Functions and trailing closures

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Goal

You will write a Swift function, pass a closure as its last argument, and use the trailing-closure shorthand. After this page you will be able to read every SwiftUI view body — they are all trailing closures stacked together.

Prerequisites

• Swift values and variables

What a function is in Swift

A function is a named block of code that takes arguments and returns a value. A closure is the same thing without a name — a block of code you can pass around as a value.

SwiftUI is built on closures. When you write VStack { Text("a") }, the part in braces is a closure passed to VStack. Reading SwiftUI without understanding closures is guessing.

The relevant syntax rules:

• Argument types are written after each parameter name, separated by a colon.
• The return type comes after ->.
• If a function's last parameter is a closure, you can write it outside the parentheses. This is called trailing closure syntax.
• If a function takes only one parameter and that parameter is a closure, you can drop the parentheses entirely.

Try it in a playground

In SwiftBasics.playground, replace the contents with:

import Foundation

func greet(name: String) -> String {
    return "Hello, " + name
}

print(greet(name: "Aarav"))

func transform(value: Int, using operation: (Int) -> Int) -> Int {
    return operation(value)
}

let doubled = transform(value: 4, using: { number in
    return number * 2
})
print(doubled)

let tripled = transform(value: 4) { number in
    number * 3
}
print(tripled)

Run it. The output:

Hello, Aarav
8
12

Read each call carefully:

• greet(name: "Aarav") — argument labels are visible at the call site. This is normal Swift.
• transform(value: 4, using: { ... }) — the closure is passed like any other argument.
• transform(value: 4) { ... } — the same call using trailing-closure syntax. The closure moves outside the parens. return is optional in a single-expression closure.

This last form is what SwiftUI uses everywhere. VStack { ... }, Button("Tap") { ... }, List { ... } — every brace block is a trailing closure.

Next → Structs