Effectful read-only properties

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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Other changes in Swift 5.5…

• Async await
• Async sequences
• Structured concurrency
• async let bindings
• Continuations for interfacing async tasks with synchronous code
• Actors
• Global actors
• Sendable and @Sendable closures
• if for postfix member expressions
• Interchangeable use of CGFloat and Double types
• Codable synthesis for enums with associated values
• lazy now works in local contexts
• Extending property wrappers to function and closure parameters
• Extending static member lookup in generic contexts

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