Guard

guard condition else {
  // false: execute some code
}
 
// true: execute some code 

func divide(_ number: Double, by divisor: Double) {

  // 나누는 수가 0인 경우를 먼저 걸러낸다.
  guard divisor != 0.0 else {
    return
  }
  
  // 핵심 로직
  let result = number / divisor
  print(result)
}

guard vs if

func divide(_ number: Double, by divisor: Double) {
  if divisor == 0.0 { return }
  let result = number / divisor
  print(result)
}

// 나누는 수가 0이 되면 안된다는 것을 명확하게 보여줌
guard divisor != 0.0 else { return }

func singHappyBirthday() {
  guard birthdayIsToday else {
    print(”No one has a birthday today.”)
    return
  }
 
  guard !invitedGuests.isEmpty else {
    print(”It’s just a family party.”)
    return
  }
 
  guard cakeCandlesLit else {
    print(”The cake’s candles haven’t been lit.”)
    return
  }
 
  print(”Happy Birthday to you!”)
}

pyramid of doom

func singHappyBirthday() {
  if birthdayIsToday {
      if !invitedGuests.isEmpty {
          if cakeCandlesLit {
              print(”Happy Birthday to you!”)
          } else {
              print(”The cake’s candles haven’t been lit.”)
          }
      } else {
          print(”It’s just a family party.”)
    }
  } else {
      print(”No one has a birthday today.”)
  }
}

Guard with Optionals (guard let)

// if-let
if let eggs = goose.eggs {
  print(”The goose laid \(eggs.count) eggs.”)
}

// 이곳에선 `eggs`에 접근할 수 없음

// guard let
guard let eggs = goose.eggs else { return }

// `eggs`에 접근 가능 
print(”The goose laid \(eggs.count) eggs.”)

// if let
func processBook(title: String?, price: Double?, pages: Int?) {
  if let theTitle = title,
    let thePrice = price,
    let thePages = pages {
    print(”\(theTitle) costs $\(thePrice) and has \(thePages) 
       pages.”)
  }
}

// guard let
func processBook(title: String?, price: Double?, pages: Int?) {
  guard let theTitle = title,
    let thePrice = price,
    let thePages = pages else {
      return
    }
    
    print(”\(theTitle) costs $\(thePrice) and has \(thePages) pages.”)
}