Allow applying custom functions during evaluation

lib/eval-module.nix

@@ -0,0 +1,16 @@
+let
+  inherit (nixpkgs) lib;
+  nixpkgs = import <nixpkgs> { config = {}; overlay = []; };
+  file = (builtins.getEnv "FILE");
+  property = (builtins.getEnv "PROPERTY");
+  processor = if (builtins.stringLength property > 0)
+    then
+      (_: lib.attrsets.getAttrFromPath (lib.strings.splitString "." property) _)
+    else
+      (_: _);
+in
+  processor (lib.evalModules {
+      modules = [
+        (import file)
+      ];
+    }).config

scripts/Common/Scripts/eval-module.fish

@@ -6,6 +6,11 @@ function evalModule --argument-names modulePath property
         set -a argv --raw
     end
 
-    set -l nixPkgs 'import <nixpkgs> { config = {}; overlay = []; }'
-    nix eval --file "$modulePath" --apply "_: (($nixPkgs).lib.evalModules { modules = [ _ ]; }).config.$property" $argv
+    FILE=(realpath "$modulePath") \
+    PROPERTY="$property" \
+    nix eval \
+        --file "$(status dirname)/../../../lib/eval-module.nix" \
+        $argv
 end
+
+evalModule $argv