Nvfetcher

February 8, 2026 Tags: inputs, un-flake

What is Nvfetcher?

It's a Haskell program that takes values from a toml file and generates fetched inputs that I can use in my config. Without having to manually update the Hash, but still remain locked on per commit basis. Same as flake.lock.

Why don't you just use flakes?

Even tho I love flakes, I can acknowledge that they have issues. Namely, in the context of inputs:

• Unchanged follows for inputs will download unnecessary code, that in context of something big as nixpkgs can cost 40MiB of download data per input and per nix flake update;

• You can't define inputs to download stuff only when needed. The ghostty-shaders input will be downloaded during update on both Desktop, a server and even on the phone. Can't decide whether it's needed there or not;

• Inputs are hella slow. No, I am not benchmarking it, just feels like this.

In this case, Nvfetcher comes in handy. Fetchers work only if you import the input.

What about Npins?

I don't like using cli interface to generate fetches. Also It has problems with AppImages.

How to use Nvfetcher?

Go read the docs in the project's repository.

TLDR: define source and fetcher in toml file, 2 files are generated, you import nix file with pkgs.callPackage, or read json file directly with builtins.fromJSON and builtins.readFile if pkgs isn't available on the scope, or you want to import a fetched nix file inside of your nix module.

For example, it works wonders for fetching neovim plugins.

# nvfetcher pins
sources = pkgs.callPackage "${self}/_sources/generated.nix" { };
heirline-components = pkgs.vimUtils.buildVimPlugin {
  name = "heirline-components.nvim";
  doCheck = false;
  src = sources.heirline-components.src;
};

Then I can call heirline-components in a package list to install it. I use self to avoid using relative paths, and keep my config modules independent of location.

And here is a custom example of using the json file directly.

sourcesJson = builtins.fromJSON (builtins.readFile ./_sources/generated.json);

modules = builtins.mapAttrs (
  name: value:
    let
      src = fetchTarball {
      url = "${value.src.url}/archive/${value.src.rev}.tar.gz";
        sha256 = value.src.sha256;
      };
    in
  value // { inherit src; }
) sourcesJson;

This snippet is in my flake.nix. I use it to create a specialArgs in my config called modules. In any file I can add modules to the top and import some file from Nvfetcher inputs.

Demonstration of one of my hosts that is NixOS-WSL based.

{ modules, ... }:
{
 flake.modules.nixos.NixwsL =
 { config, ... }:
 {
   imports = [
     "${modules.nixos-wsl.src}/modules"
   ];
   networking.hostName = "NixwsL";
   wsl = {
     enable = true;
     # if you are wondering what these config options are
     # it will be explained in another article
     defaultUser = "${config.custom.meta.user}";
     startMenuLaunchers = true;
     tarball.configPath = "${config.custom.meta.self}";
     usbip.enable = true;
     useWindowsDriver = true;
   };

   system.stateVersion = "24.05"; # Do you like bunnies?

   nixpkgs.hostPlatform = "x86_64-linux";
 };
}

"${modules.nixos-wsl.src}/modules" is equivalent to importing inputs.nixos-wsl.nixosModules.default. But it will be fetched only if I use NixOS-WSL, and not on other hosts.

Roll the credits

Okay, that's about all I know and use Nvfetcher for, as always, Thanks goes to:

• @berberman - for creating and maintaining such a nice program;

• @iynaix - for information that Nvfetcher exists;

• @sunworms - for sharing a way to parse json file directly;

• You - for reading to the end, have a nice day and reproducible builds.