Another interesting approach to copy-on-write for immutable collections (for example arrays) is where you actively mutate the array in place, but leave the original as a lazy description/view of how to get back to the before state.

From the outside the effect is the same, but the performance is optimized for accessing the updated collection and only possibly using the old value.

Great for cases where you want the immutability guarantee, but where it might be unlikely the old value is actually going to be used in the general case.