shell.nix

{ withDevTools ? true }:

let
  sources = import ./nix/sources.nix;
  pkgs = import sources.nixpkgs { };
  # ghc924 matches the version from Stack
  ghcVersion = "984";
in

let
  # Wrap Stack to configure Nix integration and target the correct Stack-Nix file
  #
  # - nix: Enable Nix support
  # - no-nix-pure: Pass environment variables, like `NIX_PATH`
  # - nix-shell-file: Specify the Nix file to use (otherwise it uses `shell.nix` by default)
  stack-wrapped = pkgs.symlinkJoin {
    name = "stack";
    paths = [ pkgs.stack ];
    buildInputs = [ pkgs.makeWrapper ];
    postBuild = ''
      wrapProgram $out/bin/stack \
        --add-flags "\
          --nix \
          --no-nix-pure \
          --nix-path=\\"nixpkgs=${pkgs.path}\\"
          --nix-shell-file=nix/stack-integration.nix \
        "
    '';
  };

  haskellBin = pkgs.haskell.lib.compose.justStaticExecutables;

  buildDeps = [
    stack-wrapped
    (haskellBin pkgs.haskell.packages."ghc${ghcVersion}".ormolu)
    # Nix for recursive calls within Stack (necessary for pure shells)
    pkgs.nix
    # Just lets us distribute project-wide commands
    pkgs.just
    # Git is used to call `git ls-files` for formatting
    pkgs.git
  ];
  devTools = [
    # Rather than:
    # (pkgs.haskell.packages."ghc${ghcVersion}".haskell-language-server)
    # I'm not completely sure what the difference is
    (pkgs.haskell-language-server.override { supportedGhcVersions = [ ghcVersion ]; })
  ];

in
pkgs.mkShell {
  buildInputs = buildDeps ++ (if withDevTools then devTools else []);

  # Configure the Nix path to our own `pkgs`, to ensure Stack-with-Nix uses the correct one rather than the global <nixpkgs> when looking for the right `ghc` argument to pass in `nix/stack-integration.nix`
  # See https://nixos.org/nixos/nix-pills/nix-search-paths.html for more information
  NIX_PATH = "nixpkgs=" + pkgs.path;
}