reword documentation on nix-path config option

[fricklerhandwerk]

also make the fallback values explicit.

Motivation

this is to clarify behavior. using the positive boolean is easier to read,
as it does not require flipping bits in your head.

Context

Follow-up on #7689 by @ncfavier instead of making a post-factum review.

Update: unfortunately that PR did not actually implement the desired behavior.

Depends on: #7871

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or bug fix: updated release notes

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/tweag-nix-dev-update-44/25546/1

[Ericson2314]

Or we trying to document the current behavior or the intended behavior? Because this to me is not quitethe intended behavior.

The rules I wrote in #7871 (comment) are not intended to be just just for nixPath but for any such configuration variable. The rules around settings files, NIX_CONF, --extra-foo, and --foo are entirely standard and additionally rules for NIX_FOO go for many other such configuration options that also have their own env vars.

Documenting this stuff per a specific option like nixPath implies that it is somehow special. It is today because of the weirdness of "restricted eval" that is in conflict with these general rules. But we should fix "restricted eval"` so as to not break those rules instead. Then we can restore the general pattern and not have special cases.

[fricklerhandwerk]

I agree this should just follow the standard pattern. But since there are four possibilities of changing the search path, I opted for making that explicit here. We don't have to do it that way, and I'm open for concrete suggestions.

[Ericson2314]

@fricklerhandwerk I am thinking of adding to BaseSettings an optEnvVar. And then we can programmatically document this for all of them! :)

(Or maybe it would be sub-class.)

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/tweag-nix-dev-update-45/26397/1

[fricklerhandwerk]

Triaged in the Nix team meeting 2023-03-31:

• this documents the desired behavior of Restore NIX_PATH taking precedence over nix-path #7871
• Should this be blocked on the implementation?
  • @edolstra: Yes. We shouldn't merge this if it isn't the current behaviour
• postponed

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-03-31-nix-team-meeting-minutes-45/27002/1

[dpulls]

🎉 All dependencies have been resolved !