I want to update whenever the file is older than 1 day (so that it does
not do anything on reboot for instance) and every 1 day
from then on (so that it keeps updating even if I don't reboot).

For that I need mcron with some condition on the mtime of
currency.units.

Here is my implementation:

	;; -*- 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))
	         (next-second)
	       ;; TODO: The following does not work while it should.  Report upstream.
	       ;; (next-hour-from (next-day) (list (tm:hour (localtime (current-time)))))
	       (+ (next-second) (- 86400 (- current-time currency-time))))))
	 ;; A string is nicer than Scheme code for `mcron --schedule' output.
	 ;; Other we could return '(system* "units_cur" currency-file)
	 (string-append "units_cur " currency-file))

I start ~mcron~ from my ~.profile~:

	mcron &

Comments are welcome :)


Note: As mentioned in the "TODO:" above, I think there is a bug in
mcron.  The following example taken from the Guix manual does not seem
to work:

     (define idutils-job
       ;; Update the index database as user "charlie" at 12:15PM
       ;; and 19:15PM.  This runs from the user's home directory.
       #~(job '(next-minute-from (next-hour '(12 19)) '(15))
              (string-append #$idutils "/bin/mkid src")
              #:user "charlie"))

Or, at the user level:

	(job '(next-minute-from (next-hour '(12 19)) '(15))
              (string-append (getenv "HOME") "/.guix-profile/bin/mkid src"))

	> mcron -s 1
	mcron: Cannot read files in your ~/.config/cron (or ~/.cron) directory.

More specifically, it seems that mcron fails to take a list as second
argument to the ~next-*~ functions.

Can anyone confirm?

--
Pierre Neidhardt