TopolearnArgument labels shape how every Swift API reads, so you need them to use or design any API.
A function is declared with func name(parameters) -> ReturnType. Each parameter has an argument label (used at the call site) and a parameter name (used inside). By default they're the same; write func move(from start: Int, to end: Int) to use different ones, or _ to omit the label: func square(_ x: Int).
Labels make calls read like phrases: move(from: 1, to: 5). Follow the API Design Guidelines: name for clarity at the point of use.
Other features:
- Default values:
func greet(_ name: String, loud: Bool = false). - Variadic parameters:
func sum(_ xs: Int...)receives an array. - inout parameters can be modified, called with
&:swap(&a, &b). - Return several values with a tuple:
-> (min: Int, max: Int). - A single-expression body can omit
return. - Functions can be overloaded by parameter types and labels.
Parameters are constants inside the function. Use @discardableResult when ignoring a return value is normal; otherwise the compiler warns about unused results.
Functions are first-class: their type looks like (Int, Int) -> Int, and they can be stored, passed and returned.
Example
func clamp(_ value: Int, between low: Int = 0, and high: Int) -> Int {
min(max(value, low), high)
}
print(clamp(15, and: 10)) // 10
print(clamp(-3, between: -1, and: 5)) // -1
func stats(_ xs: Int...) -> (min: Int, max: Int)? {
guard let lo = xs.min(), let hi = xs.max() else { return nil }
return (lo, hi)
}
if let s = stats(4, 9, 1) { print(s.min, s.max) } // 1 9
func double(_ n: inout Int) { n *= 2 }
var x = 21
double(&x)
print(x) // 42
let op: (Int, Int) -> Int = (+)
print(op(2, 3)) // 5Remember
- Argument label (outside) vs parameter name (inside);
_drops the label - Default values, variadics and
inoutwith& - Return named tuples for multiple results
Watch out for
- Overusing
inoutinstead of returning a new value — it makes code harder to follow.