Example of a Haskell packages (GHC 8.4.1) overlay.

haskell.nix

self: super:
let
  inherit (super.haskell.lib) dontCheck;
in
{
  glibc_static = super.glibc.static;
  zlib_static = super.zlib.static;
  gmp_static = super.gmp6.override { withStatic = true; };

  haskell = super.haskell // {
    packages = super.haskell.packages // {
      ghc841 = super.haskell.packages.ghc841.override {
        overrides = self: super: {
          # Internal project
          blablabla-core = super.callPackage ../../core {};
          # overcome a formatting issue in the tests
          servant-swagger = dontCheck super.servant-swagger;
        };
      };
    };
  };
}