导致 "Couldn't match type ‘BaseBackend backend0’ with ‘SqlBackend’" 错误的持久性 selectList

Question

我运行遇到以下编译错误：

• 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]) }

代码：

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

查看 runSqlite 的类型签名：

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

我假设我需要为 runSqlite 指定一个显式类型，尽管我不太确定我在 ReaderT backend (NoLoggingT (ResourceT m)) a?

中为 backend 设置了什么

Answer 1

您可以在 SqlBackend 对其进行专精。

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])

查看 runSqlite 的类型，需要满足 IsSqlBackend backend 约束条件。

IsSqlBackend的定义是：

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

然后查找IsPersistBackend。

就在 class 的定义下面，我们看到它有三个实例：

instance IsPersistBackend SqlWriteBackend
instance IsPersistBackend SqlReadBackend
instance IsPersistBackend SqlBackend

这三种类型指定了具有各种功能的后端，SqlBackend 是最通用的一种（功能未知）。如果您只需要这些，请随意使用更受限制的。

导致 "Couldn't match type ‘BaseBackend backend0’ with ‘SqlBackend’" 错误的持久性 selectList

Persistent selectList causing error of "Couldn't match type ‘BaseBackend backend0’ with ‘SqlBackend’"

haskell

haskell-persistent