ben dover.nix

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

  outputs = { self, nixpkgs, ...}:
    let
      system = "x86_64-linux";
      pkgs = import nixpkgs {
        inherit system;
        overlays = [ self.overlays.default ];
      };
    in
    {
      overlays.default = final: prev: {
        hsPkgs = pkgs.haskellPackages.override {
          overrides = self: super: {
            Cabal-described = self.callCabal2nix "Cabal-described" (pkgs.fetchgit{
              url = "https://github.com/haskell/cabal";
              rev = "cabal-install-v3.12.1.0";
              sparseCheckout = [ "Cabal-described" ];
            });

            Cabal-QuickCheck = self.callCabal2nix "Cabal-QuickCheck" (pkgs.fetchgit{
              url = "https://github.com/haskell/cabal";
              rev = "cabal-install-v3.12.1.0";
              sparseCheckout = [ "Cabal-QuickCheck" ];
            });

            Cabal-tree-diff = self.callCabal2nix "Cabal-tree-diff" (pkgs.fetchgit{
              url = "https://github.com/haskell/cabal";
              rev = "cabal-install-v3.12.1.0";
              sparseCheckout = [ "Cabal-tree-diff" ];
            });

            Cabal-tests = self.callCabal2nix "Cabal-tests" (pkgs.fetchgit{
              url = "https://github.com/haskell/cabal";
              rev = "cabal-install-v3.12.1.0";
              sparseCheckout = [ "Cabal-tests" ];
            });

            Cabal-syntax = self.callHackageDirect {
              pkg = "Cabal-syntax";
              ver = "3.12.1.0";
              sha256 = "sha256-muXJog2wj6B4Gj4Q1cLngiOCQ2+67ArFeu/wsLbv7c4=";
            } {};

            cabal-install-solver = self.callHackageDirect {
              pkg = "cabal-install-solver";
              ver = "3.12.1.0";
              sha256 = "sha256-sZgQasbMq8OP+e9zYxYK2u0WKcLZheRaCTp98R+iuws=";
            } {};

             hackage-security = self.callHackageDirect {
              pkg = "hackage-security";
              ver = "0.6.2.6";
              sha256 = "sha256-B61sYNOJXszHBA4JWFP5UZBp3UcJk9ufnGE3Li5rQNI=";
            } {};

            Cabal = self.callHackageDirect {
              pkg = "Cabal";
              ver = "3.12.1.0";
              sha256 = "sha256-wIO/GoEknor74jAcBuq2B8iK7BxXnAyKAVsKnUaQCDo=";
            } {};

            cabal-install = self.callHackageDirect {
              pkg = "cabal-install";
              ver = "3.12.1.0";
              sha256 = "sha256-iBQL6gRpe6Nzy9pnbHxMKtPQCA3edF9VMuFLaNMLetk=";
            } {};
          };
        };
      };

      devShells.${system}.default = pkgs.mkShell {
        buildInputs = with pkgs; [
          haskell.compiler.ghc910
          hsPkgs.cabal-install
        ];
      };
    };
}

thanks dad! when are you coming home?