Haskell Yesod get user credentials in subsite

Question

I'm writing a subsite for my yesod project, and I need to display the logged in user's name on that subsite (I'm using yesod-auth hardcoded where the type of AuthId master = Text).

However, the user is logged in on the master site. I'm able to get a value of type AuthId master using maybeAuthId, but I am unable to display this value because it is not an instance of Show.

Is there a type constraint I can put on my Handler to make sure the type of AuthId master derives 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|.......|]

EDIT: Here is the error message:

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=\"");
                  .... }) }'

Cubic Cubic · Accepted Answer · 2017-08-15T15:01:33

Seems to me like all you need is a Show (AuthId master) constraint in your type signature:

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

Note that this requires the FlexibleContexts language extension, which you can enable by putting {-# LANGUAGE FlexibleContexts #-} at the top of your source file.

Haskell Yesod get user credentials in subsite

1 Answers