Guard

Guard can be used to reduce indentation on the happy path. In a guard statement, the else branch must transfer control to exit the code block containing the guard statement. This can be done with return, break, continue, or throw.

func divide(x: Int, y: Int) -> Int? {
  guard y != 0 else {
    print("You cannot divide by 0.")
    return nil
  }
  let answer = x / y
  return answer
}
print(divide(x: 5, y: 0))
// Output: 
// You cannot divide by 0.
// nil

func greet(name: String?) {
  guard let name else {
    print("Hello guest!")
    return
  }
  print("Hello \(name)!")
}
greet(name: "Asma")
greet(name: nil)
// Output: 
// Hello Asma!
// Hello guest!

guard let name is shorthand for guard let name = name: it unwraps the optional into a constant of the same name. The unwrapped value can also be given a different name, as in guard let unwrapped = name.