Transformation operations that create new data structures are thus reasonable to keep even if they could be replaced with calculations. Indeed, there is a duality here between objects that wrap a data structure with a series of calculated properties and functions that transform one data structure into another. The object route is clearly better when the source data changes and you would have to manage the lifetime of the derived data structures. But if the source data is immutable, or the derived data is very transient, then both approaches are effective.
