haskell.nix/docs/user-guide/stack-projects.md

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.

Using stack-to-nix

With nix-tools installed, we can simply run the following command on a stack project:

stack-to-nix --output . --stack-yaml stack.yaml

This will produce a pkgs.nix file that looks like the following:

{
  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 to instantiate a package set.

# 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