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Idea: Function composition to declare operating-system
From: |
Théo Maxime Tyburn |
Subject: |
Idea: Function composition to declare operating-system |
Date: |
Mon, 29 Aug 2022 17:14:11 +0200 |
User-agent: |
mu4e 1.8.7; emacs 28.1.90 |
Hi guix!
I experimented on a functional approach to the operating
system declaration. My goal was to be able to pass an operating-system
instance to a composition of functions, modifying it one after another
and returning the desired operating-system object. I find this approach
more convenient because it allows for better segmentation of the system
features. I achieved to make it work for the system declaration I
usually use and I’d like to share it with you.
--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
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:
--8<---------------cut here---------------start------------->8---
((apply compose (reverse os-functions)) minimal-os)
--8<---------------cut here---------------end--------------->8---
Where `minimal-os` is roughly an (operation-system) declaration with at
least all the required fields. `os-functions` is a list of procedures
taking an operating-system as input and returning an operating-system.
An example for one such function that adds a label:
--8<---------------cut here---------------start------------->8---
(define (label-os os)
(operating-system
(inherit os)
(label (string-append (operating-system-host-name os) ":"
"refactor-functional-os" ;; my
custom tag
" "
(operating-system-default-label this-operating-system)))))
--8<---------------cut here---------------end--------------->8---
The most problematic thing is handling services. For some reason I
ignore, there is a set of services that always gets added
again to the list of services when you call (operating-system ...) with a
services field with more than one service. So you can’t just do like
above and
--8<---------------cut here---------------start------------->8---
(define (add-some-service os)
(operating-system
(inherit os)
(services (cons (simple-service ...)
(operating-system-services os)))))
--8<---------------cut here---------------end--------------->8---
because some services that need to be unique would come more than once
in the final operating-system object.
So I just added a simple function to filter them out
--8<---------------cut here---------------start------------->8---
(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))
--8<---------------cut here---------------end--------------->8---
and also force keeping or dropping of some services if needed. The list
of services that gets duplicated seems to be this one:
--8<---------------cut here---------------start------------->8---
(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))
--8<---------------cut here---------------end--------------->8---
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:
--8<---------------cut here---------------start------------->8---
(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)
))))
--8<---------------cut here---------------end--------------->8---
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?
Anyway that was a very fun hacking session :)
Happy Hacking!
Théo
- Idea: Function composition to declare operating-system,
Théo Maxime Tyburn <=