remove the Channels section

[fricklerhandwerk]

Motivation

this is a how-to guide which should not be in the reference manual. also it does not contain information that is not in the manual page on nix-channel.

additionally, slightly reword the documentation to be more concise and informative.

Context

another step towards #7769

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Priorities

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[edolstra]

it also refers to nix-env, which is discouraged for package management, because it behaves surprisingly.

nix-env is not discouraged for package management, so that's not a valid reason for removing this information.

[fricklerhandwerk]

I fixed up the wording in the PR description to be more precise. It's of course important to say who's discouraging it and why, and the commit message is a proposal to make that statement authoritatively, and thus make it a valid reason.

@edolstra what's your take on these arguments?

• https://stop-using-nix-env.privatevoid.net/
• https://discourse.nixos.org/t/depreciate-the-use-of-nix-env-to-install-packages/20139

In any case, channels and nix-env -i are non-portable and at some point inevitably confront beginners with surprising behavior, which makes them rework their setups to use Home Manager or NixOS, or give up on Nix. Especially from the Discourse thread I gather that nix-env --set (and rarely nix-env -i) is – apart from beginners who don't know better – only used on systems where proper declarative configs are not possible, and other than that the only reasonable client of that interface are NixOS, HM, and nix-darwin. On top of that, the only reason those even use channels is essentially... as far as I can see no good reason at all, it's probably historical, but still really complicates the entire setup. Here's a very old and lengthy and still on-going discussion which traces the problems involved.

[fricklerhandwerk]

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

• @edolstra: We shouldn't rip out material if there's no alternative
• @fricklerhandwerk: Usage of nix-channel is evident from the man page
  • Also upcoming PR listing all the files that are relevant to Nix would make this much clearer
• @Ericson2314: We don't want to remove things that don't have a non-experimental alternative, but a lot of people use the (stable) built-in fetchers instead of channels
• to discuss

[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

[roberth]

Controversy doesn't seem resolved.

• remove the Channels section #7973 (comment)

Maybe separate out the deletion so that we can merge the other improvements first?

Alternatively, or later, a page on nix.dev could resolve this.

[fricklerhandwerk]

Suggested change

	> **Note**
	>
	> The state of a subscribed channel is external to the Nix expressions relying on it.
	> This may limit reproducibility.
	>
	> Dependencies on other Nix expressions can be declared explicitly with:
	> - [`fetchurl`](@docroot@/language/builtins.md#builtins-fetchurl), [`fetchTarball`](@docroot@/language/builtins.md#builtins-fetchTarball), or [`fetchGit`](@docroot@/language/builtins.md#builtins-fetchGit) in Nix expressions
	> - the [`-I` option](@docroot@/command-ref/opt-common.md#opt-I) in command line invocations

We may or may not want to have this here.

[fricklerhandwerk]

At this point all the information from the Channels page is contained in the manual page on nix-channel, therefore I think it can be safely removed. A redirect will be added immediately when this is merged: NixOS/nixos-homepage#1004