我的数据结构或多或少看起来像这样（为了问题而简化）
data GameObject = GameObject { _num :: Int }
data Game = Game { _objects :: [GameObject] }
我使用makeLenses为这两者生成访问器。因此，我能够像这样进行操作 en masse ：
-- loop :: MonadState with Game inside
loop = objects.traversed.num += 1
这很棒。
但是，我无法找到相应的方法（每num打印一次）：
game <- get
let objects = _objects game
let objectNums = map _num objects
mapM_ print objectNums
我尝试过像
这样的事情uses (objects.traversed.num) >>= mapM_ print
但是我看到使用traversed和某些状态的唯一方法是使用monoids，我不想将字段组合在一起;我希望对来自我的状态的某些序列中元素中每个元素的一部分运行monadic动作。
那么，这是否可以使用一些提供的镜头功能，或者我必须自己编写，不知何故？
答案 0 :(得分：6)
怎么样？
mapMOf_ (objects . traversed . num) print
或者如果你想将它应用于状态monad的状态，
get >>= mapMOf_ (objects . traversed . num) print
？ （也许有一种更像镜头的方式将第一行与州monad的状态结合起来。如果是这样，我想亲自了解它。）
答案 1 :(得分：0)
此解决方案对镜头所看到的状态进行操作
objects . traverse . num .=>> putStrLn . ("num: " ++) . show
示例：
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TupleSections #-}
module Main where
import Control.Monad.IO.Class
import Control.Lens
-- (Getting, Lens', LensLike', lens, makeLenses, view, zoom)
import Control.Lens.Internal.Zoom -- (Focusing)
import Control.Monad.Trans.State.Lazy -- (StateT(StateT), execStateT)
viewM :: Monad m => Getting t s t -> (t -> m a) -> StateT s m a
viewM g f = StateT $ \s -> (, s) <$> f (view g s)
viewEachM ::
Monad m => LensLike' (Focusing m c) t s -> (s -> m c) -> StateT t m c
viewEachM g f = zoom g $ id .=> f
infix 4 .=>, .=>>
(.=>) :: Monad m => Getting t s t -> (t -> m a) -> StateT s m a
g .=> f = viewM g f
(.=>>) :: Monad m => LensLike' (Focusing m c) t s -> (s -> m c) -> StateT t m c
g .=>> f = viewEachM g f
data GameObject = GameObject
{ _num :: Int
} deriving (Show)
data Game = Game
{ _objects :: [GameObject]
} deriving (Show)
makeLenses ''Game
makeLenses ''GameObject
defaultGame = Game {_objects = map GameObject [0 .. 3]}
action :: StateT Game IO ()
action = do
id .=> putStrLn . ("game: " ++) . show
objects .=> putStrLn . ("objects: " ++) . show
objects .=> putStrLn . ("length of objects: " ++) . show . length
-- use .=>> as alternative
objects .=>> putStrLn . ("length of objects: " ++) . show . length
-- if you use traverse you have to use .=>>
objects . traverse . num .=>> putStrLn . ("num: " ++) . show
main :: IO ()
main = do
execStateT action defaultGame
return ()
<强>输出：
game: Game {_objects = [GameObject {_num = 0},GameObject {_num = 1},GameObject {_num = 2},GameObject {_num = 3}]}
objects: [GameObject {_num = 0},GameObject {_num = 1},GameObject {_num = 2},GameObject {_num = 3}]
length of objects: 4
length of objects: 4
num: 0
num: 1
num: 2
num: 3
解析器： lts-9.3