How to call async throwing functions

Paul Hudson    @twostraws    November 28th 2021

Updated for Xcode 15

Just like their synchronous counterparts, Swift’s async functions can be throwing or non-throwing depending on how you want them to behave. However, there is a twist: although we mark the function as being async throws, we call the function using try await – the keyword order is flipped.

To demonstrate this in action, here’s a fetchFavorites() method that attempts to download some JSON from my server, decode it into an array of integers, and return the result:

func fetchFavorites() async throws -> [Int] {
    let url = URL(string: "https://hws.dev/user-favorites.json")!
    let (data, _) = try await URLSession.shared.data(from: url)
    return try JSONDecoder().decode([Int].self, from: data)
}

if let favorites = try? await fetchFavorites() {
    print("Fetched \(favorites.count) favorites.")
} else {
    print("Failed to fetch favorites.")
}

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Both fetching data and decoding it are throwing functions, so we need to use try in both those places. Those errors aren’t being handled in the function, so we need to mark fetchFavorites() as also being throwing so Swift can let any errors bubble up to whatever called it.

Notice that the function is marked async throws but the function calls are marked try await – like I said, the keyword order gets reversed. So, it’s “asynchronous, throwing” in the function definition, but “throwing, asynchronous” at the call site. Think of it as unwinding a stack.

Not only does try await read more easily than await try, but it’s also more reflective of what’s actually happening when our code executes: we’re waiting for some work to complete, and when it does complete we’ll check whether it ended up throwing an error or not.

Of course, the other possibility is that they could have try await at the call site and throws async on the function definition, and here’s what the Swift team says about that:

“This order restriction is arbitrary, but it's not harmful, and it eliminates the potential for stylistic debates.”

Personally, if I saw throws async I’d be more likely to write it as “this thing throws an async error” or similar, but as the quote above says ultimately the order is just arbitrary.

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