[Solved] Possible nix typing bug

R-VdP · March 6, 2020, 1:49pm

Hey guys

I wrote the following module demonstrating what seems to be a typing bug in nix:

gist.github.com

https://gist.github.com/R-VdP/b18a8cc0e1d8180e1f8887af6f500993

type_error.nix

{ lib, ... }:

with lib;

let
  subOpts = { name, ... }: {
    options = {
      name = mkOption {
        type = types.int;
      };

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The property name has type int but I assign it a value of type str. Nix does not complain when I import this module in my configuration.nix though.

Am I missing something or should nix report this as a typing error?
I am facing this in my actual server config that I am using in production, but I tried to make an example that’s as small as possible to demonstrate the issue.

Thanks in advance for your inputs!

KoviRobi · March 6, 2020, 2:18pm

I get a type error, but remember that Nix is a lazy language, so that the type error doesn’t appear until you use the name. Here is what I did.

Create a file called type_error.nix next to my NixOS configuration with your GitHub Gist.
Put ./type_error.nix into the imports in configuration.nix
$ nix-repl '<nixpkgs/nixos>'

nix-repl> config.opts.test
{ _module = { ... }; name = «error: The option value `opts.test.name' in `/home/rmk35/nixos/type_error.nix' is not of type `signed integer'.»; }

R-VdP · March 6, 2020, 7:32pm

You are totally right, I was underestimating nix’ laziness and particularly the fact that type checking as well happens lazily at evaluation time.

Thanks, case closed!