Explanation of Nix/Nix Flakes to someone who is unfamiliar with them, courtesy of ChatGPT-4

nix_flakes_explanation_chatgpt4.md

Nix Flakes Explained

You

explain nix flakes to me like I'm a software developer but new to Nix.

Then explain the difference between "old Nix/NixOS" and Nix Flakes to me.

ChatGPT

Part 1: Introduction to Nix Flakes

Nix Flakes is a feature introduced in Nix, a powerful package manager for Linux and other Unix systems. It provides atomic upgrades and rollbacks, side-by-side installation of multiple versions of a package, multi-user package management, and easy setup of build environments.

A Flake is essentially a unit of Nix functionality that includes all of its dependencies. It's a way to encapsulate Nix expressions and make them more composable and reusable. Flakes aim to improve the reproducibility, discoverability, and composability of Nix expressions.

Here's a basic structure of a Flake:

{
  description = "A very cool package";

  outputs = { self, nixpkgs }: {
    packages.x86_64-linux.hello = with nixpkgs; stdenv.mkDerivation {
      name = "hello";
      src = self;
      buildPhase = "echo Hello, world! > hello";
      installPhase = "install -D hello $out/bin/hello";
    };
  };
}

This flake has a single output, packages.x86_64-linux.hello, which is a derivation that builds a "hello world" program. The flake itself is built from its source code (self), and it uses the nixpkgs package set, which is passed as an input.

The outputs function returns a set of packages and other values that are produced by the flake. In this case, there's just one package, hello.

Part 2: Difference between "old Nix/NixOS" and Nix Flakes

1. Reproducibility: In the old Nix, although you could pin nixpkgs for a project, it was not as straight-forward and required additional files such as nixpkgs.json or shell.nix. With Flakes, the dependencies are explicitly declared and pinned, improving the reproducibility of builds.
2. Composability: With old Nix, it was harder to reuse parts of other Nix expressions in your own expressions. Nix Flakes introduces a standard structure and API for Nix projects, making it easier to use one flake's outputs inside another.
3. Discoverability: In traditional Nix, there was no straightforward way to query available packages, tests, or applications. The introduction of nix registry in Nix Flakes allows you to query these outputs.
4. CLI Improvements: Nix Flakes comes with a new command-line interface (CLI), which replaces the old nix-* commands. The new CLI is more consistent and easier to use.
5. Evaluation Caching: Nix Flakes can cache the evaluation of a flake's outputs. This means that if you're building or running the same flake multiple times, Nix doesn't have to evaluate the flake's function every time.

In summary, Nix Flakes is an improvement over traditional Nix expressions that aims to make Nix more reproducible, discoverable, and composable. It provides a better way of managing dependencies, and it comes with a new command-line interface that's easier to use. It's a big step forward for the Nix ecosystem.