Installing all-hies

config.nix

let
  config = {
    allowUnfree = true;
    
    packageOverrides = pkgs: with pkgs; 
      let jdk = openjdk11; in rec {
      unstable = import <nixpkgs> { inherit config; };
      
      all = pkgs.buildEnv {
        name = "all";

        paths = [
          htop
          nox
          nix-info
          nix-index
          google-chrome
          nodejs-11_x
          binutils.bintools
          haskell.compiler.ghc864
          haskellPackages.cabal-install
          unstable.haskellPackages.stack
          unstable.haskellPackages.cabal2nix
          haskellPackages.hoogle
          haskellPackages.ghcid
          haskellPackages.phoityne-vscode
          haskellPackages.haskell-dap
        ];
      };
    };
  };
in config

How can I add:

let
  all-hies = import (fetchTarball "https://github.com/infinisil/all-hies/tarball/master") {};

to the file above?

So that I can add this to the packages list:

(all-hies.selection { selector = p: { inherit (p) ghc864; }; })

This works:

let
  all-hies = import (fetchTarball "https://github.com/infinisil/all-hies/tarball/master") {};
  config = {
    allowUnfree = true;
    
    packageOverrides = pkgs: with pkgs; 
      let jdk = openjdk11; in rec {
      unstable = import <nixpkgs> { inherit config; };
      
      all = pkgs.buildEnv {
        name = "all";

        paths = [
          htop
          nox
          nix-info
          nix-index
          google-chrome
          nodejs-11_x
          binutils.bintools
          haskell.compiler.ghc864
          haskellPackages.cabal-install
          unstable.haskellPackages.stack
          unstable.haskellPackages.cabal2nix
          haskellPackages.hoogle
          haskellPackages.ghcid
          haskellPackages.phoityne-vscode
          haskellPackages.haskell-dap
          (all-hies.selection { selector = p: { inherit (p) ghc864; }; })
        ];
      };
    };
  };
in config