If you have precise reference counts (which is often achievable in array languages because they don't have cycles), then you can dynamically do efficient in-pla

If you have precise reference counts (which is often achievable in array languages because they don't have cycles), then you can dynamically do efficient in-place updates if the array you are updating has a count of 1. Uniqueness types are only necessary if you want to make the in-place update a static guarantee (which can be crucial if the code is going to run in an environment where allocation is not possible).