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lazy now works in local contexts

Available from Swift 5.5

Paul Hudson      @twostraws

The lazy keyword has always allowed us to write stored properties that are only calculated when first used, but from Swift 5.5 onwards we can use lazy locally inside a function to create values that work similarly.

This code demonstrates local lazy in action:

func printGreeting(to: String) -> String {
    print("In printGreeting()")
    return "Hello, \(to)"
}

func lazyTest() {
    print("Before lazy")
    lazy var greeting = printGreeting(to: "Paul")
    print("After lazy")
    print(greeting)
}

lazyTest()

When that runs you’ll see “Before lazy” and “After lazy” printed first, followed by “In printGreeting()” then “Hello, Paul” – Swift only runs the printGreeting(to:) code when its result is accessed on the print(greeting) line.

In practice, this feature is going to be really helpful as a way of selectively running code when you have conditions in place: you can prepare the result of some work lazily, and only actual perform the work if it’s still needed later on.

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