That is not how functional data structures work. Check out Okasaki's book "Purely functional data structures" if you want to know what we are talking about here

That is not how functional data structures work. Check out Okasaki's book "Purely functional data structures" if you want to know what we are talking about here. There is not a whole copy of data being made. They use structural sharing and there is almost never a need for the implementation to make a data copy. You can have a piece of data that "changes" as it passes through the data flow, but in reality very tiny changes are actually happening behind the scenes, even if the perception to the programmer is that large changes are happening.