Changes to data can often lead to unexpected consequences and tricky bugs. I can update some data here, not realizing that another part of the software expects something different and now fails—a failure that’s particularly hard to spot if it only happens under rare conditions. For this reason, an entire school of software development—functional programming—is based on the notion that data should never change and that updating a data structure should always return a new copy of the structure with the change, leaving the old data pristine.
