;; -*- mode: Lisp; -*-

;; This cannot be let-bound within `job'.
(define currency-file (string-append (getenv "HOME") "/.cache/currency.units"))

(job
 (lambda (current-time)
   (let* ((seconds-in-a-day (* 60 60 24))
          (currency-time (stat:mtime (stat currency-file))))
     (if (< currency-time (- current-time seconds-in-a-day))
         ;; Use next-minute to avoid overwhelming the system in case of failure.
         (next-minute)
         (next-hour-from (next-day) (list (tm:hour (localtime currency-time)))))))
 ;; A string is nicer than Scheme code for `mcron --schedule' output.
 ;; Otherwise we could return '(system* "units_cur" currency-file)
 ;; and use job's 3rd argument as a description.
 (string-append "units_cur " currency-file))

(job '(next-hour (range 0 24 3)) "updatedb-local")