[Nix-dev] Help with ghc errors after nix-channel --update

[Peter Simons]

Hi Carlo,

 > ghc-pkg check is full of errors

I don't think that "ghc-pkg check" ever succeeded in any ghc-wrapper
based installation.

 > packages I installed no longer load in ghci

There is no obvious explanation for the errors you've described. As
random suggestion how you might obtain some additional insight, you
could add

  ghcTestEnv = self.haskellPackages_ghc783.ghcWithPackages (p: with p; [
    mtl
  ]);

to your ~/.nix-pkgs/config.nix and then run:

  $ nix-env -p /tmp/haskell -iA ghcTestEnv
  $ /tmp/haskell/ghci
  Prelude> import Control.Monad.State.Lazy

Does that succeed? If it does, you try adding 'abcnotation', too, to see
whether that makes a difference.

Generally speaking, ghc-wrapper is fundamentally broken and many
discerning Nix hackers gave up using it entirely a long time ago
(precisely because of the kind of trouble that you seem to be having).

Best regards,
Peter