servant-env.nix

let
  pkgs = import <nixpkgs> {};
  stdenv = pkgs.stdenv;
  haskellPackages = pkgs.haskell-ng.packages.ghc7102.override {
    overrides = self: super: {
      servant = self.callPackage ~/Work/opensrc/servant/servant.nix {};
    };
  };
  inputs = [
    haskellPackages.ghc-mod
    haskellPackages.cabal-install
  ];
in rec {
  servant7102Env = stdenv.lib.overrideDerivation haskellPackages.servant.env (old: {
    name = "servant-7102-env";

    # Append our inputs to the old build inputs
    buildInputs = old.buildInputs ++ inputs;
    NIX_MYENV_NAME = "servant-7102";
    shellHook = ''
      source ~/.bash_prompt
      # Run the old shell hook of the haskell environment
      ${old.shellHook}
    '';
  });
}

servant.nix

// This works with nix-shell but not if I try and nix-instantiate the servant-env.nix file
// I get this error: error: attempt to call something which is not a function but a set, at /usr/local/nix/nixpkgs/lib/customisation.nix:56:12
let
  pkgs = import <nixpkgs> {};

  haskellPackages = pkgs.haskellPackages.override {
    overrides = self: super: {
      servant-client = pkgs.haskell.lib.dontCheck (pkgs.haskell.lib.appendConfigureFlag (self.callPackage ./servant-client {}) "--ghc-options=-Werror");
      servant-docs = pkgs.haskell.lib.dontCheck (pkgs.haskell.lib.appendConfigureFlag (self.callPackage ./servant-docs {}) "--ghc-options=-Werror");
      servant-js = pkgs.haskell.lib.dontCheck (pkgs.haskell.lib.appendConfigureFlag (self.callPackage ./servant-js {}) "--ghc-options=-Werror");
      servant-server = pkgs.haskell.lib.dontCheck (pkgs.haskell.lib.appendConfigureFlag (self.callPackage ./servant-server {}) "--ghc-options=-Werror");
      servant-examples = pkgs.haskell.lib.dontCheck (pkgs.haskell.lib.appendConfigureFlag (self.callPackage ./servant-examples {}) "--ghc-options=-Werror");
      servant-blaze = pkgs.haskell.lib.dontCheck (pkgs.haskell.lib.appendConfigureFlag (self.callPackage ./servant-blaze {}) "--ghc-options=-Werror");
      servant-lucid = pkgs.haskell.lib.dontCheck (pkgs.haskell.lib.appendConfigureFlag (self.callPackage ./servant-lucid {}) "--ghc-options=-Werror");
      servant-mock = pkgs.haskell.lib.dontCheck (pkgs.haskell.lib.appendConfigureFlag (self.callPackage ./servant-mock {}) "--ghc-options=-Werror");
      servant = pkgs.haskell.lib.dontCheck (pkgs.haskell.lib.appendConfigureFlag (self.callPackage ./servant {}) "--ghc-options=-Werror");
    };
  };
  aggregated = haskellPackages.mkDerivation {
    pname = "servant-aggregated";
    version = "0.5.0";
    src = null;
    license = null;
    libraryHaskellDepends = with haskellPackages; [
      servant-client servant-docs servant-js servant-server
      servant-examples servant-blaze servant-lucid servant
      servant-mock
    ];
    librarySystemDepends = [ ];
  };

in aggregated.env