Example of a Swift function that takes a function as a parameter, nests functions, and returns a function

printOutSumOfNumber:andNumber:withFormatter:

func formatNumberAsSpelledOutString(number: Any) -> String {
  var numberString: String!
  let spellOutFormatter = NSNumberFormatter()
  spellOutFormatter.numberStyle = .SpellOutStyle
  if number is Int {
    let num = number as Int
    numberString = spellOutFormatter.stringFromNumber(num)
  } else if number is Double {
    spellOutFormatter.minimumFractionDigits = 1
    let num = number as Double
    numberString = spellOutFormatter.stringFromNumber(num)
  } else {
    numberString = "NaN"
  }
  return numberString
}

func printOutSumOfNumber(var number1: Any, var andNumber number2: Any, withFormatter formatter: (Any) -> String) -> () -> () {
  var result: String!
  func addTwoIntegers(int1: Any, int2: Any) -> Int {
    let num1 = int1 as Int
    let num2 = int2 as Int
    let sum = num1 + num2
    return sum
  }
  func addTwoDoubles(int1: Any, int2: Any) -> Double {
    let num1 = int1 as Double
    let num2 = int2 as Double
    let sum = num1 + num2
    return sum
  }
  func printResult() {
    println("The sum of \(formatter(number1)) and \(formatter(number2)) is \(result).")
  }
  switch (number1, number2) {
  case (is Int, is Int):
    number1 = number1 as Int
    number2 = number2 as Int
    let sum = addTwoIntegers(number1, number2)
    result = formatter(sum)
  case (is Int, is Double):
    number1 = Double(number1 as Int)
    number2 = number2 as Double
    let sum = addTwoDoubles(number1, number2)
    result = formatter(sum)
  case (is Double, is Int):
    number1 = number1 as Double
    number2 = Double(number2 as Int)
    let sum = addTwoDoubles(number1, number2)
    result = formatter(sum)
  case (is Double, is Double):
    number1 = number1 as Double
    number2 = number2 as Double
    let sum = addTwoDoubles(number1, number2)
    result = formatter(sum)
  default:
    result = formatter("")
  }
  return printResult
}

printOutSumOfNumber(1, andNumber: 0.23, withFormatter: formatNumberAsSpelledOutString)() // Prints "The sum of one and zero point two three is one point two three."