Multi-pattern catch clauses

Paul Hudson      @twostraws

SE-0276 introduced the ability to catch multiple error cases inside a single catch block, which allows us to remove some duplication in our error handling.

For example, we might have some code that defines two enum cases for an error:

enum TemperatureError: Error {
    case tooCold, tooHot
}

When reading the temperature of something, we can either throw one of those errors, or send back “OK”:

func getReactorTemperature() -> Int {
    90
}

func checkReactorOperational() throws -> String {
    let temp = getReactorTemperature()

    if temp < 10 {
        throw TemperatureError.tooCold
    } else if temp > 90 {
        throw TemperatureError.tooHot
    } else {
        return "OK"
    }
}

When it comes to catching errors thrown there, SE-0276 lets us handle both tooHot and tooCold in the same way by separating them with a comma:

do {
    let result = try checkReactorOperational()
    print("Result: \(result)")
} catch TemperatureError.tooHot, TemperatureError.tooCold {
    print("Shut down the reactor!")
} catch {
    print("An unknown error occurred.")
}

You can handle as many error cases as you want, and you can even bind values from your errors if needed.

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Other changes in Swift 5.3…

• Multiple trailing closures
• Synthesized Comparable conformance for enums
• self is no longer required in many places
• Type-based program entry points
• where clauses on contextually generic declarations
• Enum cases as protocol witnesses
• Refined didSet semantics
• A new Float16 type
• Swift Package Manager gains binary dependencies, resources, and more

Download all Swift 5.3 changes as a playground Link to Swift 5.3 changes

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