nix: use specific revision of servant

gistfile1.nix

let
  filterHaskellSource = builtins.filterSource (path: type:
    type != "unknown" &&
    baseNameOf path != ".git" &&
    baseNameOf path != "result" &&
    baseNameOf path != "dist");
in
{ pkgs ? import <nixpkgs> { }
, src ? filterHaskellSource ./.
}:
let
  servantSrc = filterHaskellSource <servant>;
  servant-new-impl = pkgs.haskellPackages.buildLocalCabalWithArgs {
    name = "servant";
    src = "${servantSrc}/servant/";
  };
in
pkgs.haskellPackages.buildLocalCabalWithArgs {
  inherit src;
  name = "kraken";
  args = {
    servant = servant-new-impl;
  };
}

Nice. I was thinking of:

{ pkgs ? import <nixpkgs> { config.allowUnfree = true; }
, src ?  builtins.filterSource (path: type:
    type != "unknown" &&
    baseNameOf path != ".git" &&
    baseNameOf path != "result" &&
    baseNameOf path != "dist") ./.
, servant
}:
pkgs.haskellPackages.buildLocalCabalWithArgs {
  name = "isourcerer";
  inherit src;
  args = {
    servant = servant;
  };
}

with nix-shell --arg servant "import ../servant/servant {}".

But having a default but overridable git commit would be ideal.