[Nix-dev] Announcing "nix-env-rebuild" (and some comment on "nix install -d")

[Lu Fennell]

Hi Nix-dev,

For quite some time now I'm using a glorified Haskell script that I call
"nix-env-rebuild" to manage my user profile "declaratively" (i.e.,
by specifying the list of installed packages in a file). Triggered by
the discussion on the new Nix UI, I decided to do some polishing and
release nix-env-rebuild on github:

https://github.com/lu-fennell/nix-env-rebuild

Features:
- Upfront configuration of a profile through a config file
  (~/.nixpkgs/packages.nix) similar to the "systemPackages" attribute.
- Commands similar to "nixos-rebuild": dry-run, build, switch
- Shows a summary of how the current profile differs from the
  "declared" one.
- "nix-env -i .." effectively becomes a one-shot install action
  (e.g., for test-driving packages). Running "nix-env-rebuild dry-run"
  shows which packages where added by "nix-env -i" (giving you a
  chance to add them to your package list). Running
  "nix-env-rebuild switch" removes those one-shot packages.
- The packages of "packages.nix" can be overridden or extended with
  concrete store paths (in a second configuration file,
  ~/.nixpkgs/store-path-install-list.txt). This is useful e.g. for
  blocking undesired upgrades or preventing expensive rebuilds.

So if you are interested in this style of (user)-package management, I
invite you to give nix-env-rebuild a try. I'm eagerly awaiting your
comments and bug reports :)

In particular I am interested in discussing the relation to the "nix
install" parts of Eelco's new UI proposal: It seems like the proposal
tries to support "declarative package management" through "nix install
-d" and "nix rebuild". I think that an approach akin to
nix-env-rebuild is a better fit, mainly for two reasons.

1. I think upfront configuration is more "declarative" in nature than
   "nix install -d", which is an imperative operation.

2. One-shot installs, in the sense outlined above, seem difficult in
   "nix install"/"nix install -d"/"nix rebuild": Doing a

   NIX_PATH=some/very/unstable/channel nix install bleeding-edge-package

   would rebuild all "declarative" packages against
   some/very/unstable/channel, although I might just want to
   test-drive "bleeding-edge-package".

Perhaps I am wrong thinking "nix install -d" is supposed support
"declarative package management"? (in this case I think the
"-d"-option's is name is quite misleading)

But if not, I am curious to know if you concur with my observations
and how your ideas of "declarative user package management" may differ
from mine.

Best regards,
Lu