Just a clarification for anybody else who might be reading this: there absolutely are mutable variables in pure fp languages like Haskell, you just have to explicitly ask for them (and reflect that fact in the type), i.e:

    do
       x <- newIORef 0
       writeIORef x 2
       print =<< readIORef x

will print 2. In fact, I'd argue that many times writing programs with mutable state is actually easier in Haskell/etc, especially when it comes to concurrency where something like software transactional memory [0] is invaluable (and more or less practically unusable in other ecosystems), if it fits your constraints.

[0] https://en.wikipedia.org/wiki/Software_transactional_memory