All of these can be both mutable or immutable. And converted as needed let array = [1, 2, 3] var mutableArray = [4, 5, 6] var anotherMutable = array Not sure

All of these can be both mutable or immutable. And converted as needed let array = [1, 2, 3] var mutableArray = [4, 5, 6] var anotherMutable = array Not sure how much optimisation is done under the hood, but Swift compiler loves to complain about mutable structures that are not ever edited.