Try using GHC_ENVIRONMENT for dev shells

builder/comp-builder.nix

@@ -65,6 +65,12 @@ let
     fi
   '';
 
+  envDep = pdbArg: p: ''
+    if id=$(${ghc.targetPrefix}ghc-pkg -v0 ${pdbArg} field ${p} id --simple-output); then
+      echo "package-id $id" >> $out/ghc-environment
+    fi
+  '';
+
   configFiles = runCommand "${fullName}-config" { nativeBuildInputs = [ghc]; } (''
     mkdir -p $out
     ${ghc.targetPrefix}ghc-pkg -v0 init $out/package.conf.d
@@ -91,6 +97,13 @@ let
 
     echo ${lib.concatStringsSep " " (lib.mapAttrsToList (fname: val: "--flags=${lib.optionalString (!val) "-" + fname}") flags)} >> $out/configure-flags
 
+    cat > $out/ghc-environment <<EOF
+    clear-package-db
+    package-db $out/package.conf.d
+    EOF
+    ${lib.concatMapStringsSep "\n" (p: envDep "--package-db ${p.components.library}/package.conf.d" p.identifier.name) componentDepends}
+    ${lib.concatMapStringsSep "\n" (envDep "") (lib.remove "ghc" nonReinstallablePkgs)}
+
   '' + lib.optionalString component.doExactConfig ''
     echo "--exact-configuration" >> $out/configure-flags
     echo "allow-newer: ${package.identifier.name}:*" >> $out/cabal.config
@@ -185,6 +198,7 @@ in stdenv.mkDerivation ({
   };
 
   CABAL_CONFIG = configFiles + /cabal.config;
+  # GHC_ENVIRONMENT = configFiles + /ghc-environment;
   LANG = "en_US.UTF-8";         # GHC needs the locale configured during the Haddock phase.
   LC_ALL = "en_US.UTF-8";
 
@@ -250,6 +264,15 @@ in stdenv.mkDerivation ({
     runHook postInstall
   '';
 
+  shellHook = ''
+    # don't use this variable for the component build, but do provide it in the shell.
+    export GHC_ENVIRONMENT=$out/ghc-environment
+
+    # provide this variable so that `ghc-pkg` works (cabal doesn't like it though)
+    export GHC_PACKAGE_PATH=$out/package.conf.d
+
+    ${toString shellHook}
+  '';
 }
 # patches can (if they like) depend on the version and revision of the package.
 // lib.optionalAttrs (patches != []) { patches = map (p: if builtins.isFunction p then p { inherit (package.identifier) version; inherit revision; } else p) patches; }
@@ -263,6 +286,5 @@ in stdenv.mkDerivation ({
 // lib.optionalAttrs (postCheck != "") { inherit postCheck; }
 // lib.optionalAttrs (preInstall != "") { inherit preInstall; }
 // lib.optionalAttrs (postInstall != "") { inherit postInstall; }
-// lib.optionalAttrs (shellHook != "") { inherit shellHook; }
 // lib.optionalAttrs (stdenv.buildPlatform.libc == "glibc"){ LOCALE_ARCHIVE = "${buildPackages.glibcLocales}/lib/locale/locale-archive"; }
 )

test/with-packages/default.nix

@@ -6,7 +6,7 @@
 with stdenv.lib;
 
 let
-  pkgSet = haskell.mkNewPkgSet {
+  pkgSet = haskell.mkPkgSet {
     inherit pkgs;
     # generated with:
     #   cabal new-build
@@ -66,7 +66,9 @@ in
 
   # Used for testing externally with nix-shell (../tests.sh).
   # This just adds cabal-install to the existing shell.
-  test-shell = pkgSet.config.hsPkgs.test-with-packages.components.library.overrideAttrs (oldAttrs: {
+  test-shell = let
+    inherit (pkgSet.config.hsPkgs.test-with-packages.components) library;
+  in library.overrideAttrs (oldAttrs: {
     buildInputs = (oldAttrs.buildInputs or []) ++ [ pkgs.cabal-install ];
   });
 }