I created a simple program in Haskell to simulate the Physics Ising model (http://en.wikipedia.org/wiki/Ising_model) with Metropolis algorithm: http://haskell.pastebin.com/Zt0gguEa And used Data.Vector.Unboxed.Mutable to model de 2D grid. To this a used a unidimensional vector, but created a operator to write and other to read the vector using a tuple to specify the position: newtype Grid = Grid { unG :: (Int, MVector (PrimState IO) Int) } newtype Ising = Ising { unI :: (M.MTGen, Grid) } type Pos = (Int, Int) (|>) :: Grid -> Pos -> IO Int (Grid (n,g)) |> (x,y) | x == n = U.unsafeRead g (y*n) | x == -1 = U.unsafeRead g (y*n + n - 1) | y == n = U.unsafeRead g x | y == -1 = U.unsafeRead g ((n - 1)*n + x) | otherwise = U.unsafeRead g (y*n + x) (<==) :: Pos -> Int -> (Pos, Int) pos <== val = (pos,val) (<|) :: Grid -> (Pos, Int) -> IO () Grid (n,g) <| ((x,y), val) = U.unsafeWrite g (y*n + x) val infixr 1 <== infixr 0 <| With this I can write expressions like: s <- g |> (x,y) grid <| (i,j) <== spin (rnumber `mod` 2) my problem is with the (<==) and (<|) operators, because in the way I defined I can't use: grid <| (i,j) <== spin $ rnumber `mod` 2 Is there way to solve this and don't have to use the parenthesis on the right expression? And I really would like to now with this approach to the 2D vector is a good idea or is better to use another data structur instead of the mutable vector. Comments about the code are really welcome. Edgar