private static void RunLoop(Calculator calculator)
{
while (true)
{
Console.ForegroundColor = ConsoleColor.Gray;
var line = Console.ReadLine();
if (line == null) break;
var args = SplitInput(line);
var result = calculator.Calculate(args);
Console.ForegroundColor = result.HasValue ? ConsoleColor.Green : ConsoleColor.Red;
Console.WriteLine("> " + result);
}
}