Hi Guix,

as I tried to add a custom udev rule I noticed that I could only really
do this when the udev-configuration accessor functions are exported.

In my system configuration I first created the new udev rule (that’s all
a bit verbose, unfortunately), which is almost identical to the
‘kvm-udev-rule’:

~~~~~~~~~~~~~~~~
(define (avrispmkii-udev-rule)
  "Return a directory with a udev rule that sets the group owner and
group write permissions on the node matching the AVRISPmkII
microcontroller programming device."
  (computed-file "avrispmkii-rules"
                 #~(begin
                     (use-modules (guix build utils))

                     (define rules.d
                       (string-append #$output "/lib/udev/rules.d"))

                     (mkdir-p rules.d)
                     (call-with-output-file
                         (string-append rules.d "/90-avrispmkii.rules")
                       (lambda (port)
                         (display "\
SUBSYSTEM!=\"usb\", ACTION!=\"add\", GOTO=\"avrisp_end\"

# Atmel Corp. JTAG ICE mkII
ATTR{idVendor}==\"03eb\", ATTR{idProduct}==\"2103\", MODE=\"660\", GROUP=\"dialout\"
# Atmel Corp. AVRISP mkII
ATTR{idVendor}==\"03eb\", ATTR{idProduct}==\"2104\", MODE=\"660\", GROUP=\"dialout\"
# Atmel Corp. Dragon
ATTR{idVendor}==\"03eb\", ATTR{idProduct}==\"2107\", MODE=\"660\", GROUP=\"dialout\"

LABEL=\"avrisp_end\"\n" port))))
                 #:modules '((guix build utils))))
~~~~~~~~~~~~~~~~

Then I replaced %desktop-services to amend the service of
‘udev-service-type’:

~~~~~~~~~~~~~~~~
(modify-services %desktop-services
  (udev-service-type config =>
                     (udev-configuration
                      (inherit config)
                      (rules (append (udev-configuration-rules config)
                                     (list (avrispmkii-udev-rule)))))))
~~~~~~~~~~~~~~~~

As you can see I need ‘udev-configuration’ to create a new
configuration, and ‘udev-configuration-rules’ to append my new rule to
the existing rules.

The attached patch just exports ‘udev-configuration’,
‘udev-configuration?’ (why not?), and ‘udev-configuration-rules’.

~~ Ricardo