FAQ

What about a watch mode?

This adds a lot of additional complexity and for now is better dealt with by using external file watcher tools such as Reflex & Watchman.

Can I change the colors?

nix-unit uses difftastic, which can be configured via environment variables. You can turn off colors via DFT_COLOR=never, give difftastic a hint for choosing better colors with DFT_BACKGROUND=light or see the full list of options via e.g. nix run nixpkgs#difftastic -- --help.

How do I compare derivations?

Comparing derivations directly will deep-force a derivation and its attributes, making a naive test case like:

let
  pkgs = import <nixpkgs> {};
in {
  test-hello = {
    expr = pkgs.hello;
    expected = pkgs.hello;
  };
}

output a cryptic series of lines like:

...
evaluation warning: Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail.
evaluation warning: Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail.
evaluation warning: Use `stdenv.tests` instead. `passthru` is a `mkDerivation` detail.
error: stack overflow (possible infinite recursion)

To work around this instead compare the derivation paths:

let
  pkgs = import <nixpkgs> {};
in {
  test-hello = {
    # Compare as strings
    expr = "${pkgs.hello}";
    expected = "${pkgs.hello}";
  };
}

Comparison with other tools

This comparison matrix was originally taken from Unit test your Nix code but has been adapted. Pythonix is excluded as it’s unmaintained.

1. Snapshot testing