Hint for the "Struct" exercise

The outline of what you want to do is this:

• create a mutable variable sum that is initially 0.0
• for each name name in the shopping list:
  • call the price() method to get the (optional) price
  • if the price is Some, add it to sum
  • if the price is None, return None
• return Some(sum)

Back to the exercise.