Haskell Yesod 在子站点中获取用户凭据

Question

我正在为我的 yesod 项目编写一个子站点，我需要在该子站点上显示登录用户的名称（我正在使用 yesod-auth 硬编码，其中 AuthId master = Text 的类型）。

但是，用户在主站点上登录。我可以使用maybeAuthId 获得AuthId master 类型的值，但我无法显示该值，因为它不是Show 的实例。

我可以对我的处理程序施加类型约束以确保AuthId master 的类型派生Show？

getSubsiteHomeR :: (YesodAuth master) => HandlerT Subsite (HandlerT master IO) Html
getSubsiteHomeR = do
   lift $ do
     maid <- maybeAuthId -- I want to display the value of 'maid'
     liftIO $ print maid
     defaultLayout [whamlet|.......|]

编辑：这是错误消息：

Could not deduce (Show (AuthId master))
    arising from a use of `print'
  from the context: YesodAuth master
    bound by the type signature for:
               getSubsiteHomeR :: YesodAuth master =>
                                     HandlerT Subsite (HandlerT master IO) Html
    at src/Subsite.hs:24:1-89
* In the second argument of `($)', namely `print maid'
  In a stmt of a 'do' block: liftIO $ print maid
  In the second argument of `($)', namely
    `do { maid <- maybeAuthId;
          liftIO $ print maid;
          defaultLayout
            (do { (asWidgetT . toWidget)
                    ((blaze-markup-0.8.0.0:Text.Blaze.Internal.preEscapedText . T.pack)
                       "<p>Welcome to the Subsite!</br><a href=\"");
                  (getUrlRenderParams
                   >>=
                     (\ urender_alJ6
                        -> (asWidgetT . toWidget)
                             (toHtml
                                ((\ u_alJ7 -> urender_alJ6 u_alJ7 ...)
                                   (toParent SubsiteHomeR)))));
                  (asWidgetT . toWidget)
                    ((blaze-markup-0.8.0.0:Text.Blaze.Internal.preEscapedText . T.pack)
                       "\">Subsite</br></a>\n\
                       \<a href=\"");
                  .... }) }'

Answer 1

在我看来，您需要的只是类型签名中的 Show (AuthId master) 约束：

getSubsiteHomeR :: (YesodAuth master, Show (AuthId master)) => HandlerT Subsite (HandlerT master IO) Html

请注意，这需要 FlexibleContexts 语言扩展名，您可以通过将 {-# LANGUAGE FlexibleContexts #-} 放在源文件顶部来启用它。