Criscrossing backwards and forwards would be bad, but the 'functional core, imperative shell' does work well. It is an excellent fit for Erlang/Elixir - you wri

Criscrossing backwards and forwards would be bad, but the 'functional core, imperative shell' does work well. It is an excellent fit for Erlang/Elixir - you write a lot of pure functional logic (with the equivalently easy tests), and then you 'glue' it to the outside world with as little process machinery and statefulness as possible. We are in Erlang though, so I wouldn't call it 'mutation' necessarily. The stateful bits are individual actors, creating a new internal state in response to messages.