default.nix

{ mkDerivation, aeson, attoparsec, base, bytestring
, bytestring-conversion, case-insensitive, directory, doctest
, filemanip, filepath, hspec, http-media, http-types, network-uri
, parsec, QuickCheck, quickcheck-instances, stdenv
, string-conversions, text, url, vault
}:
mkDerivation {
  pname = "servant";
  version = "0.5";
  src = ./.;
  libraryHaskellDepends = [
    aeson attoparsec base bytestring bytestring-conversion
    case-insensitive http-media http-types network-uri
    string-conversions text vault
  ];
  testHaskellDepends = [
    aeson attoparsec base bytestring directory doctest filemanip
    filepath hspec parsec QuickCheck quickcheck-instances
    string-conversions text url
  ];
  homepage = "http://haskell-servant.github.io/";
  description = "A family of combinators for defining webservices APIs";
  license = stdenv.lib.licenses.bsd3;
}

Folder structure

/servant
  /servant
    default.nix
    shell.nix
  /servant-client
    default.nix
    shell.nix
  /servant-server
    default.nix
    shell.nix

shell.nix

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.appendConfigureFlag (self.callPackage ./. {}) "--ghc-options=-Werror";
    };
  };

in haskellPackages.servant.env