It's doable but harder than in imperative languages and the resulting code is _really_ ugly. See the thread of the 1BRC https://discourse.haskell.org/t/one-billion-row-challenge-in... with example code at (I hope that's the right Gist) https://gist.github.com/AndrasKovacs/e156ae66b8c28b1b84abe6b...