Nix Flakes

Nix provides flakes as a way to expose packages, OS configuration and so on using version-pinned dependencies[1].

A flake.nix file has to define at least an outputs element[2]. inputs element is used to inject dependencies. The description field is optional:

{
  inputs = <<inputs>>;
 
  outputs = <<outputs>>;
}

flake.nix

inputs is an attribute set (like a hashmap or dictionary in other languages). We are building a NixOS configuration here, so we take the nixpkgs repository as an input to access its wast package repository and configuration system:

{
  nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
 
  <<flake-inputs>>
}

flake.nix:inputs

outputs is a function accepting the evaluated inputs as its argument, and returning another attribute set.

{ self, ... }@inputs:
{
  <<flake-outputs>>
}

flake.nix:outputs

References

[1]

“Flakes - NixOS Wiki,” Nixos.wiki. Accessed: Aug. 17, 2024. [Online]. Available: https://nixos.wiki/wiki/flakes

[2]

A. Wenger, “Making a dev shell with nix flakes,” Fasterthanli.me. Accessed: Aug. 17, 2024. [Online]. Available: https://fasterthanli.me/series/building-a-rust-service-with-nix/part-10