Feedback for https://github.com/StevenMcD/swift_katas to be pasted into Xcode playground.

StringCalculator

// Better class name. Removed unneeded ; from code lines.
class StringCalculator {
  // Made class level method
  // Removed unneeded () around return value
  class func add(numbers: String) -> Int {
    // Removed unneeded () around if condition
    if numbers.isEmpty {
      return 0
    }
    
    // No need to cast. String has method to create int value. Returns optional so forcing unwrap. Not the nicest but simplest.
    return numbers.toInt()!
  }
}

// Not sure how to call test from playground so simple value checking.
StringCalculator.add("") == 0
StringCalculator.add("1") == 1