lib: cleanup a little bit, add traceIf

lib/customisation.nix

@@ -29,8 +29,8 @@ rec {
 
      For another application, see build-support/vm, where this
      function is used to build arbitrary derivations inside a QEMU
-     virtual machine. */
-     
+     virtual machine.
+  */
   overrideDerivation = drv: f:
     let
       newDrv = derivation (drv.drvAttrs // (f drv));
@@ -56,18 +56,17 @@ rec {
   makeOverridable = f: origArgs:
     let
       ff = f origArgs;
+      overrideWith = newArgs: origArgs // (if builtins.isFunction newArgs then newArgs origArgs else newArgs);
     in
       if builtins.isAttrs ff then (ff //
-        { override = newArgs:
-            makeOverridable f (origArgs // (if builtins.isFunction newArgs then newArgs origArgs else newArgs));
+        { override = newArgs: makeOverridable f (overrideWith newArgs);
           deepOverride = newArgs:
             makeOverridable f (lib.overrideExisting (lib.mapAttrs (deepOverrider newArgs) origArgs) newArgs);
           overrideDerivation = fdrv:
             makeOverridable (args: overrideDerivation (f args) fdrv) origArgs;
         })
       else if builtins.isFunction ff then
-        { override = newArgs:
-            makeOverridable f (origArgs // (if builtins.isFunction newArgs then newArgs origArgs else newArgs));
+        { override = newArgs: makeOverridable f (overrideWith newArgs);
           __functor = self: ff;
           deepOverride = throw "deepOverride not yet supported for functors";
           overrideDerivation = throw "overrideDerivation not yet supported for functors";
@@ -102,8 +101,10 @@ rec {
       };
   */
   callPackageWith = autoArgs: fn: args:
-    let f = if builtins.isFunction fn then fn else import fn; in
-    makeOverridable f ((builtins.intersectAttrs (builtins.functionArgs f) autoArgs) // args);
+    let
+      f    = if builtins.isFunction fn then fn else import fn;
+      auto = builtins.intersectAttrs (builtins.functionArgs f) autoArgs;
+    in makeOverridable f (auto // args);
 
 
   /* Add attributes to each output of a derivation without changing the derivation itself */