WWDC22 SALE: Save 50% on all my Swift books and bundles! >>

Why are Swift’s closure parameters inside the braces?

Paul Hudson    @twostraws   

Updated for Xcode 14.0 beta 1

Both closures and functions can take parameters, but the way they take parameters is very different. Here’s a function that accepts a string and an integer:

func pay(user: String, amount: Int) {
    // code
}

And here’s exactly the same thing written as a closure:

let payment = { (user: String, amount: Int) in
    // code
}

As you can see, the parameters have moved inside the braces, and the in keyword is there to mark the end of the parameter list and the start of the closure’s body itself.

Closures take their parameters inside the brace to avoid confusing Swift: if we had written let payment = (user: String, amount: Int) then it would look like we were trying to create a tuple, not a closure, which would be strange.

If you think about it, having the parameters inside the braces also neatly captures the way that whole thing is one block of data stored inside the variable – the parameter list and the closure body are all part of the same lump of code, and stored in our variable.

Having the parameter list inside the braces shows why the in keyword is so important – without that it’s hard for Swift to know where your closure body actually starts, because there’s no second set of braces.

Save 50% in my Black Friday sale.

SAVE 50% To celebrate WWDC22, all our books and bundles are half price, so you can take your Swift knowledge further without spending big! Get the Swift Power Pack to build your iOS career faster, get the Swift Platform Pack to builds apps for macOS, watchOS, and beyond, or get the Swift Plus Pack to learn advanced design patterns, testing skills, and more.

Save 50% on all our books and bundles!

Sponsor Hacking with Swift and reach the world's largest Swift community!

BUY OUR BOOKS
Buy Pro Swift Buy Swift Design Patterns Buy Testing Swift Buy Hacking with iOS Buy Swift Coding Challenges Buy Swift on Sundays Volume One Buy Server-Side Swift (Vapor Edition) Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Advanced iOS Volume Three Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with macOS Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Server-Side Swift (Kitura Edition) Buy Beyond Code

Was this page useful? Let us know!

Average rating: 4.9/5

 
Unknown user

You are not logged in

Log in or create account
 

Link copied to your pasteboard.