如何使用镜头库编写依赖于其他镜头的复杂镜头？

此刻，我有一个WorkLog类型，并带有开始日期和结束日期。我还想添加一个持续时间镜头，它将从开始日期和结束日期派生出来。它应该是只读的，或者如果其值发生更改则更改结束日期（我想知道如何实现这两个版本，即使我只会使用一个版本）。

这是我的代码。基本上，如果你能实现workLogDurationRO and workLogDurationRW函数来获得主要传递中的所有测试，这将回答我的问题。

{-# LANGUAGE TemplateHaskell #-}
module Main where
import Control.Lens

-- Keep times simple for this example
newtype TimeStamp = TimeStamp Int deriving (Show, Eq)
newtype TimeDifference = TimeDifference Int deriving (Show, Eq)

(-.-) :: TimeStamp -> TimeStamp -> TimeDifference
(TimeStamp a) -.- (TimeStamp b) = TimeDifference (a - b)

data WorkLog = WorkLog {
  _workLogDescription :: String
  , _workLogStartTime :: TimeStamp
  , _workLogEndTime :: TimeStamp
  }

makeLenses ''WorkLog

-- | Just return the difference between the start and end time
workLogDurationRO :: Getter WorkLog TimeDifference
workLogDurationRO = error "TODO write me!"

-- | Like the read only version, but when used with a setter,
-- change the end date.
workLogDurationRW :: Lens' WorkLog TimeDifference
workLogDurationRW = error "TODO write me!"

ensure :: String -> Bool -> IO ()
ensure _ True = putStrLn "Test Passed"
ensure msg False = putStrLn $ "Test Failed: " ++ msg

main :: IO ()
main = do
  let testWorkLog = WorkLog "Work 1" (TimeStamp 40) (TimeStamp 100)
  ensure "read only lens gets correct duration" $ 
     testWorkLog^.workLogDurationRO == TimeDifference 60
  ensure "read+write lens gets correct duration" $ 
     testWorkLog^.workLogDurationRW == TimeDifference 60
  let newWorkLog = testWorkLog & workLogDurationRW .~ TimeDifference 5
  ensure "writeable lens changes end time" $ 
     newWorkLog^.workLogEndTime == TimeStamp 45

你可以写下Getter using to（你可以给-.-降低优先级以去掉括号）：

workLogDurationRO = to $ \wl -> (wl^.workLogEndTime) -.- (wl^.workLogStartTime)

但作为镜头维基 https://github.com/ekmett/lens/wiki/FAQ#wiki-using-getters说，你可能最好使用一个计算时间差的普通函数，然后你可以使用它to当您需要它作为镜头时。

您可以构建Lens'来自 getter（与上面相同）和 setter：

workLogDurationRW = lens get set
  where
    get :: WorkLog -> TimeDifference
    get wl = (wl^.workLogEndTime) -.- (wl^.workLogStartTime)

    set :: WorkLog -> TimeDifference -> WorkLog
    set wl timeDiff = wl & workLogEndTime .~ (wl^.workLogStartTime) +.+ timeDiff
      where
        TimeStamp a +.+ TimeDifference b = TimeStamp (a + b)