Notice: Undefined index: understanding-swift in /var/sites/ on line 89
Why are Swift’s closure parameters inside the braces? - a free tutorial

NEW: Start my new Ultimate Portfolio App course with a free Hacking with Swift+ trial! >>

Why are Swift’s closure parameters inside the braces?

Paul Hudson    @twostraws   

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.

Hacking with Swift is sponsored by ViRE

SPONSORED ViRE offers discoverable way of working with regex. It provides really readable regex experience, code complete & cheat sheet, unit tests, powerful replace system, step-by-step search & replace, regex visual scheme, regex history & playground. ViRE is available on Mac & iPad.

Download on the App Store

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

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!

Unknown user

You are not logged in

Log in or create account

Link copied to your pasteboard.