haskell - 无法将类型“PersistEntityBackend U”与“SqlBackend”匹配

Answer 1

Answer 2

问 ghci:info User给了我正确的提示：它显示了它的类实例。事实证明，Entity User直接使用是错误的。我怀疑虽然这创建了实体，但它生成的所有类实例Database.Persist.TH.share都丢失了。以下代码编译，还删除了 Bjartur 提到的冗余约束：

sampleUser :: User
sampleUser = User
  { userName = "admin"
  , userEmail = "admin@test.com"
  }

type U = User

connectInfo :: MySQLConnectInfo
connectInfo = undefined

runAction :: (MonadUnliftIO m, IsPersistBackend r, BaseBackend r ~ SqlBackend) => MySQLConnectInfo -> ReaderT r (LoggingT m) a -> m a
runAction connectInfo action = runStdoutLoggingT $ withMySQLConn connectInfo $ \backend ->
  runReaderT action backend

insertUser :: (PersistRecordBackend U SqlBackend) => 
             U -> ReaderT SqlBackend (LoggingT IO) (Key U) 
insertUser = insert

doDBStuff :: IO ()
doDBStuff = do
  runAction connectInfo (runMigration migrateAll)
  runAction connectInfo (insertUser sampleUser) -- compile error fixed
  return ()