Add checks-utils (#36)

* Add checks-utils overlay

Add checks-utils overlay

Fix example flake inputs

Fix string expected exception

* Update check-utils implementation

* nixpkgs-fmt

examples/checks-utils/flake.nix

@@ -0,0 +1,29 @@
+{
+  description = "Flake utils demo";
+
+  inputs.flake-utils.url = "github:numtide/flake-utils";
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        inherit (flake-utils.lib.check-utils system) isEqual hasKey;
+
+        testDataset = { key1 = "value1"; key2 = "value2"; key3 = "value3"; };
+      in
+      rec {
+        checks = {
+          valid_key1 = isEqual testDataset.key1 "value1";
+          contains_key2 = hasKey testDataset "key2";
+
+          failing_valid_key1 = isEqual testDataset.key1 "failing-data";
+          failing_contains_key2 = hasKey testDataset "failing-data";
+
+          number_formatting_isEqual = isEqual testDataset.key1 123;
+          number_formatting_hasKey = hasKey testDataset 123;
+
+          null_formatting_key1 = isEqual testDataset.key1 null;
+          null_formatting_hasKey = hasKey testDataset null;
+        };
+      }
+    );
+}