Comment by mrkeen

This is so odd.

It's not that I disagree without you about mutable fields vs params, but the industry so disproportionately on the other side of that issue that I'm surprised this objection is raised as often as it is.

Bring it on! Let's have more persistent data structures in standard libraries. Construct a Map by folding new key-vals onto another (possibly) empty Map.

(Rather than creating an empty map and then mutating its instance fields until it's full)