Excerpts from the “Functions” section of the official docuemnt 《The Swift Programming Language》

Function Parameters

Parameter names

A parameter name can have a internal name and an external name declared in the form of: external_name internal_name: type.

Explicitly specify an external name for the first parameter to enable it when calling.

func join(aString lhs: String, toString rhs: String) -> String {
  return lhs + rhs

println(join(aString: "Hello", toString: " World!"))

Swift compiler automatically generates external name from corresponding internal names of all parameters except the 1st parameter.

Use _ to suppress auto-generating external name: _ internal_name: type

func join(_ aString: String, _ toString: String) -> String {
  return aString + toString

println(join("Hello", " World!"))

Parameters with default value

Parameters with default value of the form [external_name/_] internal_name: type = default_value are better be placed at end of the parameter list.

func Foo(arg1: String, arg2: Int = 30, _ arg3: Bool) {

Foo("text", arg2: 1, true)

Variadic parameters

Declare variadic parameter in form name: type..., which, in the function body, is of type [type].

Only one variadic parameter is allowed in a parameter list.

Variable parameters

Prefix parameter names with keyword var to make them modifiable inside the function body.

It is just another small syntactic sugar brought by Swift, the change made in the function body will not propagate outside the function body.

In-Out parameters

Prefix parameters names with keyword inout to propagate the change to the parameter inside function body outside.

When specifying argument for inout parameters, prepend & to the argument name to indicate that it could be modified during the function call.

In-out parameters

func swapTwoInts(inout a: Int, inout b: Int) {
    let temporaryA = a
    a = b
    b = temporaryA

var a = 2
var b = 10
swapTwoInts(&a, &b)
println("after swapping, a = \(a), b = \(b)")

Function Return Values

Functions without a defined return type return a special value of type Void. This is simply an empty tuple, in effect a tuple with zero elements, which can be written as ().

You can use a tuple type (or a optional tuple) as the return type for a function to return multiple values as part of one compound return value.

// wrap multiple return values in an optional tuple.
func minMax(array: [Int]) -> (min: Int, max: Int)? {
  return (1, 3) // no need to specify tuple element names again

if let minMax = minMax([1,2,3]) {
  // use name to fetch tuple element.
  println("min: \(minMax.min)\tmax: \(minMax.max)")

  // use index to fetch tuple elements.
  println("min: \(minMax.0)\tmax: \(minMax.1)")

Function Types

A function type is made up of its’ parameter types and return type.

In Swift, function is first class object, so feel free to use them as other normal objects:

// declare a type alias for long function types
typealias cmp = (Int, Int) -> Bool

func whichCmp(functionPassedIn: ()-> Bool) -> cmp {
  // nested fucntion definition
  func Foo(lhs: Int, rhs: Int) -> Bool {
    return false

  func Hoo(lhs: Int, rhs: Int) -> Bool {
    return true

  // function being returned
  return functionPassedIn() ? Foo : Hoo

func aFunc() -> Bool {
  return false

// function being passed in as an argument
whichCmp(aFunc)(1334, 234)