Maybe you're thinking of the zipper data structure? [1] Where you represent the composite structure hanging from the point you want to mutate, so updating that

Maybe you're thinking of the zipper data structure? [1] Where you represent the composite structure hanging from the point you want to mutate, so updating that point is merely dropping the header and appending the updated value as a new prefix. 1 - https://en.m.wikipedia.org/wiki/Zipper_(data_structure) https://en.m.wikipedia.org/wiki/Zipper_(data_structure)