Manage macOS with Nix flakes

🚧 This post is under construction. 🚧

A flake is a directory that contains a file named flake.nix in the root directory. flake.nix specifies some metadata about the flake such as dependencies (called inputs), as well as its /outputs (the Nix values such as packages or NixOS modules provided by the flake).

To ensure reproducibility, Nix will automatically generate and use a lock file called flake.lock in the flake’s directory. The lock file contains a graph structure isomorphic to the graph of dependencies of the root flake.

– RFC 0049: Flakes

Nix flakes

According to the definition, the structure of a flake is as simple as:

.
├── flake.lock
└── flake.nix

The flake.nix has these attributes:

• description: what is the flake for?
• inputs: an attribute set specifying the dependencies of the flake.
• outputs: a function which accepts self¹ and attributes of inputs as its params, and returns the Nix values as its results.

A basic flake.nix is as simple as the following:

{
  description = "A basic flake";
  inputs = { };
  outputs = { self }: { };
}

Let’s create a flake-example directory for the flake.nix, thus, the flake-example is called a flake. Run nix build in the flake for the first time:

~/flake-example> nix build
error: flake 'path:/Users/james/flake-example' does not provide attribute 'packages.aarch64-darwin.default' or 'defaultPackage.aarch64-darwin'

The error means something is missing in the outputs, let’s set the result to self for a quick experiment:

{
  description = "A basic flake";
  inputs = { };
  outputs = { self }:
    let
      system = "aarch64-darwin";
    in {
      packages.${system}.default = self;
    };
}

Run nix build and another error rises:

~/flake-example> nix build
error: flake output attribute 'packages.aarch64-darwin.default' is not a derivation or path

The flake output attribute packages.aarch64-darwin.default should be a derivation or path². Derivation is an important concept in Nix, but to keep things simple, let’s put it aside and set the output attribute to a Nix path:

{
  description = "A basic flake";
  inputs = { };
  outputs = { self }:
    let
      system = "aarch64-darwin";
    in {
      packages.${system}.default = ./.;
    };
}

./. denotes the current directory, similarly, ./flake.nix denotes the flake.nix file in the current directory.

Run nix build and it is successful:

~/flake-example> tree
.
├── flake.lock
├── flake.nix
└── result -> /nix/store/rl108r5a3nwxyciwj0ibgdi3d55m1rxx-source

The result symbolic link is the output of the build result, which is located in the /nix/store directory.

~/flake-example> tree result
result
├── flake.lock
└── flake.nix

You can see the result directory has the same files as the flake-example directory. So our first flake is done, it makes a copy of current directory as outputs.