pkgs-lib.formats.xml: init

[Stunkymonkey]

Description of changes

add XML export format. beneficial for https://github.com/NixOS/rfcs/blob/master/rfcs/0042-config-option.md

tested via: nix-build . -A tests.pkgs-lib.formats

sonarr can be improved to have settings e.g.: master...Stunkymonkey:nixpkgs:exportarr-sonarr-test. which can be tested via nix-build . -A nixosTests.prometheus-exporters.exportarr-sonarr

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[Stunkymonkey]

to be honest: i just copied and pasted this from the json... not sure if this is a problem

[Stunkymonkey]

not sure where to note the badgerfish-notation.

[h7x4]

I'm wondering whether something could/should be done to ensure we can order attributes properly and still have control of the parent node? With the current implementation, it seems like you can either ensure the order of the child attributes by choosing to use a list, or you can use unordered attributes and be allowed to set "@ asdf" (sorry, accidentally pinged this user...) and "#asdf" properties.

[roberth]

Better XML support seems very useful. Thank you ❤️

[roberth]

Is this format suitable for markup-like XML, such as XHTML? Based on the examples here and there, that seems to be an unsolved problem.

I doubt that this is the only and best Nix representation of XML, so I'd prefer a more specific name, like xmltodict.

[Stunkymonkey]

it is the other way around: attrsToXml right?

maybe we include the badgerfish notation in the name? like badgerfishToXml?

[roberth]

Only referred to xmltodict as the project name, assuming they didn't name the input format. So they implement the badgerfish format? Then that's great!

I guess we could group representations of XML under an xml attrset, so that it's formats.xml.badgerfish? That way it's more easily discoverable in documentation and the repl.

[roberth]

Or formats.xml.fromBadgerfish.
formats.badgerFishToXml also works. We don't have nesting yet, actually.

Maybe @infinisil has an opinion, as he's started this library.
Otherwise, current name will do.

[Stunkymonkey]

I am open for suggestions. formats.xml.fromBadgerfish sounds good. but lets wait for feedback from others.

[ambroisie]

What about formats.xml { format = "badgerfish" } (probably the default value)? This seems to be the intent with having the attrset parameter.

[Stunkymonkey]

implemented the idea from @ambroisie

[Stunkymonkey]

ping @infinisil

[Stunkymonkey]

i someone is looking for different kinds of formats here is a good list: https://wiki.open311.org/JSON_and_XML_Conversion/

But when searching for implementations on GitHub; xmltodict is the most actively maintained and famous one.

So IMHO it make sense to have this as default, because it is the only thing that has "prevailed"

[roberth]

How would namespaces look?

Does definition merging work as expected?

[Stunkymonkey]

it follows: https://www.xml.com/pub/a/2006/05/31/converting-between-xml-and-json.html:

doesn't take into consideration the following:

• XML declaration
• processing instructions
• explicit handling of namespace declarations
• XML comments

i found: https://github.com/martinblech/xmltodict/blob/master/README.md#namespace-support

[Stunkymonkey]

namespaces are "mainly problematic" for parsing the other way around. it can be achieved via:

{
  "RSS": {
    "@xmlns:jwplayer": "http://support.jwplayer.com/customer/portal/articles/1403635-media-format-reference#feeds";
    "@version": "2.0";
    "Channel": {
...

to create:

<?xml version="1.0" encoding="utf-16"?>
<RSS xmlns:jwplayer="http://support.jwplayer.com/customer/portal/articles/1403635-media-format-reference#feeds" version="2.0">
  <Channel>

[roberth]

Ok, makes sense.

Handling foreign input while programming with hardcoded prefixes is basically wrong; an XML document should behave exactly the same under prefix renaming, but a lot of software requires certain prefixes, which is - in principle - wrong.
However, we will have to cater to such applications, and users who are kinda ok with such requirements, so that's just that.

Treating any node as URI + tag instead of prefix + tag makes any XML processing more robust (for an option type, the processing means implementing a merge operation for the XML representation), so that may be worth considering, but it seems that we'd end up writing a small library for that purpose; a bit of a research project.

So for now I think we only need to point out in the docs that the type deals with the concrete syntax of the XML, and all responsibility is on the user when it comes to the correctness of any use of namespace features.

[Stunkymonkey]

since this will not be the "perfect/best" xml-exporter for nix, I would suggest to get started, but name it "specially" (not xml as we both agreed above.).

Surely there are a lot of things to improve, but in the end we should enable other users to enable new NixOS-options. Migrating this to other formats seems tedious (manual work), but doable.

Documentation is currently missing completely, because I do not know where to put it. Any suggestions where to put it?

[Stunkymonkey]

ping @infinisil ? Is this not wanted?

[infinisil]

This seems like a fine addition, I'm just very behind on things to review and on holidays (well at least since last week, not since the PR was opened 😅) :)

After taking a look:

• The logic in the two functions should be deduplicated
• There should be a docs entry for the new functions in the NixOS manual (there's a sections on formats)

[Stunkymonkey]

@infinisil thanks for the feedback.

the two functions were placed to give a choice which one is preferred.

Additionally I fixed the merge-conflicts.