[tome]

I'm not sure why you say not to respond with 'use the IO monad' because that's exactly how you'd do it! As an example, here's some code that updates elements of a vector.

    import Data.Vector.Unboxed.Mutable
    
    import Data.Foldable (for_)
    import Prelude hiding (foldr, read, replicate)
    
    -- ghci> main
    -- [0,0,0,0,0,0,0,0,0,0]
    -- [0,5,10,15,20,25,30,35,40,45]
    main = do
      v <- replicate 10 0
    
      printVector v
    
      for_ [1 .. 5] $ \_ -> do
        for_ [0 .. 9] $ \i -> do
          v_i <- read v i
          write v i (v_i + i)
    
      printVector v
    
    printVector :: (Show a, Unbox a) => MVector RealWorld a -> IO ()
    printVector v = do
      list <- foldr (:) [] v
      print list

It does roughly the same as this Python:

    # python /tmp/test28.py
    # [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
    # [0, 5, 10, 15, 20, 25, 30, 35, 40, 45]
    def main():
        v = [0] * 10
    
        print(v)
    
        for _ in range(5):
            for i in range(10):
                v_i = v[i]
                v[i] = v_i + i
    
    
        print(v)
    
    if __name__ == '__main__': main()