Flakes

Flakes are an experimental feature of Nix, introduced in version 2.4.

Enable flakes

To use flakes, you must enable them in your configuration:

• system-wide on NixOS or user-wide with Home Manager:

  nix.settings.experimental-features = [ "nix-command" "flakes" ];

• on any other system, run the following commands:

  mkdir -p ~/.config/nix
  echo 'experimental-features = nix-command flakes' >> ~/.config/nix/nix.conf

What is a Nix Flake?

A Nix Flake is a file named flake.nix placed at the root of a project or a repository.

A Nix Flake contains a Nix attribute set with a schema (certain expected properties). At minimum, it must contain the following attributes:

• inputs, an attribute set
• outputs, a function from resolved inputs to an attribute set.

A minimum valid flake:

{
  inputs = { };
  outputs = inputs: { };
}

Benefits of Nix Flakes

A Nix Flake allows easy source pinning of external resources. Compare the following expressions:

{ pkgs ? import <nixpkgs> {}
}:

...

{ pkgs ? import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/06278c77b5d162e62df170fec307e83f1812d94b.tar.gz") {}
}:

...

The problem with the first one is that it depends on channels, an opaque and stateful mechanism of tracking external dependencies.

The problem with the second one is that the URL of the external resource contains arbitrary commit hashes, and it is not obvious how this should be maintained.

Contrast to a flake:

{
  inputs.nixpkgs.url = "github:NixOS/nixpkgs";

  outputs = inputs: {
    let
      system = builtins.currentSystem;
      pkgs = inputs.nixpkgs.legacyPackages.${system};
    in
    ...
  };
}

This has several advantages:

• the external resources (in this case nixpkgs) are listed cleanly
• the resources are pinned in a separate file called flake.lock
• the lock file can be updated without changing the flake.nix
• we may explicitly define the differences between various host systems