持久 selectList 导致错误“无法将类型‘BaseBackend backend0’与‘SqlBackend’匹配”

我遇到以下编译错误：

• 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]) }

Code:

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

看一下 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，尽管我不太确定我设置的是什么backend in ReaderT backend (NoLoggingT (ResourceT m)) a?

您可以专注于SqlBackend.

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])

看看类型runSqlite，有一个IsSqlBackend backend需要满足的约束。

的定义IsSqlBackend https://hackage.haskell.org/package/persistent-2.7.0/docs/Database-Persist-Sql.html#t:IsSqlBackend is:

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

然后抬头看IsPersistBackend https://hackage.haskell.org/package/persistent-2.7.0/docs/Database-Persist-Class.html#t:IsPersistBackend.

在类的定义下面，我们看到它有三个实例：

instance IsPersistBackend SqlWriteBackend
instance IsPersistBackend SqlReadBackend
instance IsPersistBackend SqlBackend

这三种类型指定具有不同功能的后端，其中SqlBackend是最通用的一种（未知功能）。如果您只需要这样，请随意使用限制更严格的。