Sorry if this is an ignorant question. I love Nix, but as a user it feels absurd that changing an option from true to false in a home-manager managed config file that then gets transpiled from Nix to TOML, for instance, takes like 10 seconds on a decently modern machine.
In my world that kind of transpilation should be instant. I get that there's more happening behind the scenes than just doing Nix -> TOML, but still. Imagine if the transpilation of config files were instant, so you could have home-manager automagically do the transpilation every time you save the file. That would be awesome, especially for programs that support hot-reloading like Hyprland or Niri.
Is this a Nix issue or a home-manager issue? How much of speedup would it yield to rewrite Nix and/or home-manager in a faster language like Zig or Rust?
Well, as you've guessed, it's not really just a transpilation. A NixOS rebuild evaluates significant parts of Nixpkgs,
<nixpkgs/nixos>as well as your configuration from scratch (technically flakes have evaluation caching, but it's not fit for purpose).Nix isn't a fast language to begin with, but the
evalModulescall that actually takes your configuration and turns it into a package that can be built is very expensive. Because NixOS imports (almost) all modules by default, it has to do deep merges of most of<nixpkgs/nixos>, which amounts to almost 32 MB of Nix code. Everything else is a rounding error compared to that.Oh, and it has to do this for Home Manager too, because Home Manager is also built on
evalModulesand also imports all the modules. As isflake-partsif you use that (though in that case the set of modules is small so you probably wouldn't notice).It would probably make the performance worse before making it better. Nix is slow but it's not because of the implementation language (which is C++), you would have to do actual interpreter engineering (e.g. writing a JIT) to make one much faster than the current one.
A builtin (so C++ instead of Nix) implementation of
evalModuleswould probably help, but nobody wants the versioning headaches that would come from that. I'm also not sure it would do that much.A more promising optimization is to make NixOS not import all the modules by default, but efforts to do this haven't really gained traction. Probably because you pretty much have to check all modules for cross dependencies.
As a user the best thing you can do right now is using Home Manager standalone and not as a NixOS module. That way, while it will still be slower than it really should be, it at least won't evaluate and merge all NixOS modules every time you change your mind about a background color.