Передайте линзу в функцию

Как правильно передать линзу в функцию с состоянием? Рассмотрим следующий код:

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE FlexibleContexts #-}

import Control.Lens
import Control.Monad.State

data Game = Game { _armies :: [Army]
                 } deriving (Show)

data Army = Army { _troops :: Int
                 } deriving (Show)

makeLenses ''Game
makeLenses ''Army

data BattleResult = Win | Defeat deriving (Show)

offend offender defender = do
  Just ot <- preuse $ offender.troops
  Just dt <- preuse $ defender.troops
  defender.troops.=0 -- doesn't work
  let eval a b
        | a >= b    = return Win
        | otherwise = return Defeat
  eval ot dt

game :: State Game ()
game = do
    armies %= (:) (Army 100)
    armies %= (:) (Army 200)
    q <- offend (armies.ix 0) (armies.ix 1)
    return ()

Отмеченная строка ведет к следующей ошибке:

Lens.hs:21:3:
    Couldn't match type ‘Const (Data.Monoid.First Int) s’
                   with ‘Identity s’
    Expected type: (Army -> Const (Data.Monoid.First Int) Army)
                   -> s -> Identity s
      Actual type: (Army -> Const (Data.Monoid.First Int) Army)
                   -> s -> Const (Data.Monoid.First Int) s
    Relevant bindings include
      defender :: (Army -> Const (Data.Monoid.First Int) Army)
                  -> s -> Const (Data.Monoid.First Int) s
        (bound at Lens.hs:18:17)
      offender :: (Army -> Const (Data.Monoid.First Int) Army)
                  -> s -> Const (Data.Monoid.First Int) s
        (bound at Lens.hs:18:8)
      offend :: ((Army -> Const (Data.Monoid.First Int) Army)
                 -> s -> Const (Data.Monoid.First Int) s)
                -> ((Army -> Const (Data.Monoid.First Int) Army)
                    -> s -> Const (Data.Monoid.First Int) s)
                -> m BattleResult
        (bound at Lens.hs:18:1)
    In the first argument of ‘(.)’, namely ‘defender’
    In the first argument of ‘(.=)’, namely ‘defender . troops’

Lens.hs:21:12:
    Couldn't match type ‘Identity Integer’
                   with ‘Const (Data.Monoid.First Int) Int’
    Expected type: (Int -> Identity Integer)
                   -> Army -> Const (Data.Monoid.First Int) Army
      Actual type: (Int -> Const (Data.Monoid.First Int) Int)
                   -> Army -> Const (Data.Monoid.First Int) Army
    In the second argument of ‘(.)’, namely ‘troops’
    In the first argument of ‘(.=)’, namely ‘defender . troops’

Если заменить строку на что-то вроде armies.ix 0.troops.=0, код нормально компилируется. Есть ли какие-то стандартные инструменты для обхода проблемы? И можно ли реализовать тот же алгоритм без использования FlexibleContexts?