Fixed-Size Sequences

Another area where tuples can be used is when you intend to constrain a type to a fixed number of items. Imagine an object that calculates various statistics for each month in a year. You need to store a certain Integer value for each month separately. The solution that comes to mind first would of course be:

var monthValuesArray: [Int]

However, in this case we don't know whether the property indeed contains 12 elements. A user of our object could accidentally insert 13 values, or 11. We can't tell the type checker that this is a fixed size array of 12 items. With a tuple, this specific constraint can easily be put into place:

var monthValues: (Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int)

The alternative would be to have the constraining logic in the object's functionality (say via a guard statement); however, this would be a run time check. The tuple check happens at compile time; your code won't even compile if you try to give 11 months to your object.