Nested Functions in Swift: A Comprehensive Guide

In the previous tutorial, I explained how to create a function in Swift, and their importance in keeping the code modular and organized. Well, functions can also be written inside other functions, and Swift’s official documentation suggests this helps to encapsulate useful functionality within a nested function scope.

In this tutorial you will learn how to create a nested function in Swift.

If you are interested in video lessons on how to write Unit tests and UI tests to test your Swift mobile app, check out this page: Unit Testing Swift Mobile App

What is the scope of a function in Swift?

Before I tell you what is a nested function, it is important that you understand what is a scope of a function.

In Swift, the scope of a function refers to the part of your code where the function and its variables can be accessed and used. Think of it like a boundary that defines where the function’s influence starts and ends. When you create a function, it’s like creating a little world for that function to live in. Inside this world, the function can have its own set of instructions (code) and variables that it can play with. These variables are like toys that the function can use while it’s doing its job.

Now, here’s the important part: the scope of a function is limited to the curly braces {} that enclose its code. Anything declared inside those curly braces, like variables or other functions, belongs to that specific function’s world.

Once you step outside those curly braces, the function’s world ends, and its toys (variables or other functions) are packed away. This means you can’t directly access those variables or use the function’s instructions from outside its world.

Let’s take a look at a simple example:

func greet() {
    let message = "Hello, Swift Learner!"
}  // End of the greet function's scope

// Outside the greet function's scope
// Uncommenting the next line will result in an error because 'message' is not known here.
// print(message)

Now that you understand the concept of a function scope, let’s take it a step further and talk about nested functions.

What are Nested Functions in Swift?

In Swift, a nested function is a function declared inside another function. It’s as if you have a mini-world inside the world of another function. This concept might sound a bit like Inception, but it’s a powerful tool for keeping your code organized and modular.

Here’s the deal: when you create a function inside another function, the inner function’s scope is within the curly braces of the outer function. It’s like having a secret helper that only works when called upon by its parent function.

Let’s break it down with an example:

func outerFunction() {
    // This is the outer function's world

    func innerFunction() {
        // This is the inner function's world, nested inside the outer function

        let secretMessage = "I'm the secret helper!"

    // Calling the inner function from the outer function

// Calling the outer function

In this example, innerFunction is nested within the scope of outerFunction. The secret message is known only inside innerFunction, and it can’t be accessed directly from outerFunction or outside its scope. It’s like having a hidden helper that comes to life when needed.

This nesting allows you to keep related functionality together, making your code more readable and organized. Just remember, the inner function’s scope is confined to the curly braces of its parent function.

Nested functions are a handy way to structure your code, creating little worlds of functionality that work together seamlessly within the larger world of your program.

Benefits of Nested Functions

Enhanced Readability

Nested functions play a crucial role in improving code readability. Unlike private functions that might be accessed from various parts of a class, nested functions stay neatly tucked within the specific function where they’re born. This distinct boundary acts as a signpost for you and future developers, clearly stating that these functions belong exclusively to their parent function. This not only makes the code more self-contained but also enhances its clarity, making it easier for you to follow and understand.

Simplified Encapsulation

One of the conveniences of nested functions is their access to all local parameters of their parent function. No more passing parameters around! This results in fewer functions for you to test, as you’ve effectively wrapped one function inside another. Moreover, when calling a nested function within a block of the non-nested function, you don’t have to worry about using “self”. The lifecycle of the nested function is tied to the lifecycle of its containing function, streamlining your coding experience.

To grasp a deeper understanding of the concept of “self” in Swift, it’s highly recommended to explore the fundamentals of classes and how to Create a New Class in the Swift programming language.

Logical Grouping of Functionality

Imagine you have multiple private functions, and some of them are called within a single public function. Nesting them under the same public function not only streamlines your code but also communicates to you that these functions are intricately related. This logical grouping makes your codebase more organized and comprehensible.

Example of Nested Functions in Action

Let’s walk through a simple example to illustrate the practical application of nested functions. Imagine you are building a weather app, and you need to format the temperature based on your preference (Celsius or Fahrenheit).

// Define a function to format temperature based on the given scale (Celsius or Fahrenheit)
func formatTemperature(_ temperature: Double, isCelsius: Bool) -> String {
    // Nested function to convert Celsius to Fahrenheit
    func celsiusToFahrenheit(_ celsius: Double) -> Double {
        return (celsius * 9/5) + 32

    // Check if the temperature is in Celsius
    if isCelsius {
        return "\(temperature)°C" // Return temperature in Celsius format
    } else {
        // If not Celsius, convert temperature to Fahrenheit using the nested function
        let fahrenheit = celsiusToFahrenheit(temperature)
        return "\(fahrenheit)°F" // Return temperature in Fahrenheit format

// Example: Format temperature of 20 degrees Celsius
let temperatureString = formatTemperature(20, isCelsius: true)

// Print the formatted temperature string
print(temperatureString) // Output: 20°C

In this example, the nested function celsiusToFahrenheit is exclusively used within the parent function formatTemperature. This encapsulation allows for clean, focused functionality, making it easier for you to comprehend and maintain.

Can I return a function from another function in Swift?

Sure! Imagine you have a little helper function tucked inside another function. This hidden helper is like a small, specialized task you want to carry around with you. Now, what if I told you that you can not only use this helper inside its parent function but also take it out and use it whenever you need it? That’s where returning a nested function, also known as a closure, comes into play.

Here’s the magic in Swift:

func createGreeter(greeting: String) -> () -> String {
    // This is the outer function's world

    func greet() -> String {
        // This is the nested function's world
        return greeting

    // Returning the nested function as a closure
    return greet

// Creating a greeter that says "Welcome"
let welcomeGreeter = createGreeter(greeting: "Welcome")

// Using the greeter closure to get the greeting message
let greetingMessage = welcomeGreeter()

// Printing the greeting message

In this example, createGreeter is the boss, and knows a little worker called greet. When you ask createGreeter("Welcome") to make a greeter for you, it not only sets up the greeter but hands you the little worker, too. This worker, the nested function greet, is now a closure that remembers its task – in this case, saying “Welcome.”

You can then take this closure, your personalized greeter, and use it whenever you want. It’s like having a greeting machine that you can turn on whenever you need a warm “Welcome.” And that, my friend, is the beauty of returning a nested function, or closure, in Swift. Happy coding!

If you’re intrigued to explore more about closures in Swift, check out our dedicated tutorial Declare a Closure in Swift.


Congratulations, you’ve taken a significant step in your Swift journey by understanding nested functions! You’ve explored their syntax, and benefits, and even walked through a practical example to solidify your understanding. For a deeper dive into functions and more practical applications, check out the tutorial on Swift Function Tutorial with Code Examples. Keep coding and exploring the endless possibilities Swift has to offer!

To learn more about Swift and apps development, check other Swift tutorials and code examples page.