rewrite-in-python: more nix validation, fix eval warning

flake.nix

@@ -16,13 +16,12 @@
     inherit (nixpkgs) lib;
 
   in {
-    apps = forAllSystems (system: pkgs: let
-      mkApp = package: {
+    apps = forAllSystems (system: pkgs: {
+      default = {
         type = "app";
-        program = lib.getExe package;
+        program = lib.getExe self.packages.${system}.default;
+        meta.description = "The 'mckart' cli tool with all map data embedded";
       };
-    in {
-      default = mkApp self.packages.${system}.default;
     });
 
     devShells = forAllSystems (_: pkgs: {
@@ -40,6 +39,8 @@
     overlays.default = final: prev: self.packages.${final.system};
 
     checks = forAllSystems (system: pkgs: {
+      inherit (self.packages.${system}) bluemap-export mapcrafter-export;
+
       hocon-include = let
         format = pkgs.formats.hocon { };
       in format.generate "minecraft-kartverket-test-hocon-include" {
@@ -69,10 +70,33 @@
         meta.mainProgram = "mckart";
       };
 
-      bluemap-export = pkgs.runCommand "bluemap-export" {
-        buildInputs = [ (self.packages.${system}.mckart) ];
+      bluemap-export = let
+        # from pkgs/pkgs-lib/formats/hocon/default.nix
+        hocon-validator =
+          pkgs.writers.writePython3Bin "hocon-validator"
+            {
+              libraries = [ pkgs.python3Packages.pyhocon ];
+            }
+            ''
+              from sys import argv
+              from pyhocon import ConfigFactory
+
+              if not len(argv) == 2:
+                  print("USAGE: hocon-validator <file>")
+
+              ConfigFactory.parse_file(argv[1])
+            '';
+      in pkgs.runCommand "bluemap-export" {
+        nativeBuildInputs = [
+          (self.packages.${system}.mckart)
+          hocon-validator
+        ];
       } ''
         mckart export-bluemap --output-dir "$out"
+
+        for file in "$out"/*.hocon; do
+          hocon-validator "$file"
+        done
       '';
 
       mapcrafter-export = pkgs.runCommand "mapcrafter-export" {