Why are Swift’s closure parameters inside the braces?

Paul Hudson @twostraws May 28th 2020

Updated for Xcode 12.0

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.