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Concurrency in top-level code

Available from Swift 5.7

Paul Hudson      @twostraws

SE-0343 upgrades Swift’s support for top-level code – think main.swift in a macOS Command Line Tool project – so that it supports concurrency out of the box. This is one of those changes that might seem trivial on the surface, but took a lot of work to make happen.

In practice, it means you can write code like this directly into your main.swift files:

let url = URL(string: "https://hws.dev/readings.json")!
let (data, _) = try await URLSession.shared.data(from: url)
let readings = try JSONDecoder().decode([Double].self, from: data)
print("Found \(readings.count) temperature readings")

Previously, we had to create a new @main struct that had an asynchronous main() method, so this new, simpler approach is a big improvement.

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