Concurrency support

[seanmrich]

Building swift-gen with swift 6 raises diagnostics around the static declarations. For example:

extension Gen where Value == Bool {
  /// A generator of random boolean values.
  public static let bool = Gen { rng in Bool.random(using: &rng) }
  //               👆
  // Static property 'bool' is not concurrency-safe because non-'Sendable' type 'Gen<Bool>' may have shared mutable state
}

The problem of course is that Gen isn't Sendable. I was going to put together a pull request to add support, but it isn't clear what the right direction would be here. Gen._run would need to be a Sendable closure, but it seems too high a requirement to require this for all users of the type. I'm not aware of a way to define Gen to be Sendable only when its closure is Sendable. Without that, perhaps these static declarations need to be computed properties (at least when in swift 6 language mode)?

[stephencelis]

@seanmrich I think a pull request requiring @Sendable is totally reasonable for the library as currently designed.

In the future we would probably rethink Gen as a protocol hierarchy, which would allow for sendability (and even hashability) at the conformance level, but we're not sure if/when we'll revisit that.

[seanmrich]

I can work on a PR for that, but it would require a tweak. We'd run into the same issue as in my earlier PR since the CI is using swift 5.3. The package's tools version would have to require at least swift 5.5 (maybe even more recent -- can't remember when the logic around static properties was introduced). Not sure if you have time to update to a more modern version.

[mbrandonw]

@seanmrich You can bump the package Swift version all the way up to 5.9 if it makes things easier. Most of our packages are 5.9+ already. Or if you want to keep things a little more backwards compatible 5.8 is also possible, but I don't think we need to go any earlier than that.

[stephencelis]

@seanmrich And sorry if I wasn't clearer in my comment, but I was suggesting bumping the CI to a newer version. Here's an example:

https://github.com/pointfreeco/swift-case-paths/blob/main/.github/workflows/ci.yml#L51-L64

[seanmrich]

That's helpful thanks. Never really used those workflows so the example is very helpful. Let me see what I can do

[seanmrich]

Pull request submitted. I did try simplifying the many zip functions using parameter packs like this:

func zip<each A: Sendable>(_ element: repeat Gen<each A>) -> Gen<(repeat each A)> {
  return Gen { rng in
    (repeat (each element)._run(&rng))
  }
}

Unfortunately the compiler thinks repeat Gen<each A> isn't Sendable. There's already an issue submitted.