cabalProject': Consider cabalProjectLocal even if cabalProject argument is null (#1588)

* `cabalProject'`: Consider `cabalProjectLocal` even if `cabalProject` argument is null

* Respond to review

Author: L-as

lib/call-cabal-project-to-nix.nix

@@ -126,17 +126,15 @@ let
   # access to the store is restricted.  If origSrc was already in the store
   # you can pass the project in as a string.
   rawCabalProject =
-    if cabalProject != null
-      then cabalProject + (
-        if cabalProjectLocal != null
-          then ''
-
-            -- Added from cabalProjectLocal argument to cabalProject
-            ${cabalProjectLocal}
-          ''
-          else ""
-      )
-      else null;
+    # Even if `cabal.project` doesn't exist, `cabal.project.local` is still used by cabal.
+    # We tested this: https://github.com/input-output-hk/haskell.nix/pull/1588
+    if cabalProject == null && cabalProjectLocal == null
+      then null
+      else (
+        # like fmap
+        let f = g: x: if x == null then "" else g x; in
+        f (x: x) cabalProject + f (x: "\n-- Added from cabalProjectLocal argument to cabalProject\n${x}") cabalProjectLocal
+      );
 
   cabalProjectIndexState =
     if rawCabalProject != null

modules/hackage-project.nix

@@ -26,9 +26,6 @@ in {
     # Avoid readDir and readFile IFD functions looking for these files in the hackage source
     # `mkOverride 1100` means this will be used in preference to the mkOption default,
     # but a `mkDefault` can still override this.
-    cabalProject = lib.mkOverride 1100 ''
-      packages: .
-    '';
     cabalProjectLocal = lib.mkOverride 1100 null;
     cabalProjectFreeze = lib.mkOverride 1100 null;
     src =