傳遞標籤值的形式，文本/字符串

Question

這是一個後續行動，我的問題在這裏：Extracting database field values inside a Handler

我想從數據庫中提取一些信息，並把它作爲一個形式的標籤值。但是，我收到一個類型錯誤。

一個簡單的演示之下（從耶索德書shell代碼）：

{-# LANGUAGE MultiParamTypeClasses #-} 
{-# LANGUAGE OverloadedStrings  #-} 
{-# LANGUAGE QuasiQuotes   #-} 
{-# LANGUAGE TemplateHaskell  #-} 
{-# LANGUAGE TypeFamilies   #-} 
import   Control.Applicative ((<$>), (<*>)) 
import   Data.Text   (Text) 
import   Data.Time   (Day) 
import   Yesod 
import   Yesod.Form.Jquery 

data App = App 

mkYesod "App" [parseRoutes| 
/HomeR GET 
|] 

instance Yesod App 

instance RenderMessage App FormMessage where 
    renderMessage _ _ = defaultFormMessage 

instance YesodJquery App 

data Person = Person 
    { personName   :: Text 
    } 
    deriving Show 

personForm :: Text -> Html -> MForm Handler (FormResult Person, Widget) 
personForm n1 = renderDivs $ Person 
    <$> areq textField n1 Nothing -- Changing n1 to "Name" works just fine. 


getHomeR :: Handler Html 
getHomeR = do 
    (widget, enctype) <- generateFormPost $ personForm "test" 
    defaultLayout 
     [whamlet| 
      <p> 
       The widget generated contains only the contents 
       of the form, not the form tag itself. So... 
     |] 


main :: IO() 
main = warp 3000 App 

當我運行程序與runhaskell，我得到以下錯誤：

Couldn't match expected type `FieldSettings site0' 
      with actual type `Text' 
In the second argument of `areq', namely `n1' 
In the second argument of `(<$>)', namely 
    `areq textField n1 Nothing' 
In the second argument of `($)', namely 
    `Person <$> areq textField n1 Nothing' 

我也試過(FieldSettings n1 Nothing Nothing Nothing [])但沒有運氣。有關如何將標籤值傳遞給areq的想法？

Answer 1

讓我們來看看areq

areq :: (RenderMessage master msg, RenderMessage master FormMessage) => Field sub master a -> FieldSettings msg -> Maybe a -> AForm sub master a 

類型，這樣areq功能估計不會有Text值作爲其第二個參數，它需要一個FieldSettings msg。那麼當你在源代碼中寫入「Name」時，它的工作原理是什麼？

在the documentation for yesod-forms中查找關於FieldSettings的文檔，我們看到它是一個具有IsString實例的數據類型。在the source code 望着具體實例中，我們看到：

instance (a ~ Text) => IsString (FieldSettings a) where 
fromString s = FieldSettings (fromString s) Nothing Nothing Nothing [] 

所以每次你寫一個字符串在你的源代碼（如果啓用了OverloadedStrings擴展名），編譯器將插入fromString實例的內容。

但是，您不想輸入字符串，而是想要從Text值創建FieldSettings。進一步看，我們看到FieldSettings的第一部分是SomeMessage，檢查documentation yet again和source我們看到SomeMessage可以通過使用SomeMessage構造函數創建。

personForm :: Text -> Html -> MForm Handler (FormResult Person, Widget) 
personForm n1 = renderDivs $ Person 
    <$> areq textField (FieldSettings (SomeMessage n1) Nothing Nothing Nothing []) Nothing