Hi,

I had similar problem popping up periodically.
So here are my 10 cents..

Théo Maxime Tyburn <theo.tyburn@gmail.com> writes:

> Hi guix!
>

[...]

>
> --BEGIN USE_CASE
> For example to add jackd to my system I need to add the 
> "realtime"
> group, add some users to this group and add a 
> pam-limits-service. If I
> want to remove this functionality from my system using the 
> declarative
> approach I have to look down my config file for places where I 
> added
> these things. Usually I partially solve this problem by putting 
> comments
> to signal the purpose of each code block in the system 
> declaration.
>
> But wouldn’t it be better if I just had a function `add-jackd` 
> that takes an
> operating-system instance and returns the os with the extra 
> functionalities ?
> --END USE_CASE

To clarify, do you ask that in the end of the day, some where
in (gnu services ...) there should be exported `add-jackd`
function? If so, I beleive that this will increase cross
dependency between things, thus decreasing flexibility.

Imagine `add-jackd` maintainer should always keep track on
what is being added into guix, that potentially may cause
conflict with jackd and/or require adjustments in `add-jackd`
function implementation.

Also, IMHO, implementation of `add-jackd` would be very
much opinionated.

>
> So that was the purpose of the experimentation. It didn’t turn 
> out to be
> too complicated to implement. At least for my use case, I just 
> needed to add two helper
> functions to extend users and services fields. The rest is 
> handled directly by
> record inheritance and by accessing the fields of the input 
> operating-system.
>
> The final declaration looks like this:
>
> ((apply compose (reverse os-functions)) minimal-os)
>

[...]

>
> (define* (extend-operating-system-services os services #:key 
> (drop '()) (keep '()))
>   (append (filter (lambda (service)
> 					(not (member (service-type-name (service-kind 
> service))
> 								 (filter (lambda (s) (not (member 
> s keep)))
> 										 (append drop 
> %fixed-system-service-types)))))
> 				  (operating-system-services os))
> 		  services))
>

I suppose this could be useful to have it in guix toolbox,
although I would prefer to have (required '(account activate ...))
or (required %fixed-system-service-types) optional argument,
instead of refering to global constant 
%fixed-system-service-types,
which might not satisfy everyone requirements.

> and also force keeping or dropping of some services if needed. 
> The list
> of services that gets duplicated seems to be this one:
>
> (define %fixed-system-service-types
>   '(account activate boot cleanup etc file-systems firmware 
>   fstab guix host-name linux-bare-metal linux-builder pam 
>   profile root-file-system session-environment setuid-program 
>   shepherd-root system user-processes))
>
> I generated the list by just checking which services get 
> duplicated, so I am not
> very sure about it. There surely is a better way to get it.
>
> Anyway I can now define a function adding desktop 
> functionalities:
>
> (define (x-os os)
>   (operating-system
>    (inherit os)
>    (services
> 	(extend-operating-system-services
> 	 os
> 	 (list
> 	  ;; slim display manager
> 	  (service slim-service-type
> 			   (slim-configuration
> 				(display ":0")
> 				(vt "vt7")
> 				(theme %default-slim-theme)
> 				(theme-name %default-slim-theme-name)
> 				(xorg-configuration
> 				 (xorg-configuration
> 				  (keyboard-layout 
> (operating-system-keyboard-layout os)))))))
> 	 #:drop '(gdm)))
>    (packages (cons*
> 			  ;; window managers
> 			  i3-wm python-py3status
> 			  emacs-nc-exwm-xdg
> 			  (operating-system-packages os)
> 			  ))))
>
> Of course there is room for some macros to make this more 
> elegant, but
> this is the rough idea.
>
> In a way it feels like treating the operating-system like a 
> service
> you can extend. Maybe it would even make sense to implement this 
> as a
> service ? Not sure about that.
>
> It seems it would also be reasonable to have something like an
> operating-system-configuration record and a way to compose some 
> before
> putting them into an operating-system record (it seems to be the
> approach rde `features` are based on). But I felt too lazy to 
> copy all the
> fields from the operating-system record definition. There might 
> be a
> way to get all the fields programatically and define a
> record/configuration though.
>
> Anyway, what do you think about this functionality? Have you 
> already experimented with similar things?
> Did I reinvent the wheel? Is there a better approach?

Did you try using (modify-services ...)? Basically, you can
achieve similar goal within services list only with it.

> Anyway that was a very fun hacking session :)
>
> Happy Hacking!
>
> Théo

Thanks in advance,
muradm