mkStackPkgSet: filter out $locals package (#393)

I found that if I added this to my stack.yaml:

    ghc-options:
      $locals: -ddump-to-file -ddump-hi

As specified in https://docs.haskellstack.org/en/stable/yaml_configuration/#ghc-options

Then my build would fail to evaluate with:

    error: The option `packages."\$locals".package.identifier.name' is used but not defined.

So to fix it, we filter out these special package globs in mkStackPkgSet.

This commit also adds a regression test for that configuration option.

overlays/haskell.nix

@@ -105,6 +105,11 @@ self: super: {
                 # The compiler referenced in the stack config
                 compiler = (stack-pkgs.extras hackage).compiler or (pkg-def hackage).compiler;
                 patchesModule = ghcHackagePatches.${compiler.nix-name} or {};
+                # Remove fake packages generated from stack keywords used in ghc-options
+                removeStackSpecial = module: if builtins.typeOf module == "set"
+                  then module // { packages = removeSpecialPackages (module.packages or {}); }
+                  else module;
+                removeSpecialPackages = ps: removeAttrs ps [ "$locals" "$targets" "$everything" ];
             in mkPkgSet {
                 pkg-def = excludeBootPackages pkg-def;
                 pkg-def-extras = [ stack-pkgs.extras
@@ -115,7 +120,7 @@ self: super: {
                 # and we should trust stackage here!
                 modules = [ { doExactConfig = true; } patchesModule ]
                        ++ modules
-                       ++ stack-pkgs.modules or [];
+                       ++ map removeStackSpecial (stack-pkgs.modules or []);
             };
 
         # Create a Haskell package set based on a Cabal configuration.

test/ghc-options/stack.nix

@@ -8,6 +8,11 @@ let
   };
   packages = project.hsPkgs;
 
+  # Get the names of all packages. This is a test to see
+  # whether there is a broken "$locals" package present.
+  hasIdentifier = p: p != null && p ? identifier;
+  packageNames = mapAttrsToList (name: p: p.identifier.name) (filterAttrs (name: hasIdentifier) packages);
+
 in recurseIntoAttrs {
   ifdInputs = {
     inherit (project) stack-nix;
@@ -19,7 +24,7 @@ in recurseIntoAttrs {
       printf "checking whether executable runs... " >& 2
       cat ${haskellLib.check packages.test-ghc-options.components.exes.test-ghc-options-exe}
 
-      touch $out
+      echo '${concatStringsSep " " packageNames}' > $out
     '';
 
     meta.platforms = platforms.all;
@@ -29,4 +34,4 @@ in recurseIntoAttrs {
       inherit project packages;
     };
   };
-}
+}

test/ghc-options/stack.yaml

@@ -5,3 +5,5 @@ packages:
 
 ghc-options:
   test-ghc-options: -DTEST_GHC_OPTION
+  # See https://github.com/ndmitchell/weeder/issues/53
+  $locals: -ddump-to-file -ddump-hi