haskell: Overriding compiler causes duplicate GHCs on PATH

[nh2]

If you use haskell.packages.ghc*.override { ghc = ... } then contrary to expectations, not only the overridden GHC will be on PATH for the build, but the un-overriden one as well, which can cause the wrong one to be picked sometimes.

See https://discourse.nixos.org/t/patching-ghc-for-your-project/9614/3?u=nh2

Steps to reproduce

Clone nixpkgs, inside create a file duplicate-ghcs.nix with contents:

let
  pkgs = import <nixpkgs> {};
in
  (pkgs.haskell.packages.ghc865.override (oldHaskellPackages: {
    ghc = pkgs.haskell.compiler.ghc865.overrideAttrs (old: {
      # x = 1;
    });
  })).dhall

Run nix-store -qR $(NIX_PATH=nixpkgs=. nix-instantiate duplicate-ghcs.nix) | grep ghc-8.6.5, you'll see 1 GHC in use (good):

/nix/store/my6xd0w33sll8ll68zyz9ppdzkvipxr7-ghc-8.6.5-src.tar.xz.drv
/nix/store/qa84fajzb5iygzgpcfz87zh971zwic48-ghc-8.6.5.drv

Then comment in the override:

-# x = 1;
+x = 1;

Now nix-store -qR $(NIX_PATH=nixpkgs=. nix-instantiate duplicate-ghcs.nix) | grep ghc-8.6.5, shows 2 GHCs in use (bad):

/nix/store/my6xd0w33sll8ll68zyz9ppdzkvipxr7-ghc-8.6.5-src.tar.xz.drv
/nix/store/qa84fajzb5iygzgpcfz87zh971zwic48-ghc-8.6.5.drv
/nix/store/yaxj0rwpys9iyh7z2am59z216dg89pb1-ghc-8.6.5.drv

---

Tested on 20.03 and current nixpkgs master commit 579daed.

CC @peti

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/patching-ghc-for-your-project/9614/4

[nh2]

When I nix-diff the 2 outputs of nix-instantiate, I also see this:

% nix-diff /nix/store/7jzms1m8d8z1bccdfjf1cx073a6xcsa6-dhall-1.32.0.drv /nix/store/h4b3jdqs5na8ccq20zyp15hcj8kyb0p7-dhall-1.32.0.drv
- /nix/store/7jzms1m8d8z1bccdfjf1cx073a6xcsa6-dhall-1.32.0.drv:{out}
+ /nix/store/h4b3jdqs5na8ccq20zyp15hcj8kyb0p7-dhall-1.32.0.drv:{out}
• The input named `Diff-0.4.0` differs
  - /nix/store/65sl8crlgvixcl0if8gp1gl1vfw0fi28-Diff-0.4.0.drv:{out}
  + /nix/store/bxspshdcbi9vxx0id80zkxh2hkmvfc6k-Diff-0.4.0.drv:{out}
  • The set of inputs named `ghc-8.6.5` do not match
      + /nix/store/yaxj0rwpys9iyh7z2am59z216dg89pb1-ghc-8.6.5.drv:{out}
  • The environments do not match:

I find it weird that I only get a + in here:

  • The set of inputs named `ghc-8.6.5` do not match
      + /nix/store/yaxj0rwpys9iyh7z2am59z216dg89pb1-ghc-8.6.5.drv:{out}

instead of a - and a +.

[cdepillabout]

My guess is that you need to override buildHaskellPackages in addition to ghc:

nixpkgs/pkgs/top-level/haskell-packages.nix

Lines 159 to 163 in b37c198

	ghc865 = callPackage ../development/haskell-modules {
	buildHaskellPackages = bh.packages.ghc865;
	ghc = bh.compiler.ghc865;
	compilerConfig = callPackage ../development/haskell-modules/configuration-ghc-8.6.x.nix { };
	};

I arrived at this guess by looking at the .drv file for dhall (well, actually I used the Diff package since it has less dependencies than dhall), and I saw that one GHC was pulled in by nativeBuildInputs, and the other GHC was pulled in by depsBuildBuild.

I don't know how things like buildPackages, targetPackages, etc work in nixpkgs (although I'd be really interested in learning about it if anyone knew any good resources), but I think that if you directly override the Haskell stuff in an overlay, buildHaskellPackages just gets overridden automatically.

The following derivation only references one GHC:

let
  myOverlay = final: prev: {
    haskell = prev.haskell // {
      packages = prev.haskell.packages // {
        ghc865 = prev.haskell.packages.ghc865.override {
          ghc = prev.haskell.compiler.ghc865.overrideAttrs (old: {
            x = 1;
          });
        };
      };
    };
  };

  # This is a checkout of nixpkgs at commit 598ffa1176.
  pkgs = import ~/git/nixpkgs {
    overlays = [ myOverlay ];
  };
in
  pkgs.haskell.packages.ghc865.Diff

[stale]

I marked this as stale due to inactivity. → More info