I wouldn't have anticipated that, but now that you say it I could see why that would lead to a debate. How would you deal with mutable data structures, for instance? What if it accesses the environment, but in a fashion you could somehow guarantee were safe? In Haskell the programmer can circumvent the system with unsafePerformIO if they know something they can't convince Haskell otherwise, but it almost seems like you'd need a "pure-but-not-really" annotation to do this kind of thing in an imperative language that actually enforced purity.