How to convert an AsyncSequence into a Sequence

Paul Hudson @twostraws November 28th 2021

Updated for Xcode 14.2

Swift does not provide a built-in way of converting an AsyncSequence into a regular Sequence, but often you’ll want to make this conversion yourself so you don’t need to keep awaiting results to come back in the future.

The easiest thing to do is call reduce(into:) on the sequence, appending each item to an array of the sequence’s element type. To make this more reusable, I’d recommend adding an extension such as this one:

extension AsyncSequence {
    func collect() async rethrows -> [Element] {
        try await reduce(into: [Element]()) { $0.append($1) }
    }
}

With that in place, you can now call collect() on any async sequence in order to get a simple array of its values. Because this is an async operation, you must call it using await like so:

extension AsyncSequence {
    func collect() async rethrows -> [Element] {
        try await reduce(into: [Element]()) { $0.append($1) }
    }
}

func getNumberArray() async throws -> [Int] {
    let url = URL(string: "https://hws.dev/random-numbers.txt")!
    let numbers = url.lines.compactMap(Int.init)
    return try await numbers.collect()
}

if let numbers = try? await getNumberArray() {
    for number in numbers {
        print(number)
    }
}

Download this as an Xcode project

Tip: Because we’ve made collect() use rethrows, you only need to call it using try if the call to reduce() would normally throw, so if you have an async sequence that doesn’t throw errors you can skip try entirely.

How to convert an AsyncSequence into a Sequence

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