nixpkgs/pkgs/development/ruby-modules/testing/testing.nix

with builtins;
let
  /*
    underTest = {
      x = {
        a = 1;
        b = "2";
      };
    };

    tests = [
      (root: false)
      {
        x = [
          (set: true)
          {
            a = (a: a > 1);
            b = (b: b == "3");
          }
        ];
      }
    ];

    results = run "Examples" underTest tests;
  */

  passed = desc: {
    result = "pass";
    description = desc;
  };

  failed = desc: {
    result = "failed";
    description = desc;
  };

  prefixName = name: res: {
    inherit (res) result;
    description = "${name}: ${res.description}";
  };

  run =
    name: under: tests:
    if isList tests then
      (concatLists (map (run name under) tests))
    else if isAttrs tests then
      (concatLists (
        map (
          subName:
          run (name + "." + subName) (if hasAttr subName under then getAttr subName under else "<MISSING!>") (
            getAttr subName tests
          )
        ) (attrNames tests)
      ))
    else if isFunction tests then
      let
        res = tests under;
      in
      if isBool res then
        [
          (prefixName name (if tests under then passed "passed" else failed "failed"))
        ]
      else
        [ (prefixName name res) ]
    else
      [
        failed
        (name ": not a function, list or set")
      ];
in
{
  inherit run passed failed;
}