Investigation: can we mutate previous values in memoized functions?

[rbalicki2]

• We keep track of references and all that, can we handle mutation?
• e.g. can a memoized function accept an Option<PreviousValue>? (Or, more likely, it would receive another struct, so that we can track calls to get_mut etc.)
• Perhaps the tracked/untracked distinction is relevant here. In particular, if I have a HashMap, and only modify one item in the map, I don't want everything else to re-execute!