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Idiomatic Nix

There are quite a few resources out there that share best practices, but no single unified place to find them all. I’m going to try to build on nix.dev’s Best practices, by doing some research as well as examining the code of some of the leaders in the NixOS world. (Tweag, numtide, etc.)

✔️ mdbook-nix-repl for interactive code blocks

I’ve added a flake.nix to the mdbook-nix-repl repo, you can add it as a flake input:

  1. flake.nix:
# flake.nix
{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";

    mdbook-nix-repl.url = "github:saylesss88/mdbook-nix-repl?dir=server";
  };

  outputs = { self, nixpkgs, mdbook-nix-repl, ... }: {
    nixosConfigurations.magic = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      modules = [
        ./configuration.nix

        mdbook-nix-repl.nixosModules.default
      ];
    };
  };
}
  1. configuration.nix:
{ pkgs, ... }:
{
  imports = [
  ];

  # This option is now provided by the module you imported from the flake
  custom.nix-repl-server = {
    enable = true;
    port = 8080;
    tokenFile = "/etc/nix-repl-server.env";
  };
}
  1. Copy the value of NIX_REPL_TOKEN in theme/index.hbs, and add create file /etc/nix-repl-server.env:
# Create the file with strict permissions (root read-only)
sudo touch /etc/nix-repl-server.env
sudo chmod 600 /etc/nix-repl-server.env

# Edit it to add: NIX_REPL_TOKEN=your_token_from_index_hbs
sudo vim /etc/nix-repl-server.env

Expected format:

NIX_REPL_TOKEN=9deb7efadb74b9e962e7911bb5caf3b3fef275a1b915b526
  1. Rebuild, and the server will now be running at boot.

All the following examples are interactive, press play to see the result.(The following examples come directly from nix.dev)

rec {
    a = 1;
    b = a + 2;
}

Use this instead:

let
  a = 1;
in {
    a = a;
    b = a + 2;
}

💡 TIP Self-reference can be achieved by explicitly naming the attribute set:

 let
   argset = {
     a = 1;
     b = argset.a + 2;
  };
in
  argset

Updating nested attribute sets

{ a = 1; b = 2; } // { b = 3; c = 4; }

Updates are shallow, names on the right take precidence:

{ a = { b = 1; }; } // { a = { c = 3; }; }

let pkgs = import <nixpkgs> {}; in
pkgs.lib.recursiveUpdate { a = { b = 1; }; } { a = { c = 3;}; }