Cleanup cabal-project-to-nix

Author: angerman

default.nix

@@ -140,6 +140,7 @@ let
       inherit (pkgs) runCommand cabal-install ghc;
       inherit (pkgs.haskellPackages) hpack;
       inherit (self) nix-tools;
+      inherit (pkgs) symlinkJoin;
     };
   });
 

lib/cabalProjectToNix.nix

@@ -1,4 +1,4 @@
-{ mkHackageIndex, pkgs, runCommand, nix-tools, cabal-install, ghc, hpack }:
+{ mkHackageIndex, pkgs, runCommand, nix-tools, cabal-install, ghc, hpack, symlinkJoin }:
 let defaultGhc = ghc;
     defaultCabalInstall = cabal-install;
 in { hackageIndexState, src, ghc ? defaultGhc, cabal-install ? defaultCabalInstall }:
@@ -10,7 +10,10 @@ let
         type == "directory" ||
         pkgs.lib.any (i: (pkgs.lib.hasSuffix i path)) [ ".project" ".cabal" "package.yaml" ];
     };
-  plan = runCommand "plan" {
+  plan = if (builtins.compareVersions cabal-install.version "2.4.0.0") < 0
+         # cabal-install versions before 2.4 will generate insufficient plan information.
+         then throw "cabal-install (current version: ${cabal-install.version}) needs to be at least 2.4 for plan-to-nix to work without cabal-to-nix"
+         else runCommand "plan" {
     buildInputs = [ ghc hpack ];
   } ''
     tmp=$(mktemp -d)
@@ -19,12 +22,34 @@ let
     chmod +w -R .
     find . -name package.yaml -exec hpack "{}" \;
     HOME=${mkHackageIndex hackageIndexState} ${cabal-install}/bin/cabal new-configure
-    HOME=$out ${nix-tools}/bin/plan-to-nix --plan-json dist-newstyle/cache/plan.json -o nix-plan
-    cp -r nix-plan $out
+
+    export LANG=C.utf8 # Needed or stack-to-nix will die on unicode inputs
+    mkdir -p $out
+
+    # ensure we have all our .cabal files (also those generated from package.yaml) files.
+    # otherwise we'd need to be careful about putting the `cabal-generator = hpack` into
+    # the nix expression.  As we already called `hpack` on all `package.yaml` files we can
+    # skip that step and just package the .cabal files up as well.
+    #
+    # This is also important as `plan-to-nix` will look for the .cabal files when generating
+    # the relevant `pkgs.nix` file with the local .cabal expressions.
+    ${pkgs.rsync}/bin/rsync -a --prune-empty-dirs --include '*/' --include '*.cabal' --exclude '*' $tmp/ $out/
+
+    # make sure the path's in the plan.json are relative to $out instead of $tmp
+    # this is necessary so that plan-to-nix relative path logic can work.
+    substituteInPlace $tmp/dist-newstyle/cache/plan.json --replace "$tmp" "$out"
+
+    # run `plan-to-nix` in $out.  This should produce files right there with the
+    # proper relative paths.
+    (cd $out && ${nix-tools}/bin/plan-to-nix --plan-json $tmp/dist-newstyle/cache/plan.json -o .)
+
+    # move pkgs.nix to default.nix ensure we can just nix `import` the result.
+    mv $out/pkgs.nix $out/default.nix
   '';
 in
   runCommand "plan-and-src" {} ''
     mkdir $out
-    cp -r ${src}/* $out
-    ln -sf ${plan} $out/nix-plan
+    # todo: should we clean `src` to drop any .git, .nix, ... other irelevant files?
+    ${pkgs.xorg.lndir}/bin/lndir -silent "${src}" "$out"
+    ${pkgs.rsync}/bin/rsync -a ${plan}/ $out/
   ''