Erlang / Elixir are 100% immutable inside the code (no var can ever be mutable).

However, they have a mutable in-process cache (living in the BEAM VM) that many people have written libraries around for stuff like mutable arrays, matrices, graphs, and many others.

It goes like this: do 99.5% functional programming and you have the imperative / stateful tools for when they are absolutely necessary.

This works. Extremely well.