Proposal: auto-instantiate services with a default value

[ludo@gnu.org (Ludovic Courtès)]

Hello Guix!

Currently on GuixSD, users have to explicitly specify all the services
they need, recursively.  For instance, if you add:

  (service rottlog-service-type)

to your ‘services’ field, you also need to add:

  (service mcron-service-type)

because the former happens to require the latter.  If you don’t do that,
you get an error message (see ‘fold-services’ in (gnu services)), and
then you go back to your config and add that line.

This was intentional, but it’s annoying, especially when considering
higher-level, complex services—imagine a MediaGoblin service: you
wouldn’t want to manually specify a dozen services that, to you, are an
implementation detail you’d rather ignore.

We discussed some time ago the possibility to automatically instantiate
services.  Now that service types have default value, I think it’s a
viable approach: if a service is required, and *if* it provides a
default value, then we can automatically instantiate it.

So in the example above, you’d just specify ‘rottlog-service-type’ and
an ‘mcron-service-type’ would be automatically added if it’s not already
there.

How does that sound?

Ludo’.