func / Argument Labels / Default Parameters

// Basic function definition
func functionName(argumentLabel parameterName: Type) -> ReturnType {
    return value
}

// Omitting the argument label (use _)
func functionName(_ parameterName: Type) -> ReturnType { }

// Default parameter
func functionName(parameterName: Type = defaultValue) { }

// Variadic parameter
func functionName(parameterName: Type...) { }

// Basic function
func greet(name: String) -> String {
    return "Hello, \(name)!"
}
print(greet(name: "Taro"))

// Separate argument label and parameter name
func move(from start: String, to end: String) {
    print("Moving from \(start) to \(end)")
}
move(from: "Tokyo", to: "Osaka")  // Call using argument labels

// Omit label with _
func multiply(_ a: Int, _ b: Int) -> Int {
    return a * b
}
print(multiply(3, 4))  // Call without labels

// Default parameter
func log(_ message: String, level: String = "INFO") {
    print("[\(level)] \(message)")
}
log("Started")              // [INFO] Started
log("Error occurred", level: "ERROR") // [ERROR] Error occurred

// Variadic parameter (received as an array)
func sum(_ numbers: Int...) -> Int {
    return numbers.reduce(0, +)
}
print(sum(1, 2, 3, 4, 5))  // 15

// Implicit return (single expression)
func square(_ n: Int) -> Int { n * n }  // return can be omitted
print(square(7))  // 49

Swift functions allow you to separate the argument label (the external name used by callers) from the parameter name (the internal name used in the function body). This enables readable call sites like move(from: "A", to: "B").

In single-expression functions, the return keyword can be omitted (Swift 5.1 and later). Only one variadic parameter (...) is allowed per function. Default parameters are conventionally placed at the end of the parameter list.

For inout parameters and multiple return values, see inout / Return Types / Multiple Return Values.