How to write a shell.nix file which combines other nix-shell environments?

Question

I want to write a shell.nix file that will provide me a development environment which includes developer tools + haskell dependencies. I would like to keep my nix expressions separate - for now that means one file containing information about my editor, and one file containing information about my haskell project.

I have tried to set up a really basic environment, but can't get it to work. I have two files which I can create a nix-shell from already:

# my-haskell-shell.nix - generated by cabal2nix --shell

{ nixpkgs ? import <nixpkgs> {}, compiler ? "default", doBenchmark ? false }:

let

  inherit (nixpkgs) pkgs;

  f = { mkDerivation, base, cabal-install, hpack, stdenv }:
      mkDerivation {
        pname = "hello-world";
        version = "0.1.0.0";
        src = ./.;
        isLibrary = false;
        isExecutable = true;
        libraryToolDepends = [ hpack ];
        executableHaskellDepends = [ base ];
        executableToolDepends = [ cabal-install hpack ];
        preConfigure = "hpack";
        license = stdenv.lib.licenses.mit;
      };

  haskellPackages = if compiler == "default"
                       then pkgs.haskellPackages
                       else pkgs.haskell.packages.${compiler};

  variant = if doBenchmark then pkgs.haskell.lib.doBenchmark else pkgs.lib.id;

  drv = variant (haskellPackages.callPackage f {});

in
  if pkgs.lib.inNixShell then drv.env else drv

# my-neovim-shell.nix

with import <nixpkgs> {};

let
  my-neovim = neovim.override { vimAlias = false; };

in
stdenv.mkDerivation {
  name = "neovim-dev-env";
  buildInputs = [
    my-neovim
  ];
}

Both of these produce useful shell environments! (That is, nix-shell my-haskell-env.nix and nix-shell my-neovim-env.nix give me cabal and neovim respectively, but not both).

My attempt at producing a shell.nix which will provide an environment with both neovim and cabal available is this:

# shell.nix
with import <nixpkgs> {};
let
  my-neovim-shell-env = import ./my-neovim-shell.nix;
  my-haskell-shell-env = import ./my-haskell-shell.nix {};
in
stdenv.mkDerivation {
  name = "my-new-env";
  buildInputs = [
    my-neovim-shell-env
    my-haskell-shell-env
  ];
}

This does not work however. It seems to try to build the neovim environment:

 $ nix-shell
these derivations will be built:
  /nix/store/k9ygid1wl75vf2nq7jzfh32mv5f8i956-neovim-dev-env.drv
building '/nix/store/k9ygid1wl75vf2nq7jzfh32mv5f8i956-neovim-dev-env.drv'...
unpacking sources
variable $src or $srcs should point to the source
builder for '/nix/store/k9ygid1wl75vf2nq7jzfh32mv5f8i956-neovim-dev-env.drv' failed with exit code 1
error: build of '/nix/store/k9ygid1wl75vf2nq7jzfh32mv5f8i956-neovim-dev-env.drv' failed

I don't know how to fix this; what am I doing wrong? I suspect maybe I should not be using mkDerivation?

score 1 · Answer 1 · answered Jan 20 '19 at 20:23

Managed to find a solution. Maybe there is a better way, but this works:

Suppose you have a package.yaml, then

 $ cabal2nix . > app.nix

# shell.nix
let
  p = import <nixpkgs> {};
  app = p.haskellPackages.callPackage (./app.nix) {};
  lib = p.haskell.lib;
in
lib.overrideCabal app (old: { buildTools = [p.my-nvim] ++ (old.buildTools or []); })

(where I've added my-nvim to my ~/.config/nixpkgs/overlays/)

Now I have access to the my-nvim package and ghc, etc:

$ nix-shell

$ nvim # works
$ ghc # works

Inspiration from https://github.com/p-implies-q/nix-util

How to write a shell.nix file which combines other nix-shell environments?

1 Answers1