default.nix

{ mkDerivation, base, lens, stdenv, webdriver }:
mkDerivation {
  pname = "myproject";
  version = "0.1.0.0";
  src = ./.;
  isLibrary = false;
  isExecutable = true;
  executableHaskellDepends = [ base lens webdriver ];
  license = stdenv.lib.licenses.unfree;
  hydraPlatforms = stdenv.lib.platforms.none;
}

myproject.cabal

-- Initial myproject.cabal generated by cabal init.  For 
-- further documentation, see http://haskell.org/cabal/users-guide/

name:                myproject
version:             0.1.0.0
-- synopsis:            
-- description:         
license:             AllRightsReserved
license-file:        LICENSE
author:              name
maintainer:          email
-- copyright:           
category:            Web
build-type:          Simple
extra-source-files:  ChangeLog.md
cabal-version:       >=1.10

executable myproject
  main-is:             Main.hs
  -- other-modules:       
  -- other-extensions:    
  build-depends:       base
                     , lens
                     , webdriver
  hs-source-dirs:      src
  default-language:    Haskell2010

shell.nix

{ nixpkgs ? import <nixpkgs> {}, compiler ? "default", doBenchmark ? false }:

let

  inherit (nixpkgs) pkgs;
  webdriver = pkgs.haskellPackages.callHackage "webdriver" "0.8.5" {};
  f = { mkDerivation, base, lens, stdenv, webdriver }:
      mkDerivation {
        pname = "myproject";
        version = "0.1.0.0";
        src = ./.;
        isLibrary = false;
        isExecutable = true;
        # buildInputs = [ pkgs.chromeDriver ]; # this is invalid
        executableHaskellDepends = [ base lens webdriver ];
        license = stdenv.lib.licenses.unfree;
        hydraPlatforms = stdenv.lib.platforms.none;
      };

  haskellPackages = if compiler == "default"
                       then pkgs.haskellPackages
                       else pkgs.haskell.packages.${compiler};

  variant = if doBenchmark then pkgs.haskell.lib.doBenchmark else pkgs.lib.id;

  drv = variant (haskellPackages.callPackage f {});

in

  if pkgs.lib.inNixShell then drv.env else drv