How to throw errors using strings

Swift version: 5.2

Paul Hudson    @twostraws    May 28th 2019

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.

SPONSORED ViRE offers discoverable way of working with regex. It provides really readable regex experience, code complete & cheat sheet, unit tests, powerful replace system, step-by-step search & replace, regex visual scheme, regex history & playground. ViRE is available on Mac & iPad.

Download on the App Store

Sponsor Hacking with Swift and reach the world's largest Swift community!

Available from iOS 8.0

Similar solutions…

• How to add warnings and errors to your code using #warning and #error
• Common SwiftUI errors and how to fix them
• How to concatenate strings to make one joined string
• How to create rich formatted text strings using NSAttributedString
• How to join an array of strings in a natural way

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.