A switch statement is a common place you might want to use an enum. Switch statements must be exhaustive. This means that each possible case must be addressed explicitly by the control flow. Using an enum type in the switch statement can prevent us from accidentally forgetting to address a case.

Let’s look at this switch control flow that uses a String type value:

let myPet = "kitty"

switch myPet {
case "goldfish":
    print("I’m not fluffy but I like to swim")
case "kitty":
    print("I’m fluffy but I’m not a big swimmer")
case "puppy":
    print("I’m fluffy and I love to swim")
default:
    print("I'm not really sure what I am")
}

While this works, there is some room for improvement. Because switch statements must be exhaustive, we need to include the default case to cover situations where myPet is equal to a value other than “goldfish”, “kitty”, or “puppy”. However, if we know all of the possible values of myPet, then we should always be able to print out the right description. Also, if you add additional cases to the switch statement it could introduce unexpected behavior down the road.

Let see how we can improve the readability and robustness of this code by using an enum:

enum Pet {
   case goldfish
   case kitty
   case puppy
}

let myPet = Pet.puppy

switch myPet {
    case .goldfish:
         print("I’m not fluffy but I like to swim")
    case .kitty:
         print("I’m fluffy but I’m not a big swimmer")
    case .puppy:
        print("I’m fluffy and I love to swim")
}

// Prints: I'm fluffy and I love to swim

We replaced the String myPet with an enumeration type called Pet. Since we have defined Pet to have exactly three cases, we only need to cover those three cases in the switch statement in order to be exhaustive. There is no need for a default case because Pet cannot be anything other than a goldfish, kitty, or puppy. By correlating enumeration cases with switch cases, the code becomes more readable since there is no need to handle unexpected pet types.