From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ricardo Wurmus <rekado@elephly.net>
Subject: Re: A simple battery level alert mcron job for your Guix, v2
Date: Wed, 31 Jul 2019 17:23:16 +0200
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To: Maxim Cournoyer <maxim.cournoyer@gmail.com>
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Maxim Cournoyer <maxim.cournoyer@gmail.com> writes:

> The following mcron job accomplish this.  I've implemented it in Guile
> Scheme this time around and was puzzled that I had to use program-file
> to make it work, otherwise srfi-26's cut would error on undefined symbol
> '<>'.
>
> The problem was that passing the code as a lambda would have the syntax
> imported not a the top level, which is the only valid place to
> define/import syntax definitions, per the Guile manual.  Using
> `program-file' works around that, by placing the logic at the top level
> of a standalone script, while allowing you to define everything at the
> level of your Guix config file.

Ah, thanks for investigating this.  When you first asked about this I
was really quite puzzled.

> 	 (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")))))))

Perhaps and-let* (SRFI 2) would be of interest here.

-- 
Ricardo