[dahfizz]

> For example, if you have a tree and you want to change one leaf on the tree, the only thing you really need to copy is the part of the tree that's actually different now - for the unchanged branches, the new tree just has a pointer to branches of the old tree, so they can be (and are) reused without copying.

This still only works if a child has one / a known-ahead-of-time number of parents. If you need to update an object that N objects point to, you need to update all N references.

[pyrale]

It just doesn't really happen that much in functional programs, to be honest.

The very concept of n objects pointing to the same, commonly mutated data is something that can be adressed easily by having that data contained by a parent structure. Since functional programs don't think in terms of "methods" within each object trying to access data, but in terms of external function manipulating all the data you need, the only change is the way you'll pass your data to your functions.