在链的末尾惯用地返回一个 Maybe

如何解决在链的末尾惯用地返回一个 Maybe

我有以下代码片段，它有点工作：

launchTask :: (DeviceRepo m) => TaskSpec -> m (Maybe Task)
launchTask taskSpec@TaskSpec { taskSpecImage = image,taskSpecRequirement = requirement
                             } = do
  mayDevice <- getDeviceMatchingRequirement requirement
  case mayDevice of
    nothing -> return nothing
    Just device -> do
      mayContainer <- createContainer device requirement
      case mayContainer of
        nothing -> return nothing
        Just container ->
          return $
          Just
            Task
              { taskName = pack image,taskStatus = TaskStatusRunning,taskSpec = taskSpec,taskContainerId = containerId container
              }

不过，我觉得这不是很地道。堆叠主要计算（来自 DeviceRepo m）和可能的 Maybe monad 的正确方法是什么，以便如果缺少任何值，则立即返回 nothing ？

编辑：我尝试使用 MaybeT，但类型不匹配

launchTask2 :: (DeviceRepo m) => TaskSpec -> m (Maybe Task)
launchTask2 taskSpec@TaskSpec { taskSpecImage = image,taskSpecRequirement = requirement
                              } = runMaybeT launcher
  where
    launcher :: MaybeT m Task
    launcher =
      getDeviceMatchingRequirement requirement >>= \device ->
        createContainer device requirement >>= \container ->
          return
            Task
              { taskName = pack image,taskContainerId = containerId container
              }

    • Couldn't match expected type ‘Device’
                  with actual type ‘Maybe Device’
    • In the first argument of ‘createContainer’,namely ‘device’
      In the first argument of ‘(>>=)’,namely
        ‘createContainer device requirement’
      In the expression:
        createContainer device requirement
          >>=
            \ container
              -> return
                   Task
                     {taskName = pack image,taskContainerId = containerId container}
   |
58 |         createContainer device requirement >>= \container ->
   |                         ^^^^^^

解决方法

您可能正在寻找 MaybeT monad transformer 中的 transformers package：

import Control.Monad.Trans.Maybe(MaybeT(MaybeT))

launchTask' :: DeviceRepo m => TaskSpec -> MaybeT m Task
launchTask' taskSpec@TaskSpec { taskSpecImage = image,taskSpecRequirement = requirement } = do
    device <- MaybeT (getDeviceMatchingRequirement requirement)
    container <- MaybeT (createContainer device requirement)
    return Task
        { taskName = pack image,taskStatus = TaskStatusRunning,taskSpec = taskSpec,taskContainerId = containerId container
        }

因此这是一个函数 MaybeT m Task，您可以使用 runMaybeT 将 MaybeT m a 转换为 m (Maybe a)：

import Control.Monad.Trans.Maybe(runMaybeT)

lanchTask :: DeviceRepo m => TaskSpec -> m (Maybe Task)
launchTask = runMaybeT . lanchTask'