Xcode Playground can only run partially - swift

Question

As shown above, in the Xcode playground, I am trying to extend the protocol Exercise to have two more computed properties, caloriesBurnedPerMinute and description. When I want to run this part of code in the playground, the "play button" at the extreme bottom left corner appears as grey, indicating that it cannot run the code until line 20. However, if I just want to run the code until line 14, as shown below, the "play button" appears as blue, indicating that it can run the code until line 14.
May I know if there is anything wrong with the second extension of protocol Exercise?
edited: the following is the code.
import Cocoa
protocol Exercise: CustomStringConvertible {
var name: String { get }
var caloriesBurned: Double { get }
var minutes: Double { get }
}
extension Exercise {
var caloriesBurnedPerMinute: Double {
return caloriesBurned / minutes
}
}
extension Exercise {
var description: String {
return "Exercise(\(name), burned \(caloriesBurned) calories in \(minutes) minutes)"
}
}

Answer 1

There is nothing wrong with your code, That is just Xcode 10's new feature. See here.
It's a way of running your code line by line, but there are some limitations. If you hover over the line number "20", you will see a grey line:
That means Xcode can't only run that part of the code, presumably because how Xcode handles extension declarations.
Just add some code that actually runs, as opposed to just declarations, and press the play button above the bottom panel. Your code will run just fine.

Answer 2

You need to wrap the Double variables in a String cast like String(caloriesBurned):
extension Exercise {
var description: String {
return "Exercise(\(name), burned \(String(caloriesBurned)) calories in \(String(minutes)) minutes)"
}
}
I don't know why, but it helps