How to check for valid method input using the guard keyword

Swift version: 5.0

Paul Hudson    @twostraws    March 28th 2019

The guard keyword was introduced in Swift to signal early returns, which is a coding technique that effectively means "make sure all these things are set up before I start doing the real work in my function, others bail out."

For example, if you want to ensure a submit() is only ever run if an existing name property has a value, you would do this:

func submit() {
    guard name != nil else { return }

    doImportantWork(name)
}

This might seem like a job for a regular if statement, and to be fair that's correct – the two are very similar. The advantage with guard, however, is that it makes your intention clear: these values need to be set up correctly before continuing.

The guard keyword is also helpful because it can be used to check and unwrap optionals that remain unwrapped until the end of the method. For example:

func betterSubmit() {
    guard let unwrappedName = name else { return }

    doImportantWork(unwrappedName)
}

So, if name is nil the method will return; otherwise, it will be safely unwrapped into unwrappedName.

Available from iOS 7.0

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