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Functions

By · Reviewed & last updated · Part of Swift Tutorial

Swift functions get external + internal names, default values, variadics, throws, and trailing closures. Together they make APIs read like a sentence.

Real Swift function signatures

EXAMPLE
// 1) Argument labels — external label optional with _
func greet(_ name: String, withPrefix prefix: String = "Hi") -> String {
    "\(prefix), \(name)"
}
greet("Ada")                     // 'Hi, Ada'
greet("Bo", withPrefix: "Hello") // 'Hello, Bo'

// 2) Variadic args
func total(_ values: Int...) -> Int {
    values.reduce(0, +)
}
total(1, 2, 3, 4)                // 10

// 3) Multiple returns via tuple
func bounds(_ xs: [Int]) -> (min: Int, max: Int)? {
    guard let first = xs.first else { return nil }
    return xs.reduce((first, first)) { (acc, x) in
        (Swift.min(acc.0, x), Swift.max(acc.1, x))
    }
}
if let b = bounds([5, 1, 7, 3]) {
    print(b.min, b.max)         // 1 7
}

// 4) inout — pass by reference
func shift(_ x: inout Int, by delta: Int) {
    x += delta
}
var count = 0
shift(&count, by: 5)
print(count)                      // 5

// 5) throws — typed errors
enum ParseError: Error { case empty, invalid }

func parsePositive(_ s: String) throws -> Int {
    guard !s.isEmpty            else { throw ParseError.empty }
    guard let n = Int(s), n > 0 else { throw ParseError.invalid }
    return n
}

do {
    let n = try parsePositive("42")
} catch ParseError.invalid {
    /* … */
}

// 6) Closures + trailing-closure syntax
let nums = [1, 2, 3, 4, 5]
let doubled = nums.map { \$0 * 2 }
let filtered = nums.filter { n in n.isMultiple(of: 2) }
let sum = nums.reduce(0, +)

// 7) @escaping closure params
func fetch(_ url: URL, then: @escaping (Data) -> Void) {
    URLSession.shared.dataTask(with: url) { data, _, _ in
        guard let data else { return }
        then(data)
    }.resume()
}

Why it matters

Trailing-closure syntax + meaningful argument labels are why Swift APIs read so well. When you design a function, choose labels that complete the sentence at the call site.

Tip: Tweak the snippet with Try it Yourself », then sit the quiz at the bottom of the page.

Example

Example
func add(_ a: Int, _ b: Int) -> Int { a + b }
func divmod(_ a: Int, _ b: Int) -> (Int, Int) { (a/b, a%b) }
let (q, r) = divmod(17, 5)
Try it Yourself »

Discussion

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