NEW! Pre-order my latest book, Testing Swift! >>

< Previous: Variadic functions   Next: Running throwing functions >

Writing throwing functions

Sometimes functions fail because they have bad input, or because something went wrong internally. Swift lets us throw errors from functions by marking them as throws before their return type, then using the throw keyword when something goes wrong.

First we need to define an enum that describes the errors we can throw. These must always be based on Swift’s existing Error type. We’re going to write a function that checks whether a password is good, so we’ll throw an error if the user tries an obvious password:

enum PasswordError: Error {
    case obvious
}

Now we’ll write a checkPassword() function that will throw that error if something goes wrong. This means using the throws keyword before the function’s return value, then using throw PasswordError.obvious if their password is “password”.

Here’s that in Swift:

func checkPassword(_ password: String) throws -> Bool {
    if password == "password" {
        throw PasswordError.obvious
    }

    return true
}

Build for watchOS

Take your existing Swift skills to Apple's tiniest platform – check out Hacking with watchOS!

< Previous: Variadic functions   Next: Running throwing functions >
MASTER SWIFT NOW
Buy Practical iOS 12 Buy Pro Swift Buy Swift Design Patterns Buy Practical iOS 11 Buy Swift Coding Challenges Buy Server-Side Swift (Vapor Edition) Buy Server-Side Swift (Kitura Edition) Buy Hacking with macOS Buy Advanced iOS Volume One Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with Swift Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Beyond Code

Was this page useful? Let me know!

Click here to visit the Hacking with Swift store >>