Embracing Functional Programming Features in Swift

KD Knowledge Diet
3 min readFeb 14, 2024

Functional programming (FP) is a paradigm that treats computation as the evaluation of mathematical functions and avoids changing state and mutable data. Swift, while not a purely functional language, embraces many functional programming concepts, allowing developers to write cleaner, safer, and more reliable code. Let’s explore the functional features that Swift offers and how they can enhance your programming style.

First-Class Functions and Closures

At the heart of Swift’s functional programming features are first-class functions, which means functions can be passed as arguments to other functions, returned as values from other functions, and assigned to variables.

Closures, Swift’s anonymous functions, capture and store references to variables and constants from the context in which they are defined. This is akin to lambda expressions in other languages and is a powerful tool for creating concise, reusable code blocks.

Example of a Closure:

let greet = { (name: String) in
print("Hello, \(name)!")
greet("World") // Prints "Hello, World!"


Swift encourages the use of immutable values with `let` keyword, making your code safer and clearer. Immutable values, once set, cannot change, which means you can trust that they won’t be altered unexpectedly, reducing the likelihood of bugs.

Pure Functions

A pure function is one that, given the same input, will always return the same output and does not have any observable side effects. Swift’s emphasis on value types (like structs and enums) naturally encourages the use of pure functions.

Higher-Order Functions

Swift’s arrays and other collections offer higher-order functions that take functions as input. These include `map`, `filter`, `reduce`, and more, which provide powerful ways to handle collections of data with less code and in a more expressive way.

Example Using `map`, `filter`, and `reduce`:

let numbers = [1, 2, 3, 4, 5]
let squaredNumbers = numbers.map { $0 * $0 } // [1, 4, 9, 16, 25]
let evenNumbers = squaredNumbers.filter { $0 % 2 == 0 } // [4, 16]
let sumOfEvenNumbers = evenNumbers.reduce(0, +) // 20

Pattern Matching

Pattern matching in Swift, often used with the `switch` statement, is another concept borrowed from functional programming. It allows for matching against a variety of patterns, not just constants, and can be used with value binding, where values within a compound data structure are extracted as part of the pattern matching.

Optionals and Optional Chaining

Swift’s optionals are a functional way to handle the absence of a value. Optional chaining is a process for querying and calling properties, methods, and subscripts on an optional that might currently be `nil`.

Example of Optional Chaining:

class Person {
var residence: Residence?
class Residence {
var numberOfRooms = 1
let john = Person()
let roomCount = john.residence?.numberOfRooms // `roomCount` is an Int? containing `nil`


Swift’s adoption of functional programming principles is part of what makes it a modern and powerful language. By combining FP concepts with an object-oriented approach, Swift provides a hybrid model that allows developers to choose the best tool for the job. Whether it’s using pure functions for predictable behavior, leveraging higher-order functions for succinct collection processing, or utilizing immutability for safety, Swift’s functional features empower developers to write better code.



KD Knowledge Diet

Software Engineer, Mobile Developer living in Seoul. I hate people using difficult words. Why not using simple words? Keep It Simple Stupid!