c.nix

{ nixpkgs ? (import <nixpkgs> {}), haskellPackages ? nixpkgs.haskell.packages.lts-5_14 }:

let
  common = import ../common/common.nix { inherit haskellPackages; };
  ...
  servant07 = nixpkgs.haskell.packages.ghc7103.servant_0_7;
  servant-server07 = nixpkgs.haskell.packages.ghc7103.servant-server_0_7;
  customizedPackages = haskellPackages.override {
    overrides = self: super: {
        servant = nixpkgs.haskell.packages.ghc7103.servant_0_7
          { inherit self; } // { name = "servant"; };
        ...
      };
  };