I think this is one of the things that flakes aims to solve. I'm playing around with a new flake-based setup and I'm using flake-utils-plus[1], and I use the overlays builder to make it easy to refer to packages that come from flake inputs in my configuration.nix and similar places, like. I just added the latest Nixops from master to make sure I was giving you a real answer, and I use an overlay like this in my flake.nix:

      (final: prev: {
        nixops = inputs.nixops.defaultPackage.${prev.system};
      })

after adding the flake input for nixops:

      nixops.url = "github:NixOS/nixops";

to my system flake's `inputs` attribute. And after a `nixos-rebuild switch`, nixops reports the highly following mysterious version number :

   nixops --version
  NixOps @version@

If you're not using Nix flakes, you can do something similar with Nix and Nixpkg's fetchers as well, also using overlays, using the `nixpkgs.overlays` option in NixOS. This is how, for example, the community Emacs overlay recommends folks use it[2].

What you wanna do for projects that don't offer a flake.nix or a ready-made overlay is to use a fetcher like in the Emacs example, but write your own overlay function that invokes Nixpkgs' `callPackage` function on whatever Nix expression inside the repo represents the package you want, or imports them from a release or default.nix as appropriate. Home-manager uses this to define its own little overlay[3], the one that gets used in its flake.nix, and its package is just its default.nix[4].

Unfortunately, in the pre-flakes world, you just have to read the Nix code and figure out how to translate things to get the attributes you want into scope. For example, this works for nix-processmgmt:

      (final: prev: {
        nix-processmgmt-tools = (import (
          (builtins.fetchTarball "https://github.com/svanderburg/nix-processmgmt/archive/6def8584c6b028c922c550859a07b989d21d6f73.tar.gz")
            + "/tools/default.nix")
          { pkgs = prev.pkgs; }
        );
      })

and then you can add, e.g., `nix-processmgmt-tools.common` to your `environment.systemPackages`.

I think maybe the reason instructions aren't given for some projects is that they see those parts of the guide as mostly for beginners or casual experimenters, and they expect advanced or ‘serious’ users to be able to figure it out without much trouble. To some extent I think this is because different people choose to pin packages in the pre-flakes world through a variety of different mechanisms, and the authors of these packages and tools don't know in advance how you want to do it.

In the case of nix-processmgmt, I think Sander doesn't actually expect to have any users! In such cases, an imperative install that users are expected to play with for a little while and then just throw away is supposed to be enough.

You're right, though, that it's odd and disappointing that instructions for the preferred way of doing things are sometimes simply not given. A polite pull request or issue report would probably be well-received. My recommended strategy, if you get stuck, would be to ask for help getting those packages into scope declaratively on Discourse or Matrix, and then to offer the authors of these out-of-tree packages pull requests to modify their READMEs accordingly. :)

PS: You don't necessarily _have_ to use overlays for these. You can also drop expressions that use builtins.fetchTarball and then use `callPackage` or import from those sources directly into lists of packages like `environment.systemPackages`.

—

1: https://github.com/gytis-ivaskevicius/flake-utils-plus

2: https://github.com/nix-community/emacs-overlay#quickstart

3: https://github.com/nix-community/home-manager/blob/master/ov...

4: https://github.com/nix-community/home-manager/blob/master/de...