guard_let.swift

func printSummary(_ programmer: Programmer) {
    guard let laptop = programmer.laptop else {     // set the value of laptop if `programmer.laptop` is not nil 
        print("\(programmer.name) have no laptop")  // and will go line 6. If `programmer.laptop` is nil, it will
        return                                      // enter into else case and return to caller of this function.
    }
    guard let laptopFan = laptop.fan else {         // Syntax
        print("\(laptop.name) contains no fan")     // guard let constant = expression else {
        return                                      //   return returnValue
    }                                               // }
    print("\(laptop.name) contains \(laptopFan.name) fan")
}

printSummary(programmer) //guard let should be used with some function. Because it is based on early exit.