我正在为yesod项目编写子网站,我需要在该子网站上显示登录用户的名称(我使用yesod-auth硬编码,其中AuthId master = Text
的类型)。
但是,用户已登录主站点。我可以使用AuthId master
获取maybeAuthId
类型的值,但我无法显示此值,因为它不是Show
的实例。
我可以在我的处理程序上放置一个类型约束,以确保AuthId master
的类型派生Show
吗?
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:以下是错误消息:
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=\"");
.... }) }'
答案 0 :(得分: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,您可以将{-# LANGUAGE FlexibleContexts #-}
放在源文件的顶部来启用。