Using dependencies with async let

[finestructure]

I ran into a problem using a dependency with async let. It took me a moment to discover the (I think) proper way of dealing with this and so I thought I'd share my solution in case it might help others in the same situation.

I was transitioning code like the following (using the "old" Current-based dependency injection style):

async let metadata = try await Current.fetchMetadata(owner, repository)
async let license = await Current.fetchLicense(owner, repository)

to the new @Dependency style:

@Dependency(\.github) var github
async let metadata = try await github.fetchMetadata(owner, repository)
async let license = await github.fetchLicense(owner, repository)

Turns out this doesn't actually work in Swift 6 language mode. It fails to compile with:

Sending 'github' into async let risks causing data races between async let uses and local uses

and that makes sense if you think about it. Under the hood, async let is a Task and we're sharing github into two different Tasks here.

At first I came up with a clumsy workaround:

func _fetchMetadata() async throws(Github.Error) -> Github.Metadata {
    @Dependency(\.github) var github
    return try await github.fetchMetadata(owner, repository)
}
func _fetchLicense() async -> Github.License? {
    @Dependency(\.github) var github
    return await github.fetchLicense(owner, repository)
}
async let metadata = try await _fetchMetadata()
async let license = await _fetchLicense()

but then I realised that I can simply be more surgical with the dependency variables:

@Dependency(\.github.fetchMetadata) var fetchMetadata
@Dependency(\.github.fetchLicense) var fetchLicense
async let metadata = try await fetchMetadata(owner, repository)
async let license = await fetchLicense(owner, repository)

Hope that helps!

[stephencelis]

@finestructure Dependency is Sendable and so is your GitHub client (a requirement of the library), so sharing it across two endpoints is fine. This looks more like an unfortunate property of property wrappers being var. If you copy the value over to a let it works:

@Dependency(\.github) var _github
let github = _github
async let metadata = try await github.fetchMetadata(owner, repository)
async let license = await github.fetchLicense(owner, repository)

I wonder if this is worth bringing up on the Swift forums. If folks have an idea of how we could make things play more nicely, we're all ears!