[regulation_d]

When I first started writing Elixir, I had no idea how much I would come to love immutable data as a language feature. "functional" can mean so many things, but for me immutable data as part of the runtime (i.e. not bolted on later) is one of the most important things. Never having to ask: did I pass by reference or value? Never having worry about data changing out from underneath you. It's pretty amazing.

Sure there are times that a need for performance calls for mutable data. But for me those times are pretty rare, and when they happen it's usually easy enough to quarantine the mutation.