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

[Peter Simons]

Hi Carlo,

 > I wrote the nix expression for abcnotation, and it's not yet present
 > in the channel. What would be the right way to add an expression made
 > by me in this setup?

run the commands

  $ cabal2nix cabal://prettify >~/.nixpkgs/prettify.nix
  $ cabal2nix cabal://abcnotation >~/.nixpkgs/abcnotation.nix

and define in ~/.nixpkgs/config.nix:

  packageOverrides = super: let self = super.pkgs; in
  {
    haskellPackages = super.haskellPackages.override {
      extension = self: super: {
        abcnotation = self.callPackage ./abcnotation.nix {};
        prettify = self.callPackage ./prettify.nix {};
      };
    };
  };

This will make abcnotation (and prettify) available like any other
package defined in Nixpkgs, and you can use them with 'ghcWithPackages'.


 > More generally, I'd like a configuration in which I'm able to:
 >
 > 1) install packages from nixos-unstable,
 > 2) from a nixpkgs local repo,
 > 3) from expressions mantained by me
 > 4) using ghc-mod and similar in emacs

You can do all those things easily with ghcWithPackages. Just add
'ghcMod' to the package set, and add these definitions to your
~/.bashrc:

  NIX_GHC_VERSION=$(ghc --numeric-version)
  export NIX_GHC="$HOME/.nix-profile/bin/ghc"
  export NIX_GHCPKG="$HOME/.nix-profile/bin/ghc-pkg"
  export NIX_GHC_DOCDIR="$HOME/.nix-profile/share/doc/ghc/html"
  export NIX_GHC_LIBDIR="$HOME/.nix-profile/lib/ghc-${NIX_GHC_VERSION}"

With that setup, ghc-mod will work just fine (for those packages that
you've declared).

I hope this helps
Peter