if_let.swift

var optionalValue: Int?     
if let value = optionalValue { // unwraps optionalValue and assigns                                    
                               // it to variable named value
 print(value)                  // type of value = Int
else {
 print("not able to unwrap optionalValue as it is nil")
}
  
//Let's rewrite the sum function using if-let
func sum(_ optionalAugend: Int?, _ optionalAddend: Int?) -> Int? {
   if let augend = optionalAugend,
      let addend = optionalAddend {// `,` is AND operation. 
                                   // If both variables are unwrapped, enter into if-case         
        return augend + addend      
    }                               
    return nil                     
}