[amichail]

You don't need to do anything non-trivial for imperative/OO game programming.

If something non-trivial is required for functional programming, then it is more difficult.

[xenoblitz]

As far as I know there is nothing non-trivial that I had to use apart from introducing state when it was needed by the game via monads and muteable variables. At first they were tricky because I was used to taking such things for granted.

What I can tell you is that the backend (which really needs no state) is much more easier to write in Haskell IMHO.