Implicit returns from single-expression functions

Paul Hudson      @twostraws

SE-0255 has removed a small but important inconsistency in the language: single-expression functions that return a value can now remove the return keyword and Swift will understand it implicitly.

In previous versions of Swift, single-line closures that returned a value you could omit the return keyword because the only line of code that was there must be the one that returned a value. So, these two pieces of code were identical:

let doubled1 = [1, 2, 3].map { $0 * 2 }
let doubled2 = [1, 2, 3].map { return $0 * 2 }

In Swift 5.1, this behavior has now been extended to functions as well: if they contain a single expression – effectively a single piece of code that evaluates to a value – then you can leave off the return keyword, like this:

func double(_ number: Int) -> Int {
    number * 2
}

That will probably cause some people to do a double take at first, but I’m sure it will become second nature over time.

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Other changes in Swift 5.1…

• Improvements to synthesized memberwise initializers
• Universal Self
• Opaque return types
• Static and class subscripts
• Warnings for ambiguous none cases
• Matching optional enums against non-optionals
• Ordered collection diffing
• Creating uninitialized arrays

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