ReadLine()

In order to catch errors that are thrown, we use the do-catch construct. 
Let’s demonstrate this by writing a function which will read in user input 
from the command line (using Swift 2.0’s nifty new readLine function), 
and try to parse it as an IP address:

func tryNumericisingIP() {
  // Get the value from the user
  print("Enter four-part IP here: > ", terminator: "")
  let maybeInput = readLine(stripNewline: true)

  // Only continue if we actually received a value
  guard let input = maybeInput else {
    return
  }

  // Try numericising the user-provided IP. Handle any errors.
  do {
    let numericValue = try numericiseIPAddress(input)
    print("Your IP address, \(input) is equal to \(numericValue)")
  } catch IPError.TooFewComponents {
    print("IP address had fewer than 4 components")
  } catch IPError.TooManyComponents {
    print("IP address had more than 4 components")
  } catch let IPError.OctetWasBad(octet) {
    print("Octet number \(octet) was invalid - either non-numeric or out of bounds")
  } catch {
    // This shouldn't be necessary, but the compiler complains about 
    //  exhaustiveness. Maybe an early beta seed bug.
    print("Encountered an unknown error \(error)")
  }
}