Swift Academy · Lesson

Functions as First-Class Values

Storing functions in variables, passing as arguments and returning from functions.

Lesson 4 of 412 steps

Functions as First-Class Values is a free Swift Academy lesson on CoddyKit. This is lesson 4 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Swift Academy learning path, and your progress syncs across the web and the CoddyKit app. The Swift Academy course includes 4 lessons in total.

Welcome

In Swift, functions are first-class citizens — you can store them in variables, pass them as arguments, return them from other functions, and nest them inside other functions.

Function Types

Every function has a type defined by its parameter and return types: ```swift func add(_ a: Int, _ b: Int) -> Int { a + b } let operation: (Int, Int) -> Int = add print(operation(3, 4)) // 7 ``` The type of `add` is `(Int, Int) -> Int`.

Passing Functions as Arguments

```swift func apply(_ f: (Int) -> Int, to value: Int) -> Int { f(value) } func double(_ n: Int) -> Int { n * 2 } print(apply(double, to: 5)) // 10 ``` This is the foundation of higher-order functions like `map` and `filter`.

Returning Functions

```swift func multiplier(by factor: Int) -> (Int) -> Int { return { value in value * factor } } let triple = multiplier(by: 3) print(triple(7)) // 21 ``` Returning a function creates a *closure* that captures `factor`.

Nested Functions

Functions can be defined inside other functions: ```swift func chooseStep(goForward: Bool) -> (Int) -> Int { func forward(_ n: Int) -> Int { n + 1 } func backward(_ n: Int) -> Int { n - 1 } return goForward ? forward : backward } let step = chooseStep(goForward: true) print(step(3)) // 4 ```

Closures as Shorthand

Anonymous closure literals can stand in for any function type: ```swift let square: (Int) -> Int = { $0 * $0 } print(square(6)) // 36 let doubled = [1, 2, 3].map { $0 * 2 } // [2, 4, 6] ``` `$0` refers to the first argument in the shorthand closure form.

Storing Functions in Collections

```swift let operations: [(Int, Int) -> Int] = [ { $0 + $1 }, { $0 - $1 }, { $0 * $1 } ] for op in operations { print(op(10, 3)) } // 13, 7, 30 ```

Trailing Closure Syntax

When the last argument is a function, you can write it outside the parentheses: ```swift func withLogging(_ label: String, action: () -> Void) { print("Start: \(label)") action() print("End: \(label)") } withLogging("fetch") { fetchData() } ```

Function Identity and Equality

You cannot compare two function values with `==` in Swift — functions are not Equatable: ```swift // let same = add == add ❌ ``` If you need to compare, wrap the function in a struct or class that conforms to Equatable.

Currying Pattern

Simulate currying by returning partially-applied functions: ```swift func add(_ a: Int) -> (Int) -> Int { { b in a + b } } let add5 = add(5) print(add5(3)) // 8 print(add5(10)) // 15 ``` Useful for creating specialised functions from generic ones.

Quick Check

What is the function type for a function that takes two `Int`s and returns an `Int`?

Answer it in the interactive lesson

Recap

Key takeaways: • Functions have types like `(ParamTypes) -> ReturnType` • Store in variables, pass as arguments, return from functions • Closures (`{ }`) are anonymous function values • Trailing closure syntax cleans up call sites • Returning functions from functions creates closures that capture variables Next: the optional chaining and unwrapping lesson.

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Frequently Asked Questions

Is the “Functions as First-Class Values” lesson free?

Yes — any 3 lessons of the Swift Academy learning path, including “Functions as First-Class Values”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Swift Academy course includes 4 lessons in total.

What will I learn in “Functions as First-Class Values”?

Storing functions in variables, passing as arguments and returning from functions. You practise Swift Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Swift Academy?

No prior experience is required. Swift Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 4 of 4.

How long does the “Functions as First-Class Values” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

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