Is a default case necessary in a method?

the Swift eBook states:

a switch statement must be exhaustive when considering an enumeration’s members

since you provided a case for every member, that switch is exhaustive since every possible enumeration case is present. there does not need to be a default because all the values are covered.

However, had you not provided a case for every member and left a few out, you would need to provide a default case in order to be exhaustive

When it is not appropriate to provide a case for every enumeration member, you can provide a default case to cover any members that are not addressed explicitly

the example from the eBook below only provides one case and then a default value for the rest. this switch is exhaustive since .Earth is covered in the case, and everything else is covered by the default

let somePlanet = Planet.Earth
switch somePlanet {
case .Earth:
    println("Mostly harmless")
default:
    println("Not a safe place for humans")
}
// prints "Mostly harmless