从Haskell MongoDB驱动程序检索错误状态

我有一个MongoDB，其中有一个集合

用户

，其中包含

电子邮件

上的唯一索引：

import Data.Bson        (Value (Int32))
import Database.MongoDB (Index (..), createIndex, (=:))

createIndex $ Index "user" [ "email" =: Int32 1 ] "email" True False

我编写了一个函数，如果电子邮件地址未被使用，则插入新用户，如果电子邮件地址已被占用，则应失败：

import           Data.Bson        (Value (ObjId))
import           Database.MongoDB (Action, ObjectId, (=:))
import qualified Database.MongoDB as M (insert)

data User = User
  { email     :: Text
  , firstName :: Text
  , lastName  :: Text
  } deriving Show

data MongoEntity a = MongoEntity ObjectId a

createIfNotExists :: User
                  -> Action Handler (Either Text (MongoEntity User))
createIfNotExists (user@User {..}) = do
  value <- M.insert "user" [ "email" =: email
                           , "firstName" =: firstName
                           , "lastName" =: lastName
                           ]
  case value of
    ObjId objectId -> return (Right $ MongoEntity objectId user)
    _ -> return $ Left "no document"

---

在

值之后，我从未使用过此模块，但阅读文档后，我能想到的最好方法是：

更改
createIfNotExists

createIfNotExists :: MonadIO m =>
                     User -> Action m (Either String (MongoEntity User))

使用
Control.Exception.Base
中的
catchJust
。作者似乎不鼓励使用
Control.Monad.Error.Class

main :: IO ()
main = do
  pipe <- connect (host "127.0.0.1")
  let user = User "jd@bar.baz" "John" "Doe"
  eitherEntity <- catchJust writeFailureErrorStr
                  (access pipe master "user" $ createIfNotExists user)
                  (return . Left)
  close pipe

writeFailureErrorStr  :: Failure -> Maybe String
writeFailureErrorStr (WriteFailure _err str) = Just str
writeFailureErrorStr _other = Nothing

main:：IO（）
main=do
pipe根据Jean-Baptiste Potonnier的回答，在阅读了Michael Snoyman（和）的两篇文章后，我将代码改写如下：

import           Control.Exception.Lifted (handleJust)
import           Data.Bson                (Value (ObjId))
import qualified Data.Bson                as B (Value)
import qualified Data.Text                as T (pack)
import           Database.MongoDB         (Action, Failure (..), (=:))
import qualified Database.MongoDB         as M (insert)

createIfNotExists :: User -> Action Handler (Either Failure (MongoEntity User))
createIfNotExists user =
  handleJust writeFailureHandler (return . Left) $ do
    value <- insertUser user
    return $
      case value of
        ObjId objectId -> Right $ MongoEntity (T.pack $ show objectId) user
        _ -> error "Unexpected missing document"

  where
    writeFailureHandler :: Failure -> Maybe Failure
    writeFailureHandler writeFailure@WriteFailure{} = Just writeFailure
    writeFailureHandler _ = Nothing

insertUser :: User -> Action Handler B.Value
insertUser User{..} =
  M.insert "users" [ "email" =: email
                   , "firstName" =: firstName
                   , "lastName" =: lastName
                   ]

import Control.Exception.Lifted（handleJust）
导入数据.Bson（值（ObjId））
导入符合条件的数据。Bson为B（值）
导入符合条件的数据。文本为T（打包）
导入Database.MongoDB（操作、失败（..），（=：）
将符合条件的Database.MongoDB作为M导入（插入）
createIfNotExists:：用户->操作处理程序（失败（MongoEntity用户））
createIfNotExists用户=
handleJust writeFailureHandler（return.Left）$do
value Right$MongoEntity（T.pack$show objectId）用户
_->错误“意外丢失文档”
哪里
writeFailureHandler:：失败->可能失败
writeFailureHandlerwriteFailure@WriteFailure{}=刚刚写入文件
writeFailureHandler=无
insertUser:：User->Action Handler B.值
插入器用户{..}=
M.插入“用户”[“电子邮件”=：电子邮件
，“firstName”=：firstName
，“lastName”=：lastName
]

使用该软件包。
谢谢。我进一步阅读了库源代码，并倾向于捕捉异常。由于MongoDB不支持事务，使用find后跟insert会打开一个可能的争用条件。顺便说一句，以下是MongoDB错误代码列表：
import           Control.Exception.Lifted (handleJust)
import           Data.Bson                (Value (ObjId))
import qualified Data.Bson                as B (Value)
import qualified Data.Text                as T (pack)
import           Database.MongoDB         (Action, Failure (..), (=:))
import qualified Database.MongoDB         as M (insert)

createIfNotExists :: User -> Action Handler (Either Failure (MongoEntity User))
createIfNotExists user =
  handleJust writeFailureHandler (return . Left) $ do
    value <- insertUser user
    return $
      case value of
        ObjId objectId -> Right $ MongoEntity (T.pack $ show objectId) user
        _ -> error "Unexpected missing document"

  where
    writeFailureHandler :: Failure -> Maybe Failure
    writeFailureHandler writeFailure@WriteFailure{} = Just writeFailure
    writeFailureHandler _ = Nothing

insertUser :: User -> Action Handler B.Value
insertUser User{..} =
  M.insert "users" [ "email" =: email
                   , "firstName" =: firstName
                   , "lastName" =: lastName
                   ]