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目前，我有一个WorkLog类型，有开始和结束日期。我还想添加一个持续时间镜头，它将根据开始日期和结束日期得出。它应该是只读的，或者如果它的值发生变化则更改结束日期（我想知道如何实现这两个版本，即使我只会使用一个）。

这是我的代码。基本上，如果您可以实现workLogDurationROandworkLogDurationRW函数以使所有测试通过主要测试，那将回答我的问题。

{-# 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
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2 回答 2

6

您可以编写Getterusing to（您可以给予-.-较低的优先级以摆脱括号）：

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

但正如镜头维基所说，你可能最好使用一个计算时间差的普通函数，然后你可以to在需要它作为镜头时使用它。

Lens'您可以从 getter（与上面相同）和 setter构建：

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)
于 2014-01-21T12:50:38.023 回答
1 
workLogDurationRO :: Getter WorkLog TimeDifference
workLogDurationRO f w@(WorkLog d s e) = fmap (const w) (f $ e -.- s)


workLogDurationRW :: Lens' WorkLog TimeDifference
workLogDurationRW f (WorkLog d s@(TimeStamp y) e) = 
   fmap (\(TimeDifference x) -> WorkLog d s (TimeStamp $ y + x)) (f $ e -.- s)
于 2014-01-21T12:52:43.477 回答