Command-line overrides for installables

[max-privatevoid]

(This should probably be turned into an RFC at some point)

Is your feature request related to a problem? Please describe.

nixpkgs provides the override and overrideAttrs patterns to easily customize preexisting derivations. These have never been particularly usable over the command line, usually requiring patterns like

$ nix build --impure --expr '(import <nixpkgs> {}).package.override { ... }'

With flakes standardizing the concept of installables, maybe we can have a go at this?

Describe the solution you'd like

Similar to nix eval's --expr, there should be a way to apply overrides to derivations supporting them.

Some examples of how this could look like:

$ nix build nixpkgs#package --override-arg withSomething true --override-arg withSomethingElse false

$ nix build nixpkgs#package --override-argstr channel dev

$ nix build nixpkgs/nixos-21.11#package --override-pkg libabstract nixpkgs/nixos-unstable#libabstract-impl

# old is magically pulled into scope
$ nix build nixpkgs#package --overrideattr patches 'old.patches ++ [ ./myfeature.patch ]'

# if we're really stretching it
$ nix build nixpkgs#package --overrideattr-append patches ./myfeature.patch

$ nix build /nix/store/xxxxxxxx-package --override-argstr foo bar
error: argument '...' cannot be evaluated

https://github.com/max-privatevoid/nix/tree/cli-overrides currently implements a rudimentary base for this concept. It allows you to do this:

$ nix build nixpkgs#ffmpeg --apply-to-installable 'x: x.override { libmfxSupport = true; }'

Things to figure out:

• The override pattern is from nixpkgs and not "official". Should the pattern be specified as part of Nix itself? ([RFC 0067] Common override interface derivations rfcs#67)
• Can installable in --override-pkg installable be a non-value (e.g. --override-pkg /nix/store/xxxxxxx-newpackage)?
• Come up with better argument names (maybe reuse --arg, --argstr?)
• How to apply functions to apps
• No way to differentiate between configuration arguments (withSomething) and dependency arguments. This is a nixpkgs issue.
• Nested overrides i.e. equivalent of pkg1.override { dep1 = pkg2.override { dep2 = newPkg3; }; }
• Use URL parameters instead? (see Support flake references to patches #3920)

Describe alternatives you've considered

Wait for https://gist.github.com/edolstra/29ce9d8ea399b703a7023073b0dbc00d#user-interface

[stale]

I marked this as stale due to inactivity. → More info

[Kranzes]

Why are feature request issues being made stale???

[hraban]

Why are feature request issues being made stale???

Because they've gone without activity for a long time, in other words "stale"? I hate auto-closing bots as much as the next guy, but just labelling someting "stale" when in fact it is stale, without closing the issue, seems only fair to me.