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

check-utils.nix

@@ -0,0 +1,22 @@
+systemOrPkgs:
+let
+  str = it: if it == null then "null" else (toString it);
+  system = systemOrPkgs.system or systemOrPkgs;
+  test = name: command: derivation {
+    inherit name system;
+    builder = "/bin/sh";
+    args = [ "-c" command ];
+  };
+in
+{
+
+  isEqual = a: b:
+    if a == b
+    then test "SUCCESS__${str a}__IS_EQUAL__${str b}" "echo success > $out"
+    else test "FAILURE__${str a}__NOT_EQUAL__${str b}" "exit 1";
+
+  hasKey = attrset: key:
+    if attrset ? ${str key}
+    then test "SUCCESS__${str key}__EXISTS_IN_ATTRSET" "echo success > $out"
+    else test "FAILURE__${str key}__DOES_NOT_EXISTS_IN_ATTRSET_SIZE_${str(builtins.length (builtins.attrNames attrset))}" "exit 1";
+}

default.nix

@@ -135,14 +135,18 @@ let
   # This function tries to capture a common flake pattern.
   simpleFlake = import ./simpleFlake.nix { inherit lib; };
 
+  # Helper functions for Nix evaluation
+  check-utils = import ./check-utils.nix;
+
   lib = {
     inherit
       allSystems
+      check-utils
       defaultSystems
       eachDefaultSystem
       eachSystem
-      flattenTree
       filterPackages
+      flattenTree
       mkApp
       simpleFlake
       ;

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;
+        };
+      }
+    );
+}

filterPackages.nix

@@ -6,7 +6,8 @@ let
   nameValuePair = name: value: { inherit name value; };
   filterAttrs = pred: set:
     listToAttrs (
-      concatMap (name:
+      concatMap
+        (name:
           let v = set.${name}; in
           if pred name v then [ (nameValuePair name v) ] else [ ]
         )