nix-playgraound/05-haskell/flake.nix

{
  # Builds a tiny Haskell executable from a local Cabal package, and
  # provides a matching dev shell for editing and running it.
  description = "A minimal Haskell project with Cabal and flakes";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
  };

  outputs =
    { self, nixpkgs, ... }:
    let
      system = "x86_64-linux";
      pkgs = import nixpkgs { inherit system; };
      inherit (pkgs) haskellPackages;
      project = haskellPackages.callCabal2nix "mini-haskell" ./. { };
      checkedProject = pkgs.haskell.lib.doCheck project;
    in
    {
      packages.${system}.default = project;

      apps.${system}.default = {
        type = "app";
        program = "${self.packages.${system}.default}/bin/mini-haskell";
        meta.description = "Run the minimal Haskell example.";
      };

      devShells.${system}.default = pkgs.mkShell {
        packages = [
          haskellPackages.ghc
          pkgs.cabal-install
          pkgs.haskell-language-server
        ];
      };

      # `doCheck` turns on the Cabal test suite when this derivation is built.
      checks.${system}.test-suite = checkedProject;
    };
}