Migrate to Nix flakes

[fwcd]

While Nix derivations, which we use to package up the programs, produce mostly reproducible builds via nix-build, the Nix ecosystem increasingly seems to be moving in the direction of flakes¹², which provide some conveniences and even more reproducible builds by e.g. locking the nixpkgs revision.

Note

Flakes are another, higher-level notion of package, that in of itself only define a set of inputs (dependencies) and outputs (an attribute set)¹. By convention, however, keys such as packages and apps are used to declare derivations and executables in the output, such that commands like nix build and nix run know what to work with.

For this reason we will migrate this repo to flakes too. Specifically:

• Each day will become a flake, declaring a default derivation ("package") and an executable ("app"), such that we can use nix run to build and run the program
• The default derivation will be moved into derivation.nix using nixpkgs-style dependency injection (i.e. { stdenv, ...}: ... instead of { pkgs }).
  • This seems to be the idiomatic approach, it requires us to use callPackage³ instead of import, however, and also means that we can no longer build the days with the legacy nix-build command. In principle, one could write a wrapper .nix that invokes callPackage, like the flake does⁴, but to keep things uniform, we will only provide a flake for now.
  • With this approach, we will also inject vendored derivations (e.g. b-compiler or pakcs) from the flake, which are now also declared in the nixpkgs-style format³.

Footnotes

1. A great write-up of Nix flakes: https://www.reddit.com/r/NixOS/comments/131fvqs/can_someone_explain_to_me_what_a_flake_is_like_im/ ↩ ↩²

2. Another great write-up: https://christine.website/blog/nix-flakes-1-2022-02-21/ ↩

3. A nice explanation and motivation of callPackage: https://summer.nixos.org/blog/callpackage-a-tool-for-the-lazy/ ↩ ↩²

4. A good walkthrough of (non-flake) Nix packaging, along with an extensive example can be found here: https://unix.stackexchange.com/a/717169/556223 ↩