Some packages need libc to include headers like <time.h>, etc. To satisfy that dependency, we add a reference to the non-existent attribute 'libc', which ought to be overridden as follows in haskell-packages.nix: foo = callPackage ../development/libraries/haskell/foo { libc = pkgs.stdenv.gcc.libc; }; On impure platforms, such as Darwin, that reference will be 'null', but on Linux it will refer to glibc, as expected.