功能超出范围

Question

我怀疑为什么会发生这种情况。我一直在关注“Yesod Web”电子书，但有一个脚手架的网站。当我到达想要在“messages”文件中应用“plural”函数的位置时，编译器返回此错误：

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

其中复数是在与我称之为“哈姆雷特”的hs文件相同的hs文件中声明的。但是，如果我在“Foundation.hs”文件的第52行之前移动函数声明，那么错误消失了，它让我可以有效地进行编译。为什么会发生？

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用户手册：http://www.haskell.org/ghc/docs/7.4.2/html/users_guide/template-haskell.html#id684916

在剧中分期限制在第二个项目符号点描述：

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.)