Here we will look into how to generate the `pkgs.nix` file for a
`stack.yaml` project. For the full integration see the
[previous page](./projects.md).

## Using `stack-to-nix`

With [nix-tools installed](../user-guide.md), we can simply run the
following command on a stack project:

```bash
stack-to-nix --output . --stack-yaml stack.yaml
```

This will produce a `pkgs.nix` file that looks like the following:
```nix
{
  resolver = "lts-12.17";
  extras = hackage:
    {
      packages = {
        "o-clock" = hackage.o-clock."0.1.1".revisions.default;
        ...
      } // {
        my-package = ./my-package.nix;
        ...
      };
    };
}
```

This file contains the stackage resolver, as well as set of extra
packages.  The extras specifies which `extra-deps` (here:
`o-clock-0.1.1`) we wanted to add over the stackage snapshot, and what
local packages we want (here: `my-package`).

## Creating the package set

Import the generated `pkgs.nix` and pass to
[`mkStackPkgSet`](../reference/library.md#mkstackpkgset) to
instantiate a package set.

```nix
# default.nix
let
  # Import the Haskell.nix library,
  haskellNix = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz) {};
  nixpkgs = import haskellNix.sources.nixpkgs-1909 haskellNix.nixpkgsArgs;
  haskell = nixpkgs.haskell-nix;

  # Instantiate a package set using the generated file.
  pkgSet = haskell.mkStackPkgSet {
    stack-pkgs = import ./pkgs.nix;
    pkg-def-overlays = [];
    modules = [];
  };
in
  pkgSet.config.hsPkgs
```