TicTacToe in Haskell
A haskell beginner using Control.Lens... Certainly better than my first program which happened to a tic tac toe solver, so I can compare. We all know Why functional programming matters, don't we?

Make your imports more specific:

import Control.Lens (set,ix)
import Data.List (transpose)
import Data.Maybe (catMaybes)


You will note I replaced your isNothing by pattern matching.
You only use type Position = (Int, Int) once. I would not declare this one.

instead of

    fDiag = zipWith (\ i x -&gt; i !! (n - x - 1)) grid [0..]


I recommend

    fDiag = zipWith (!!) (reverse grid) [0..] -- or
    fDiag = zipWith (!!) grid [n-1,n-2..]


Why force isWin to take Game? Grid would suffice.
I suggest you to write a program that uses this module, then you know better.

OK, now for

move :: Game -&gt; Position -&gt; Game


I'd write

move :: Int -&gt; Int -&gt; Game -&gt; Maybe Game


Instead of throwing an error, return Nothing. Then you can compute all possible moves with

moves :: Game -&gt; [Game]
moves game = catMaybes [move x y game | x&lt;-[0..2], y&lt;-[0..2]]


and I put Game in the last position so you can compose functions like move 2 0 . move 1 1 $ newGame. Actually, that won't work because move now encapsulates in Maybe. But now you can write:

  test = Just newGame &gt;&gt;=
              move 1 1 &gt;&gt;= -- first move in center
              move 2 1 &gt;&gt;=
              move 2 0 &gt;&gt;=
              move 0 2 &gt;&gt;= -- forced
              move 0 0     -- setting up the trap


because Maybe is a monad, and &gt;&gt;= allows forward chaining instead of .! The modified move looks like this:

move :: Int -&gt; Int -&gt; Game -&gt; Maybe Game
move i j (Game grid player) = case grid !! i !! j of
   Just _ -&gt; Nothing
   Nothing -&gt; Just $ Game {
     board = set (ix i . ix j) (Just player) grid,
     curTurn = nextPlayer player
  }

nextPlayer X = O
nextPlayer O = X


I also made nextPlayer a top level function, I'd export it, it looks useful.