The alternative would be calamitous. If len returned an unsigned number, then i too would be a uint, and the condition i >= 0 would always be true by definition. After the third iteration, in which i == 0, the i-- statement would cause i to become not −1, but the maximum uint value (for example, 264−1), and the evaluation of medals[i] would fail at run time, or panic (§5.9), by attempting to access an element outside the bounds of the slice.
