Comment by grumpyprole

Pure functional programming is really about tracking and controlling effects by making them first-class, not avoiding them altogether. This has numerous benefits for reasoning, correctness and optimisation. Haskell allows mutable local variables, 'ST' references which can be used to build functions that are observably pure on the outside. The Haskell type checker will guarantee that no mutable state escapes. This is true "state encapsulation". It is completely acceptable for pure FP programming.