Haskell packages not listed in ghc-pkg when installed through Nix

Question

I have installed GHC through Nix:

$ nix-env -i ghc

Then I have installed the aeson package:

$ nix-env -f "<nixpkgs>" -iA haskellPackages.aeson

And pointed GHC to the Nix package folder:

$ export GHC_PACKAGE_PATH=~/.nix-profile/lib/ghc-8.0.1/package.conf.d/

Which seems to work:

$ ghc-pkg list
/Users/zoul/.nix-profile/lib/ghc-8.0.1/package.conf.d
    Cabal-1.24.0.0
    array-0.5.1.1
    …

But the aeson package is missing from the list of packages above and can’t be loaded. Even though there’s clearly something there:

$ ls /Users/zoul/.nix-profile/lib/ghc-8.0.1/ | grep ^ae
aeson-0.11.2.1

What am I doing wrong?

Answer 1

You cannot install Haskell libraries in Nix that way because the ghc compiler you're using does not search your user's profile for libraries. Consequently, installing a library there has no effect. This topic is explained in great detail in the Nixpkgs user manual. I'm citing the relevant bit from "8.5.2.2. How to install a compiler with libraries":

GHC expects to find all installed libraries inside of its own lib directory. This approach works fine on traditional Unix systems, but it doesn’t work for Nix, because GHC’s store path is immutable once it’s built. We cannot install additional libraries into that location. As a consequence, our copies of GHC don’t know any packages except their own core libraries, like base, containers, Cabal, etc.

We can register additional libraries to GHC, however, using a special build function called ghcWithPackages. That function expects one argument: a function that maps from an attribute set of Haskell packages to a list of packages, which determines the libraries known to that particular version of GHC. For example, the Nix expression ghcWithPackages (pkgs: [pkgs.mtl]) generates a copy of GHC that has the mtl library registered in addition to its normal core packages:

$ nix-shell -p "haskellPackages.ghcWithPackages (pkgs: [pkgs.mtl])"

 [nix-shell:~]$ ghc-pkg list mtl
 /nix/store/zy79...-ghc-7.10.2/lib/ghc-7.10.2/package.conf.d:
     mtl-2.2.1

This function allows users to define their own development environment by means of an override. After adding the following snippet to ~/.nixpkgs/config.nix,

{
  packageOverrides = super: let self = super.pkgs; in
  {
    myHaskellEnv = self.haskell.packages.ghc7102.ghcWithPackages
                     (haskellPackages: with haskellPackages; [
                       # libraries
                       arrows async cgi criterion
                       # tools
                       cabal-install haskintex
                     ]);
  };
}

it’s possible to install that compiler with nix-env -f "<nixpkgs>" -iA myHaskellEnv.