惯用地 return 可能在链的末尾

Question

我有以下代码片段，它有点管用：

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
              , taskStatus = TaskStatusRunning
              , taskSpec = taskSpec
              , 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, taskStatus = TaskStatusRunning,
                      taskSpec = taskSpec, taskContainerId = containerId container}
   |
58 |         createContainer device requirement >>= \container ->
   |                         ^^^^^^

Answer 1

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

import Control.Monad.Trans.Maybe(<b>MaybeT(MaybeT)</b>)

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(<b>runMaybeT</b>)

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

惯用地 return 可能在链的末尾

Idiomatically return a Maybe at the end of a chain

haskell

monads

monad-transformers