lib(util): add functions for finding and acessing fragments ("paths") through a tree of attrSets

flake.nix

@@ -9,6 +9,7 @@
 
   outputs = { self, nixpkgs, flake-utils }: with flake-utils.lib; eachSystem (with system; [ x86_64-linux aarch64-linux ]) (curSystem:
     let
+      util = import ./lib/util.nix;
       pkgs = nixpkgs.legacyPackages.${curSystem};
 
       imageName = "pihole/pihole";

lib/util.nix

@@ -0,0 +1,22 @@
+{
+  collectAttrFragments = predicate: attrs: with builtins; let
+    _collectAttrFragments = attrs:
+      concatMap (key: _collectAttrFragmentsBelowKey key attrs.${key}) (attrNames attrs)
+    ;
+    _collectAttrFragmentsBelowKey = key: value:
+      if predicate value then [ [key] ]
+      else if isAttrs value then
+        map (fragment: [key] ++ fragment) (_collectAttrFragments value)
+      else [ ]
+    ;
+    in _collectAttrFragments attrs
+  ;
+
+  accessValueOfFragment = attrs: fragment: with builtins; let
+    _accessValueOfFragment = value: fragment:
+      if fragment == [] then value
+      else _accessValueOfFragment (value.${head fragment}) (tail fragment)
+    ;
+    in _accessValueOfFragment attrs fragment
+  ;
+}