[Nix-dev] Fixing module arguments (PROPOSAL MAY BREAK EXISTING NIXOS CONFIGS, PLEASE READ)

[Nicolas Pierron]

Hi all,

On Wed, Feb 19, 2014 at 4:44 PM, Shea Levy <shea at shealevy.com> wrote:
> Hi all,
>
> The Problem
> -----------
>
> I've been doing a bit of work to make it possible to define nixops
> networks modularly in the same way we do NixOS configs (with each
> machine in a network being a submodule),

I think this is a good idea, that's what I tried to motivate during
the making of disnix, but apparently failed to explain / explicit to
get it adopted.

> and in doing so I've repeatedly
> run into issues due to the fact that NixOS's modules take a bunch of
> arguments beyond config, options, and lib, in particluar that they take
> 'pkgs'. The problem with these extra arguments in general is that they
> are superfluous and non-modular: superfluous, because any internal
> values we want to make accessible to all modules can be put into config;

Yes for packages, even if they are extremely convenient to have as a
first argument, but no for the module logic. (mkIf, mkOption, ...)

Also, having pkgs as an argument is a convenient shortcut for all
expressions.  One way to transition would be to define pkgs as being
extracted from the result.

let
  moduleWithPkgs = result: m: args: m (args // { pkgs =
result.config.nixpkgs.pkgs });
in
  ...

I have always made changes such as we can give a LONG grace period for
migrating.  People might remember the "require" attributes, the
rename.nix file[1], or the "pkgs.types.loaOf".

The reason why I did so is the same reason why I switched to NixOS in
the first place, *not having to* update, *but choosing when I want to*
update.

This is exactly the same idea we have behind the "--rollback" option.
Even if the rollback option is not evaluating Nix code, we still want
users to experiment with their old configuration for a while, such as
they are not forced to update at one time, but within a grace period.

In terms of solution, I have no problem adding a pkgs argument to all
sub-modules arguments, even if it defaults to an error message when
the "pkgs" option does not exist.  We can do that while we ensure that
all mkFunctions are extracted from the "lib" attribute, and not
"pkgs.lib".  Then, later we can remove this pkgs argument.

[1] https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/rename.nix

-- 
Nicolas Pierron
http://www.linkedin.com/in/nicolasbpierron - http://nbp.name/