Usage with Nix Flakes

Hello!

I was just wondering how I would go about using this with Nix Flakes, as in what module I would have to import in nixpkgs.lib.nixosSystem.modules? Would mach-nix.nixosModules.mach-nix work?

Thank you kindly for the help!

[nat543207]

Flake inputs actually have some special handling that causes toString flake-input-name to return a Nix store path; as a result, you can import Nix files from flake inputs directly. I’ve been using that strategy in my own flake-based projects; a working (but very simple) example flake that does this with mach-nix is shown below:

{
  description = "Using mach-nix from a flake";

  inputs = {
    nixpkgs.url = github:NixOS/nixpkgs/nixos-unstable;
    mach-nix.url = github:DavHau/mach-nix/f80ee22; # v3.0.0 release tag
  };

  outputs = { self, nixpkgs, mach-nix }: let
    pkgs = (import nixpkgs { system = "x86_64-linux"; }).pkgs;
    mach-nix-utils = import mach-nix {
      inherit pkgs;
      python = "python3";
    };
  in {

    packages.x86_64-linux.customPython = mach-nix-utils.mkPython {
      requirements = ''
        pytest
      '';
    };

    defaultPackage.x86_64-linux = self.packages.x86_64-linux.customPython;
  };
}

mkPython works just fine, which you can see for yourself by initializing a repo with the above flake and running

[nat543207@hostname|tmp] nix shell .#customPython                                                                                                                                                             
[nat543207@hostname|tmp] python
Python 3.8.6 (default, Sep 23 2020, 13:54:27)
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pytest
>>> pytest.fail('Intentional failure')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/nix/store/6qbpa7k8mlgzb8gf67lms6ypx4q6vfp2-python3-3.8.6-env/lib/python3.8/site-packages/_pytest/outcomes.py", line 153, in fail
    raise Failed(msg=msg, pytrace=pytrace)
Failed: Intentional failure
>>>

However, functions like buildPythonApplication that rely on extractor don’t run correctly for some arguments (e. g. URLs) due to a <nixpkgs> import that can’t be evaluated in pure-mode by the flake. ~I’ve got a local patch to replace that import with the pkgs value used to initialize mach-nix; it hasn’t caused me any issues yet, so I’ll see if I can put a PR up for it tomorrow.~ This can be worked around by providing pname, version, and requirements manually in the argument attrset, which causes the logic that depends on the extractor to be skipped. My local patch doesn’t actually resolve this issue, and it looks like doing so would require digging deeper into the code than I have time for right now.

[DavHau]

I merged some commits of @nat543207 and @newkozlukov and added the library functions to flakes.nix.