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Question

我懷疑爲什麼會發生這種情況。我一直在關注「Yesod Web」電子書，但有一個腳手架的網站。當我到達想要在「messages」文件中應用「plural」函數的位置時，編譯器返回此錯誤：

Foundation.hs：52：1：not in scope：`plural'

其中複數是在與我稱之爲「哈姆雷特」的hs文件相同的hs文件中聲明的。但是，如果我在「Foundation.hs」文件的第52行之前移動函數聲明，那麼錯誤消失了，它讓我可以有效地進行編譯。爲什麼會發生？

module Handler.UserProfile where 

import Import 
import Data.Maybe (fromMaybe) 
import Data.Text (pack, unpack) 

viewCountName :: Text 
viewCountName = "UserProfileViews" 

readInt :: String -> Int 
readInt = read 

plural :: Int -> String -> String -> String 
plural 1 x _ = x 
plural _ _ y = y 

getUserProfileR :: Handler RepHtml 
getUserProfileR = do 

    viewCount <- lookupSession viewCountName 
       >>= return . (1 +) . readInt . unpack . fromMaybe "0" 
    setSession viewCountName (pack $ show viewCount) 

    maid <- maybeAuth 
    --msg <- getMessageRender 
    let user = case maid of 
        Nothing -> "(Unknown User ID)" --show MsgHello -- 
        Just (Entity _ u) -> userEmail u 
    defaultLayout $ do 
     setTitleI MsgUserProfile 
     $(widgetFile "nhUserProfile") 

Answer 1

看看在GHC用戶手冊：http://www.haskell.org/ghc/docs/7.4.2/html/users_guide/template-haskell.html#id684916

在劇中分期限制在第二個項目符號點描述：

You can only run a function at compile time if it is imported from another module. That is, you can't define a function in a module, and call it from within a splice in the same module. (It would make sense to do so, but it's hard to implement.)