I don't understand why you hate the IO monad so much. I mean I've seen very large codebases doing web apps and almost everything is inside the IO monad. It's not as "clean" and not following best practices, but still gets the job done and is convenient. Having pervasive access to IO is just the norm in all other languages so it's not even a drawback.

But let's put that aside. You can instead use the ST monad (not to be confused with the State monad) and get the same performance benefit of in-place update of values.