将 Servant.Generic 路由与 ReaderT（池连接）IO 结合使用

Question

我正在使用 servant-generic-0.1.0.3 和 servant-server-0.13.0.1 来执行以下操作：

data Site route = Site
  { page :: route :-
      "page" :> Capture "x" Int :> Capture "y" Int :> Get '[JSON] [Int]
  , home :: route :-
      Raw
  } deriving (Generic)

type API = ToServant (Site AsApi)

siteServer :: Pool Connection -> Site AsServer
siteServer pool = Site
  { page = \x y ->
      liftIO $ withResource pool $ \conn -> someDbFunction conn x y
  , home = serveDirectoryWebApp "static"
  }

api :: Proxy API
api = Proxy

app :: Pool Connection -> Application
app pool = serve api (toServant $ siteServer pool)

效果很好，然后我尝试使用 ReaderT 来避免将 Pool Connection 传递给 siteServer，所以我添加了 AppM 并替换了 siteServer这个：

type AppM = ReaderT (Pool Connection) IO

siteServer :: ServerT API AppM
siteServer = Site
  { page = do
      pool <- ask
      \x y ->
        liftIO $ withResource pool $ \conn -> someDbFunction conn x y
  , home = serveDirectoryWebApp "static"
  }

但是当我尝试编译它时出现了一堆错误。

我遵循了 servant cookbook 中显示的相同步骤，但我无法使用通用路由进行此操作，尽管它在使用常规路由时有效。

我是否遗漏了一些可以使这项工作成功的东西？

Answer 1

至少对于 servant-* >= 0.14（见 here）支持的记录式路由，如果你想使用 Handler 之外的另一个 monad，你会想看看在 AsServerT 和 genericServerT.

应用于您的示例，这意味着 siteServer 应定义如下（未进行类型检查，但应该非常接近正确）。

siteServer :: Site (AsServerT AppM)
siteServer = Site
  { page = ... something in AppM ...
  , home = ... something in AppM ...
  }

-- turning that into a usual chain of :<|>-separated handlers
oldStyleServer :: ServerT API AppM
oldStyleServer = genericServerT siteServer

-- bringing it all back in Handler
oldStyleServerInHandler :: Pool Connection -> Server API -- same as ServerT API Handler
oldStyleServerInHandler conns = hoistServer (Proxy @API) appToHandler oldStyleServer
  where appToHandler = liftIO . flip runReaderT conns
        -- or something along those lines

-- serving it
app :: Pool Connection -> Application
app conns = serve (Proxy @API) (oldStyleServerInHandler conns)

编辑： 由于您将 servant-* < 0.14 与 servant-generic 一起使用，因此应将 genericServerT 替换为 toServant。

将 Servant.Generic 路由与 ReaderT（池连接）IO 结合使用

Using Servant.Generic routes with ReaderT (Pool Connection) IO

haskell

servant