I will say plainly that I think there are situations in which mutability offers more elegant solutions than immutability, but I think most languages that offer it do it badly. I’m most experienced programming the Erlang platform via Elixir, and I think it offers a really nice midpoint between locality of state and purity. Within a process everything is immutable, and mutation requires sending a message to a process that will have a function specifying an explicit, pure state transformation from that message. Just about the only thing I don’t love about Elixir is the lack of real types.

I’m also very pragmatic and to the example of a web browser I would say, most applications are not web browsers. The overwhelming majority aren’t, in fact. I’ve chosen at this point in my career to mostly focus on enterprise software development, which I believe was Rich’s original field as well, and I’ve seen an enormous number of solutions with too much state cast about everywhere that benefit massively from centralizing the state high in the tree and really thinking through the data model carefully. So I stand by the principle I advocated originally, but it’s not universally applicable. It’s my belief that one of the core virtues of software development is knowing when to apply which principles.