swift2nix: Run Swift inside Nix builds

Posted on October 5, 2020

While working on a Swift project, I didn’t find any tool that would allow Swift to run inside Nix builds. Even thought you can run Swift, the real problem arises when using the package manager. It has many of the same problems that other package managers have when trying to integrate with Nix, more on this below.

I wrote a simple little tool called swift2nix that allows you trick Swift’s package manager into assuming everything is set up. Here’s the example from swift2nix’s README file:

  niv-sources = import ./nix/sources.nix;
  pkgs = import niv-sources.nixpkgs { };
  src = pkgs.nix-gitignore.gitignoreSource [ ] ./.;
  swift2nix = pkgs.callPackage niv-sources.swift2nix {
    package-resolved = ./Package.resolved;
in pkgs.stdenv.mkDerivation {
  inherit src;
  name = "swift-test";
  buildInputs = with pkgs; [ swift ];
  phases = [ "unpackPhase" "buildPhase" ];
  buildPhase = ''
    # Setup dependencies path to satisfy SwiftPM
    mkdir .build
    ln -s ${swift2nix.env.dependencies-state-json} .build/dependencies-state.json
    ln -s ${swift2nix.env.checkouts} .build/checkouts

    # Run the tests
    swift test
    touch $out

The key parts are lines 15~17: we just fake enough files inside .build/ that Swift believes it has already downloaded and checked-out all dependencies, and just moves on to building them.

I’ve worked on it just enough to make it usable for myself, so beware of unimplemented cases. Patches welcome.


What swift2nix does is just provide you with the bare minimum that Swift requires, and readily get out of the way:

  1. I explicitly did not want to generated a Package.nix file, since Package.resolved already exists and contains the required information;
  2. I didn’t want to have an “easy” interface right out of the gate, after fighting with “*2nix” tools that focus too much on that.

The final actual code was so small (46 lines) that it made me think about package managers, “*2nix” tools and some problems with many of them.

Problems with package managers

I’m going to talk about solely language package managers. Think npm and cargo, not apt-get.

Package managers want to do too much, or assume too much, or just want to take control of the entire build of the dependencies.

This is a recurrent problem in package managers, but I don’t see it as an intrinsic one. There’s nothing about a “package manager” that prevents it from declaring what it expects to encounter and in which format. The declaring part is important: it should be data, not code, otherwise you’re back in the same problem, just like lockfiles are just data. Those work in any language, and tools can cooperate happily.

There’s no need for this declarative expectation to be standardized, or be made compatible across languages. That would lead to a poor format that no package manager really likes. Instead, If every package manager could say out loud what it wants to see exactly, than more tools like swift2nix could exist, and they would be more reliable.

This could even work fully offline, and be simply a mapping from the lockfile (the Package.resolved in Swift’s case) to the filesystem representation. For Swift, the .build/dependencies-state.json comes very close, but it is internal to the package manager.

Even though this pain only exists when trying to use Swift inside Nix, it sheds light into this common implicit coupling that package managers have. They usually have fuzzy boundaries and tight coupling between:

  1. resolving the dependency tree and using some heuristic to pick a package version;
  2. generating a lockfile with the exact pinned versions;
  3. downloading the dependencies present on the lockfile into some local cache;
  4. arranging the dependencies from the cache in a meaningful way for itself inside the project;
  5. work using the dependencies while assuming that step 4 was done.

When you run npm install in a repository with no lockfile, it does 1~4. If you do the same with cargo build, it does 1~5. That’s too much: many of those assumptions are implicit and internal to the package manager, and if you ever need to rearrange them, you’re on your own. Even though you can perform some of those steps, you can’t compose or rearrange them.

Instead a much saner approach could be:

  1. this stays the same;
  2. this also stays the same;
  3. be able to generate some JSON/TOML/edn which represents the local expected filesystem layout with dependencies (i.e. exposing what the package manager expects to find), let’s call it local-registry.json;
  4. if a local-registry.json was provided, do a build using that. Otherwise generate its own, by downloading the dependencies, arranging them, etc.

The point is just making what the package manager requires visible to the outside world via some declarative data. If this data wasn’t provided, it can move on to doing its own automatic things.

By making the expectation explicit and public, one can plug tools à la carte if desired, but doesn’t prevent the default code path of doing things the exact same way they are now.

Problems with “*2nix” tools

I have to admit: I’m unhappy with most of they.

They conflate “using Nix” with “replicating every command of the package manager inside Nix”.

The avoidance of an “easy” interface that I mentioned above comes from me fighting with some of the “*2nix” tools much like I have to fight with package managers: I don’t want to offload all build responsibilities to the “*2nix” tool, I just want to let it download some of the dependencies and get out of the way. I want to stick with npm test or cargo build, and Nix should only provide the environment.

This is something that node2nix does right. It allows you to build the Node.js environment to satisfy NPM, and you can keep using NPM for everything else:

ln -s ${node2nix-package.shell.nodeDependencies}/lib/node_modules ./node_modules
npm test

Its natural to want to put as much things into Nix as possible to benefit from Nix’s advantages. Isn’t that how NixOS itself was born?

But a “*2nix” tool should leverage Nix, not be coupled with it. The above example lets you run any arbitrary NPM command while profiting from isolation and reproducibility that Nix provides. It is even less brittle: any changes to how NPM runs some things will be future-compatible, since node2nix isn’t trying to replicate what NPM does, or fiddling with NPM’s internal.

A “*2nix” tool should build the environment, preferably from the lockfile directly and offload everything else to the package manager. The rest is just nice-to-have.

swift2nix itself could provide an “easy” interface, something that allows you to write:

nix-build -A swift2nix.release
nix-build -A swift2nix.test

The implementation of those would be obvious: create a new pkgs.stdenv.mkDerivation and call swift build -c release and swift test while using swift2nix.env under the hood.


Package managers should provide exact dependencies via a data representation, i.e. lockfiles, and expose via another data representation how they expect those dependencies to appear on the filesystem, i.e. local-registry.json. This allows package managers to provide an API so that external tools can create mirrors, offline builds, other registries, isolated builds, etc.

”*2nix” tools should build simple functions that leverage that local-registry.json1 data and offload all the rest back to the package manager itself. This allows the “*2nix” to not keep chasing the package manager evolution, always trying to duplicate its behaviour.

  1. This local-registry.json file doesn’t have to be checked-in the repository at all. It could be always generated on the fly, much like how Swift’s dependencies-state.json is.