* (car timestamp) 60 60)
         (* (cadr timestamp) 60)
         (list-ref timestamp 2))
      0))

(timestamp-to-seconds (parse-timestamp "1y")) ;=> 0
(timestamp-to-seconds (parse-timestamp "1s")) ;=> 1
(timestamp-to-seconds (parse-timestamp "1m2s")) ;=> 62
(timestamp-to-seconds (parse-timestamp "1h2m3s")) ;=> 3723