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[bug#36875] doc: Document the use of `program-file' for mcron jobs.

Message ID 875znhbugu.fsf@gmail.com
State Accepted
Headers show
Series [bug#36875] doc: Document the use of `program-file' for mcron jobs. | expand

Commit Message

Maxim Cournoyer Aug. 1, 2019, 12:27 a.m. UTC 
Hello!

This follows up to my second post under the thread at
(https://lists.gnu.org/archive/html/help-guix/2019-07/msg00180.html).

It aims to shed some light on (more) complex mcron job Guile scripting
with the aid of Guix features (such as program-file).
Thanks,

Maxim

Comments

Ricardo Wurmus July 31, 2019, 7:39 p.m. UTC | #1 
Hi Maxim,

> This follows up to my second post under the thread at
> (https://lists.gnu.org/archive/html/help-guix/2019-07/msg00180.html).
>
> It aims to shed some light on (more) complex mcron job Guile scripting
> with the aid of Guix features (such as program-file).

I wonder if this can be worked around some other way, e.g. by adding
srfi-26 to the “with-imported-modules” clause.

I don’t fully understand the problem as described in the addition to the
manual.  What does “imported syntax definitions wouldn't work correctly”
mean?

It would be useful to state that this is something to do with srfi-26.

About the patch:

- please replace tabs with spaces.

- “Beep the system when the battery reaches %MIN-LEVEL or less battery
  percent.” sounds odd.  How about “Beep when the battery percentage
  falls below %MIN-LEVEL.”?

- Can the example be simplified further?  Is (setenv "LC_ALL" "C")
  really needed here?

- instead of let* and when I’d probably use and-let*.

--
Ricardo
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Patch

From 4d2c5929f056e547b6bd138f69bd1e09e7cfc89f Mon Sep 17 00:00:00 2001
From: Maxim Cournoyer <maxim.cournoyer@gmail.com>
Date: Thu, 1 Aug 2019 07:34:17 +0900
Subject: [PATCH] doc: Document the use of `program-file' for mcron jobs.

* doc/guix.texi (Scheduled Job Execution): Explain why using `program-file'
for an mcron job can be necessary.  Add an example.
---
 doc/guix.texi | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/doc/guix.texi b/doc/guix.texi
index e6047a4909..418dbce16b 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -12444,6 +12444,41 @@  gexps to introduce job definitions that are passed to mcron
                   %base-services)))
 @end lisp
 
+For more complex jobs defined in Scheme, it is safer to pass the job as a
+script to mcron; otherwise, imported syntax definitions wouldn't work
+correctly, as these must be strictly imported at the top level of a Guile
+module.  This can be achieved using the @code{program-file} procedure from the
+@code{(guix gexp)} module, as shown in the example below.
+
+@lisp
+(define %battery-alert-job
+  ;; Beep the system when the battery reaches %MIN-LEVEL or less
+  ;; battery percent.
+  #~(job
+     '(next-minute (range 0 60 1))
+     #$(program-file
+	"battery-alert.scm"
+	(with-imported-modules (source-module-closure
+				'((guix build utils)))
+	  #~(begin
+	      (define %min-level 20)
+	      (use-modules (guix build utils)
+			   (ice-9 popen)
+			   (ice-9 regex)
+			   (ice-9 textual-ports)
+			   (srfi srfi-26))
+	      (setenv "LC_ALL" "C")
+	      (let* ((input-pipe (open-pipe* OPEN_READ
+					     #$(file-append acpi "/bin/acpi")))
+		     (output (get-string-all input-pipe))
+		     (m (string-match "Discharging, ([0-9]+)%" output))
+		     (level (and=> m (compose string->number
+					      (cut match:substring <> 1)))))
+		(when (and=> level (cut <= <> %min-level))
+		  (format #t "warning: Battery level is low (~a%)~%" level)
+		  (invoke #$(file-append beep "/bin/beep") "-r5"))))))))
+@end lisp
+
 @xref{Guile Syntax, mcron job specifications,, mcron, GNU@tie{}mcron},
 for more information on mcron job specifications.  Below is the
 reference of the mcron service.
-- 
2.21.0