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Effectful read-only properties

Available from Swift 5.5

Paul Hudson      @twostraws

SE-0310 upgrades Swift’s read-only properties to support the async and throws keywords, either individually or together, making them significantly more flexible.

To demonstrate this, we could create a BundleFile struct that attempts to load the contents of a file in our app’s resource bundle. Because the file might not be there, might be there but can’t be read for some reason, or might be readable but so big it takes time to read, we could mark the contents property as async throws like this:

import Foundation

enum FileError: Error {
    case missing, unreadable

struct BundleFile {
    let filename: String

    var contents: String {
        get async throws {
            guard let url = Bundle.main.url(forResource: filename, withExtension: nil) else {
                throw FileError.missing

            do {
                return try String(contentsOf: url)
            } catch {
                throw FileError.unreadable

Because contents is both async and throwing, we must use try await when trying to read it:

func printHighScores() async throws {
    let file = BundleFile(filename: "highscores")
    try await print(file.contents)
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