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我正在使用servant-generic-0.1.0.3并servant-server-0.13.0.1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，然后我尝试使用它ReaderT来避免传递Pool Connection到siteServer，所以我添加AppM和替换siteServer如下：

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遇到了一堆错误。

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。

我是否错过了可以使这项工作发挥作用的东西？

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1 回答 1

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至少对于servant-* >= 0.14（见这里）支持的记录风格的路线，如果你想使用另一个monad而不是Handler，你会想看看AsServerTand genericServerT。

应用于您的示例，此方法siteServer应定义如下（未经过类型检查，但应非常接近正确）。

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：由于您使用servant-* < 0.14和servant-generic，您应该替换genericServerT为toServant.

于 2018-07-22T16:09:09.870 回答