That’s true of all software written in a purely functional style. By definition, any software that is purely functional does not have explicit mutable state. I

That’s true of all software written in a purely functional style. By definition, any software that is purely functional does not have explicit mutable state. In practice, it is often useful to have parts of a system written in a pure functional style but other parts that do use mutable state, either explicitly or via some mechanism that simulates it in a functional context.