Service customization

[Ludovic Courtès]

Hi Guix!

A fairly common pattern in GuixSD is: You want to use whatever is in
‘%desktop-services’, say, but you want to tweak the configuration of
some of the services in there.

This is where the ‘modify-services’ macro comes in, inspired by Taylan’s
‘modify-phases’:

     Occasionally, instead of using the base services as is, you will want
  to customize them.  For instance, to change the configuration of
  ‘guix-daemon’ and Mingetty (the console log-in), you may write the
  following instead of %BASE-SERVICES:

       (modify-services %base-services
         (guix-service-type config =>
                            (guix-configuration
                             (inherit config)
                             (use-substitutes? #f)
                             (extra-options '("--gc-keep-outputs"))))
         (mingetty-service-type config =>
                                (mingetty-configuration
                                 (inherit config)
                                 (motd (plain-file "motd" "Hi there!")))))

  The effect here is to change the options passed to ‘guix-daemon’ when it
  is started, as well as the “message of the day” that appears when
  logging in at the console.  *Note ‘modify-services’: Service Reference,
  for more on that.

Simple but convenient, no?

Ludo’.