Re: Code sharing between system and home services (was Re: On the naming of System and Home services modules.)

[Maxime Devos <maximedevos@telenet.be>]

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Xinglu Chen schreef op vr 24-09-2021 om 15:35 [+0200]:
> On Thu, Sep 23 2021, Ludovic Courtès wrote:
> 
> > Hi,
> > 
> > Xinglu Chen <public@yoctocell.xyz> skribis:
> > 
> > > Some services might be useful to have in both Guix System and Guix Home;
> > > for instance, Guix System currently has a service for configuring
> > > Syncthing, and I think it makes sense to also have one for Guix Home,
> > > this would mean that people not using Guix System (me :-)) could also
> > > have Guix manage Syncthing.  With the current approach, we would have to
> > > copy and paste quite a bit of code, and if the Syncthing service for
> > > Guix System changes, then the one for Guix Home might have to change as
> > > well.
> > 
> > Silly question, but why do we need to have two different configuration
> > record types in the first place?
> 
> The problem is that the configuration records for system and home
> service don’t necessarily have the same fields.  The Syncthing service
> for Guix System has a ‘user’ and a ‘group’ field, which is not really of
> any use in Guix Home, as the only user would be the user invoking ‘guix
> home’.
> 
> > Sharing configuration between Home and System sounds important to me: it
> > means users can easily move services from one to the other, which is
> > pretty big deal.  It also means we’d have much less code to maintain.
> 
> Agreed, that’s what I would like to see as well.
> 
> > Would that be feasible?  (Apologies if this has already been
> > discussed!)
> 
> Since it might not make sense to have the same records fields for a
> system service and home service, I proposed (in the mail you replied to)
> a ‘define-configuration’ form that would generate a configuration record
> for a system service and optionally one for a home service, without
> having to maintain two records separately.
> 
> --8<---------------cut here---------------start------------->8---
> (define-configuration syncthing-configuration
>   (package
>    (package syncthing)
>    "Syncthing package to use.")
>   (arguments
>    (list-of-strings ’())
>    "Command line arguments to pass to the Syncthing package.")
>   (log-flags
>    (integer 0)
>    "Sum of logging flags.")
>   (user
>    (maybe-string 'disabled)
>    "The user as which the Syncthing service is to be run."
>    (home-service? #f))  ; not for Guix Home
>   (group
>    (string "users")
>    "The group as which the Syncthing service is to be run."
>    (home-service? #f))  ; likewise ^^
>   (home
>    (maybe-string 'disabled)
>    "Common configuration and data directory.")
>   (home-service? #t))
> --8<---------------cut here---------------end--------------->8---
> 
> It would generate <syncthing-configuration> and
> <home-syncthing-configuration>.  The only difference being that
> <home-syncthing-configuration> doesn’t have a ‘user’ and a ‘group’
> field.

The 'parent' mechanism (rnrs records syntactic) 'parent' could be used
here (after adapting it to define-configuration), to define three record types:

The record type with all fields common to the home configuration and system configuration
(<common-syncthing-configuration> + common-syncthing-configuration?)
and the record types for the home and system configuration
(<syncthing-configuration> + syncthing-configuration? and <home-syncthing-configuration>
+ home-syncthing-configuration?).

Using this mechanism, all syncthing-configuration? and home-syncthing-configuration?
are common-syncthing-configuration?.

Greetings,
Maxime.

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