Hi!
Carlo Zancanaro <ca...@zancanaro.id.au> skribis:
> I have a question related to this, but about a broader issue.
>
> On Sat, Apr 15 2017, Ludovic Courtès wrote:
>> (service openssh-service-type)
>
> Is this supposed to be the preferred way to add a service to your
> operating system? Or, put another way, as a service writer: should I
> consider a *-service-type to be sufficient to complete the work of
> creating a service?
Yes.
> Looking at various services that we have defined at the moment
> (according to what is documented), most services have a procedure (ie.
> *-service), and maybe also a *-service-type. For example, in "Database
> Services" we have postgresql-service and mysql-service, with no
> corresponding *-service-type. But then we have redis-service-type
> without a *-service to go with it.
>
> I wonder if we would be better off creating *-service procedures for
> each *-service-type and documenting them as the canonical way to create
> services. That way we could handle things like default arguments through
> the usual scheme mechanisms, rather than our own special case.
It used to be that we would write ‘-service’ procedures. Then when (gnu
services) was added, each of these procedures would boil down to:
(define (foo-service #:key (foo foo) (number 42))
(service foo-service-type
(foo-configuration (foo foo) (number number))))
There are two issues here:
1. The default values here are redundant with those we also specify in
<foo-configuration>.
2. The ‘-service’ procedures are a bit opaque. For things like
‘modify-services’, we want to expose the fact that we have service
objects with an associated <foo-configuration> value, rather than
hide it inside a procedure.
For these reasons, I’ve been progressively suggesting that we avoid
‘-service’ procedures altogether, and deprecate the existing ones.
There are still many of them though, as you write; we should remove
them (patches welcome! easy task for a GuixSD newcomer! :-)).
The default value thing in this thread is about making the ‘service’
form less verbose and closer to what we had with ‘-service’ procedures.
Clearly there’s a parallel between procedures with keyword parameters
and services/configurations records with default field values. The key
here is that we want first-class objects that we can inspect and derive
from when customizing a system.
Does that clarify things?
Thanks,
Ludo’.
