Adding Overlays with Flakes

For a more in depth walkthrough of this example, check out our blog post

This flake shows how to use an overlay Nix flake to override a Nixpkgs package and use it in your devbox configuration.

In this example, using the default yarn from Nixpkgs will cause yarn start to fail. To fix this issue, we use an overlay to modify the yarn package to use NodeJS 16 instead of it's default NodeJS 14.

   overlay = (final: prev: {
      yarn = prev.yarn.override { nodejs = final.pkgs.nodejs-16_x; };
   });

The yarn-overlay flake exports the modified yarn package in it's outputs. We can then use this package in our devbox shell by adding it to packages in our devbox.json file.

{
   "packages": [
      "path:./yarn-overlay#yarn"
      "fnm"
   ]
   ...
}

Note: you will need Devbox 0.4.7-dev or later for this to work. You can try it out by exporting DEVBOX_VERSION=0.4.7-dev before running devbox shell.

You can use the flake.nix in the yarn-overlay directory as a template for creating your own overlays.