持久的SelectList導致的錯誤 「無法匹配型 'BaseBackend backend0' 與 'SqlBackend'」

Question

我運行到下面的編譯錯誤：

• Couldn't match type ‘BaseBackend backend0’ with ‘SqlBackend’ 
    arising from a use of ‘runSqlite’ 
    The type variable ‘backend0’ is ambiguous 
• In the expression: runSqlite ":memory:" 
    In the expression: 
    runSqlite ":memory:" 
    $ do { records <- selectList [UserUsername ==. "abc"] [LimitTo 10]; 
      liftIO $ print (records :: [Entity User]) } 
    In an equation for ‘selectAll’: 
     selectAll 
     = runSqlite ":memory:" 
      $ do { records <- selectList [UserUsername ==. "abc"] [LimitTo 10]; 
       liftIO $ print (records :: [Entity User]) } 

代碼：

selectAll :: IO() 
selectAll = runSqlite ":memory:" $ do 
    records <- selectList [UserUsername ==. "abc"] [LimitTo 10] 
    liftIO $ print (records :: [Entity User]) 

有看看runSqlite的類型簽名：

runSqlite  
:: (MonadBaseControl IO m, MonadIO m, IsSqlBackend backend) 
=> Text 
-> ReaderT backend (NoLoggingT (ResourceT m)) a 
-> m a 

我假設我需要指定runSq的顯式類型lite，儘管我不太確定我在ReaderT backend (NoLoggingT (ResourceT m)) a中爲backend設置了什麼？

Answer 1

你可以在SqlBackend專門。

asSqlBackendReader :: ReaderT SqlBackend m a -> ReaderT SqlBackend m a 
asSqlBackendReader = id 

selectAll :: IO() 
selectAll = runSqlite ":memory:" . asSqlBackendReader $ do 
    records <- selectList [UserUsername ==. "abc"] [LimitTo 10] 
    liftIO $ print (records :: [Entity User]) 

---

綜觀runSqlite類型，還有一個約束IsSqlBackend backend滿足。

的IsSqlBackend的定義是：

type IsSqlBackend backend = 
    (IsPersistBackend backend, BaseBackend backend ~ SqlBackend) 

再看看IsPersistBackend。

略低於類的定義，我們可以看到，它有三種情況：

instance IsPersistBackend SqlWriteBackend 
instance IsPersistBackend SqlReadBackend 
instance IsPersistBackend SqlBackend 

三種類型的各種功能指定後端，與SqlBackend是最常用的一個（未知功能）。如果這是您所需要的，您可以隨意使用更受限制的。