[karmakaze]

> The solution to this incremental scope problem is a lot less clear to me and something I struggle with personally in scientific computing projects. When variety #2 pops up, how does one decide whether to restructure everything in case of a variety #3, or take the quicker (potentially more efficient) path and avoid unnecessary complexity?

Short-answer rule-of-three (but really it depends).

I get what you mean, but this is more about implementation choices than identification of variety. Varieties can be grouped different ways that makes sense for implementation challenges or development timelines and later refactored to be more effective. The identification of the varieties themselves is distinct from the choice of grouping/implementation. Things that have little effect on behaviors (eg. color only affects one aspect of rendering) can be a property of a thing. More complex/varied characteristics may be better served by separating into distinct things.

Sometimes a 'bag of properties' can serve for all things (similar to JS Object). That's how I believe reddit's data model evolved at one time. I think HN isn't much different as it seems that id's of posts, comments, etc share numbering.