What is a throwing function?

Swift version: 5.10

Paul Hudson @twostraws May 28th 2019

Throwing functions are those that will flag up errors if problems happen, and Swift requires you to handle those errors in your code.

To make a throwing function, just write throws before your function’s return value. You should define the error types you can throw, so users of this function know what to expect.

As an example, try adding this custom error enum to a playground:

enum LoginErrors: Error {
    case badUsername
    case badPassword
}

We can now use that to create a throwing function called login(). If the username is empty we’ll throw badUsername, if the password is empty we’ll throw badPassword, and if both are non-empty we’ll return true:

func login(username: String, password: String) throws -> Bool {
    if username.isEmpty { throw LoginErrors.badUsername }
    if password.isEmpty { throw LoginErrors.badPassword }
    return true
}

Because that function throws errors, it must be called using either try, try?, or try!.

There’s a subtle difference between throwing functions and functions that return optionals, but it’s usually a better idea to use throwing functions if there might be several different reasons why the function failed.