尝试使用持久性时,没有Control.Monad.Logger.MonadLogger的实例

Question

我正在构建一个使用scotty和persistent编译的麻烦的Web应用程序.这是我的代码:

runDb :: SqlPersist (ResourceT IO) a -> IO a
runDb query = runResourceT . withSqliteConn "dev.app.sqlite3" . runSqlConn $ query

readMessage :: KeyBackend (PersistEntityBackend Post) Post -> IO (Maybe Post)
readMessage postID = runDb $ get postID

我收到此错误消息:

Message.hs:30:30:
No instance for (Control.Monad.Logger.MonadLogger IO)
  arising from a use of `get'
Possible fix:
  add an instance declaration for
  (Control.Monad.Logger.MonadLogger IO)
In the second argument of `($)', namely `get postID'
In the expression: runDb $ get postID
In an equation for `readMessage':
    readMessage postID = runDb $ get postID

我发现了这个问题,但接受的答案是使用旧版本monad-logger也会迫使我使用许多其他软件包的旧版本,scotty并且persistent包括在内,我不想这样做.另一个答案建议使用runNoLoggingT,我无法工作.我无法弄清楚要把它放到什么样的地方.

Answer 1

runNoLoggingT 有类型

runNoLoggingT :: NoLoggingT m a -> m a

MonadLogger只要m上面是一个实例,它就是有效的MonadIO.所有以下堆栈都是实例MonadIO

SqlPersist (ResourceT IO)
            ResourceT IO
                      IO

所以以下所有堆栈都是有效的实例 MonadLogger

NoLoggingT (SqlPersist (ResourceT IO))
SqlPersist (NoLoggingT (ResourceT IO))
SqlPersist (ResourceT (NoLoggingT IO))

我推荐第三个,然后我们只是编辑

runDb :: SqlPersist (ResourceT (NoLoggingT IO)) a -> IO a
runDb = runNoLoggingT 
      . runResourceT 
      . withSqliteConn "dev.app.sqlite3" 
      . runSqlConn