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How to throw errors using strings

Swift version: 5.1

Paul Hudson    @twostraws   

Throwing functions in Swift are the main way we have of signaling that an operation failed, but sometimes it can be annoying to define a whole new error enum just to report a simple failure.

With a small extension to String you can make the whole process easier:

extension String: LocalizedError {
    public var errorDescription: String? { return self }
}

With that change you can now throw strings as errors, and they work just like regular errors. For example, you can create a throwing function like this one:

func doDangerousThing() throws {
    throw "I'm sorry, Dave, I can't do that."
}

Then attempt to run it and print any errors that are thrown, like this:

do {
    try doDangerousThing()
} catch {
    print(error.localizedDescription)
}

This only really works for errors that have one case, because you can’t match specific string errors.

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Available from iOS 8.0

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