🐦 Swift

Closures

Lesson 4 of 5 ~5 min

Closures are self-contained blocks of functionality that can be passed around and used in your code. They're similar to functions but written in a more compact syntax. Closures are used everywhere in Swift — in completion handlers, array transformations, and UI callbacks.

Closure Syntax

A closure captures and references variables from the surrounding context. Here's how the syntax evolves from full form to shorthand:

Editor

Swift

// Full closure syntax
let greet = { (name: String) -> String in
  return "Hello, \(name)!"
}
print(greet("Archon"))

// Shorthand argument names ($0, $1)
let numbers = [5, 2, 8, 1, 9]

// Sorted with closure
let sorted = numbers.sorted { $0 < $1 }
print("Sorted: \(sorted)")

// Map — transform each element
let doubled = numbers.map { $0 * 2 }
print("Doubled: \(doubled)")

// Filter — keep elements matching condition
let big = numbers.filter { $0 > 4 }
print("Big numbers: \(big)")

// Reduce — combine into single value
let sum = numbers.reduce(0) { $0 + $1 }
print("Sum: \(sum)")

Output

Try it

Modify the filter to keep only even numbers ($0 % 2 == 0) instead of numbers greater than 4.

Shorthand Arguments

// $0 refers to the first argument
// $1 refers to the second argument
// $2, $3... and so on

let words = ["hello", "world"]

// Full form
let upper1 = words.map { (word: String) -> String in
  return word.uppercased()
}

// Short form with $0
let upper2 = words.map { $0.uppercased() }

// Trailing closure syntax
let upper3 = words.map {
  $0.uppercased()
}

// All produce: ["HELLO", "WORLD"]

Map, Filter, Reduce

→ map — Transforms every element in a collection and returns a new array.
→ filter — Tests each element and keeps only those that pass the test.
→ reduce — Combines all elements into a single value using an accumulator.
→ forEach — Iterates over a collection without producing a result.

Quiz

What does $0 refer to in a closure?

Challenge

Use map to transform an array of strings to uppercase. Try using both the full closure syntax and the $0 shorthand.

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