导致“无法匹配类型'BaseBackend backend0'与'SqlBackend'的错误的持久selectList” [英] Persistent selectList causing error of "Couldn't match type ‘BaseBackend backend0’ with ‘SqlBackend’"

问题描述

 •无法将类型'BaseBackend backend0'与'SqlBackend'匹配'使用'runSqlite'导致的
 
类型变量'backend0'不明确
•在表达式中：runSqlite：memory：
在表达式中：
 runSqlite：memory：
 $ do {records<  -  selectList [UserUsername ==。 abc] [LimitTo 10]; 
 liftIO $ print（records :: [Entity User]）} 
在等式'selectAll'中：
 selectAll 
 = runSqlite：memory：
 $做{记录<  - 选择列表[UserUsername ==。 abc] [LimitTo 10]; 
 liftIO $ print（records :: [Entity User]）} 
  

p>

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

看看runSqlite的类型签名：

  runSqlite 
 ::（MonadBaseControl IO m，MonadIO m，IsSqlBackend后端）
 =>文本
  - > ReaderT后端（NoLoggingT（ResourceT m））a 
  - > ma 
  

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

解决方案

您可以在 SqlBackend 中对其进行专门化处理。

  asSqlBackendReader :: ReaderT SqlBackend ma  - > ReaderT SqlBackend m a 
 asSqlBackendReader = id 
 $ b $ selectAll :: IO（）
 selectAll = runSqlite：memory：。 asSqlBackendReader $ do 
 records<  -  selectList [UserUsername ==。 abc] [LimitTo 10] 
 liftIO $ print（records :: [Entity User]）
  

看看 runSqlite 的类型，有一个 IsSqlBackend后端约束来满足。

IsSqlBackend 为：

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

然后查找 IsPersistBackend 。

在类的定义下面，我们看到它有三个实例：

$ pre $实例IsPersistBackend SqlWriteBackend
实例IsPersistBackend SqlReadBackend
i nstance IsPersistBackend SqlBackend

这三种类型指定具有各种功能的后端，其中 SqlBackend 是最常用的（未知功能）。如果这是您所需要的，请随意使用更受限制的。

I'm running into the below compile error:

• 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])

Having a look at the type signature of runSqlite:

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

I would assume I need to specify an explicit type for runSqlite, though I'm not too sure what I set for backend in ReaderT backend (NoLoggingT (ResourceT m)) a?

解决方案

You can specialize it at 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])

---

Looking at the type of runSqlite, there is a IsSqlBackend backend constraint to satisfy.

The definition of IsSqlBackend is:

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

Then look up IsPersistBackend.

Just below the definition of the class, we see that it has three instances:

instance IsPersistBackend SqlWriteBackend
instance IsPersistBackend SqlReadBackend
instance IsPersistBackend SqlBackend

The three types specify backends with various capabilities, with SqlBackend being the most general one (unknown capabilities). Feel free to use a more restricted one if that's all you need.

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