how about

{
  jupyterLib.mkKernels = {
    python = {
      # the env should be a individual python env
      env = pkgs.python3.withPackages [ ];
      # if env == {}, then env = pkgs.python3.withPackages [ ] ++ [python.pkgs.ipykernel];
      withPackages = [];
      override = {
        ...
      };
    };
    haskell = {
      # env = pkgs.haskellPackages.ghcWithPackages (p: with p; [ ihaskell ]);
      # if env == {}, then env = pkgs.haskellPackages.ghcWithPackages (p: with p; [ ihaskell ]) ++ ghcWithPackages p);
      ghcWithPackages = p: [ p.ihaskell ];
      override = {
        display_name = "Custom Haskell";
      };
    };
    ...
  };
}