OptionalStringExtension.swift

protocol StringType {}
extension String: StringType {}

extension Optional where Wrapped: StringType {
    var something: Int {
        switch self {
        case .None: return 10
        case .Some(_): return 20
        }
    }
}

let str: String? = nil
print(str.something) // 10

let str2: String? = "abc"
print(str2.something) // 20