This is the #1 misconception I see surrounding pure functional programming - that once you run into mutable state, you run into problems. It is entirely the op

This is the #1 misconception I see surrounding pure functional programming - that once you run into mutable state, you run into problems. It is entirely the opposite. PFP exposes the murky aspects of mutability, provides abstractions around it, and ensures that you're not tripping over yourself in regards to mutability.