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我正在尝试使用 Obelisk 构建一个基本的 Web 应用程序，并且一直在关注本教程：https ://github.com/obsidiansystems/obelisk/pull/733/files#diff-fe2dadb00af0583d3841e501760804eaR457

我不明白为什么当我的 Obelisk 安装是一个默认的 Obelisk 项目时我会收到这种类型的错误，并且只对上面的教程进行了修改以实现“配置文件”和“事件”/“投入工作”页面。

我的最终目标是这些路由/链接通过我在下面发布的 Frontend.hs 中定义的两个按钮进行连接（例如elAttr "p" ("class" =: "button is-link is-large is-fullwidth") $ text "My Profile"）。

这是我得到的错误：

frontend/src/Frontend.hs:41:41-60: error:
    • Couldn't match type ‘T.Text’ with ‘RoutedT t EventId m ()’
      Expected type: RoutedT t a m ()
        Actual type: T.Text
    • In the second argument of ‘(<>)’, namely ‘T.pack (show uidVal)’
      In the expression: text "This event : " <> T.pack (show uidVal)
      In the second argument of ‘($)’, namely
        ‘\ (EventId uidVal)
           -> text "This event : " <> T.pack (show uidVal)’
    • Relevant bindings include
        dUserId :: Dynamic t EventId
          (bound at frontend/src/Frontend.hs:39:13)
   |
41 |                 text "This event : " <> T.pack (show uidVal)
   |                                         ^^^^^^^^^^^^^^^^^^^^
frontend/src/Frontend.hs:48:40-59: error:
    • Couldn't match type ‘T.Text’ with ‘RoutedT t UserId m ()’
      Expected type: RoutedT t a m ()
        Actual type: T.Text
    • In the second argument of ‘(<>)’, namely ‘T.pack (show uidVal)’
      In the expression: text "This user : " <> T.pack (show uidVal)
      In the second argument of ‘($)’, namely
        ‘\ (UserId uidVal) -> text "This user : " <> T.pack (show uidVal)’
    • Relevant bindings include
        dUserId :: Dynamic t UserId
          (bound at frontend/src/Frontend.hs:46:13)
   |
48 |                 text "This user : " <> T.pack (show uidVal)
   |                                        ^^^^^^^^^^^^^^^^^^^^

这是我的 Common.Route 文件的摘录，这是本教程中唯一实现一半的部分......所以也许这就是问题所在？

data FrontendRoute :: * -> * where
  FrontendRoute_Main :: FrontendRoute ()
  -- This type is used to define frontend routes, i.e. ones for which the backend will serve the frontend.
  FrontendRoute_MyProfile :: FrontendRoute UserId
  FrontendRoute_PutInWork :: FrontendRoute EventId

fullRouteEncoder
  :: Encoder (Either Text) Identity (R (FullRoute BackendRoute FrontendRoute)) PageName
fullRouteEncoder = mkFullRouteEncoder
  (FullRoute_Backend BackendRoute_Missing :/ ())
  (\case
      BackendRoute_Missing -> PathSegment "missing" $ unitEncoder mempty)
  (\case
      FrontendRoute_Main -> PathEnd $ unitEncoder mempty
      -- FrontendRoute_MyProfile -> PathSegment "user" _todo
      -- FrontendRoute_PutInWork -> PathSegment "event" _todo
      )

这也是 Frontend.hs 的主要功能：

frontend :: Frontend (R FrontendRoute)
frontend = Frontend
  { _frontend_head = do
      el "title" $ text "The App Name"
      elAttr "link" ("href" =: static @"bulma.css" <> "type" =: "text/css" <> "rel" =: "stylesheet") blank
  , _frontend_body = subRoute_ $ \case          -- https://github.com/obsidiansystems/obelisk/pull/733/files#diff-fe2dadb00af0583d3841e501760804eaR425
        FrontendRoute_PutInWork -> do
            text "We're on the Put In Work page! But which event?"
            dUserId <- askRoute
            dyn_ $ ffor dUserId $ \(EventId uidVal) ->
                text "This event : " <> T.pack (show uidVal)
            routeLink (FrontendRoute_Main :/ ()) $
                text "To the main page!"
        FrontendRoute_MyProfile -> do
            text "We're on the My Profile page! But which user?"
            dUserId <- askRoute
            dyn_ $ ffor dUserId $ \(UserId uidVal) ->
                text "This user : " <> T.pack (show uidVal)
            routeLink (FrontendRoute_Main :/ ()) $
                text "To the main page!"
        FrontendRoute_Main -> do
            elClass "div" "content" $ do
            elAttr "h1" ("class" =: "content") $ text "Welcome to the app!"
            el "h2" $ text $ T.pack commonStuff
            elAttr "p" ("class" =: "button is-primary is-large is-fullwidth") $ text "Put In Work"
            elAttr "p" ("class" =: "button is-link is-large is-fullwidth") $ text "My Profile"

            elAttr "p" ("class" =: "box") $ text "News about the app and the world (box is color-coded based on content type), special offers from partners, prompts that there is a new partner nearby, and prompts to push yourself harder eventually.  At launch, it's just a welcome text / helper note."

            routeLink (FrontendRoute_MyProfile :/ User 42) $
                text "Visit the page of User # 42"
    return ()

}
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1 回答 1

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这也是我在编写 Reflex 应用程序时常犯的一个错误。

解决方案是使用$. 为什么？让我们看看固定性声明，

有 3 种固定性，非、左和右关联性（分别为中缀、中缀和中缀）和十个优先级，包括 0 到 9（0 级绑定最紧密，9 级绑定最紧密） .

通过玩 GHCi repl，

λ> :i (<>)
class Semigroup a where
  (<>) :: a -> a -> a
  ...
        -- Defined in ‘GHC.Base’
infixr 6 <>
λ> :i ($)
($) :: (a -> b) -> a -> b       -- Defined in ‘GHC.Base’
infixr 0 $
λ>

GHC 手册说“普通构造函数应用程序比中缀构造函数应用程序具有更高的优先级（因此 a : Foo a 解析为 a : (Foo a)）。 ”，因此在你的：

text "This event : " <> T.pack (show uidVal)

它被解释为

(text "This event : ") <> T.pack (show uidVal)

这显然是错误的。您可以通过明确限定表达式的范围来解决此问题：

text ("This event : " <> T.pack (show uidVal))

或者，只需使用上述$运算符（在反射代码库中通常首选）：

text $ "This event : " <> T.pack (show uidVal)
于 2020-05-17T16:11:35.757 回答