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我以为我手头有 Yesod 类型，可以满足我的简单需求。哈！这里有两个现在可以编译的函数，但目前并没有完全按照我的意愿进行，我不知道如何进行。

    addNote' selectedProduct = do  
      _ <- runDB $ insert <=< liftIOHandler $ addNote selectedProduct
      return ()


--    addNote :: SelectedProduct -> IO (NotesGeneric SqlPersist)
addNote (MKsp tid firmware version _ requester (Textarea note)) = do
--   currentTime <- getCurrentTime
   return $ Notes tid requester firmware version note undefined

我有addNote注释掉的类型签名是有原因的，如果我取消注释它，或者调用 getCurrentTime，我会收到我将在下面显示的错误。

我不知道是什么类型的addNote'。GHC 表示如下：

    Warning: Top-level binding with no type signature:
               addNote' :: forall master (monad :: * -> *) sub sub1 master1.
                           (YesodPersistBackend master
                              ~
                            Control.Monad.Trans.Reader.ReaderT (HandlerData sub1 master1),
                            YesodPersist master,
                            PersistBackend
                              (Control.Monad.Trans.Reader.ReaderT (HandlerData sub1 master1))
                              (GGHandler sub master IO),
                            Control.Monad.IO.Class.MonadIO monad) =>
                           SelectedProduct
                           -> Control.Monad.Trans.Reader.ReaderT
                                (HandlerData sub master) monad ()

此外，GHC 说的类型addNote是这样的：

           addNote :: forall (m :: * -> *).
                      Monad m =>
                      SelectedProduct -> m (NotesGeneric SqlPersist)

如果我取消注释该getUTCTime行或addNote.

Couldn't match expected type `Control.Monad.Trans.Reader.ReaderT
                                (HandlerData sub0 master0) IO b0'
            with actual type `IO (NotesGeneric SqlPersist)'
Expected type: GGHandler sub0 master0 IO b0
  Actual type: IO (NotesGeneric SqlPersist)
In the return type of a call of `addNote'
In the second argument of `($)', namely `addNote selectedProduct'

如何让类型匹配？

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1 回答 1

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Just replace liftIOHandler with liftIO. liftIOHandler is a dirty hack that's only necessary in very rare circumstances, and fortunately is completely gone starting with Yesod 0.10.

于 2012-01-25T13:25:45.010 回答