警告：模式匹配（ES）的字符串匹配时重叠

Question

我试图在哈斯克尔简单的URL路径和周围的警告不能得到：

Warning: Pattern match(es) are overlapped 
      In a case alternative: "/" -> ... 
Ok, modules loaded: Main. 

的片段：

{-# LANGUAGE OverloadedStrings #-} 
import Network.Wai 
import Network.Wai.Handler.Warp (run) 
import Network.Wai.Middleware.Debug (debug) 
import Network.HTTP.Types (statusOK, status404) 
import qualified Data.ByteString as B 
import qualified Data.ByteString.Lazy as L 
import Data.ByteString.Char8 (unpack) 
import Data.ByteString.Lazy.Char8 (pack) 
import qualified Data.Text.Lazy as T 
import Control.Monad.IO.Class (liftIO, MonadIO) 

application req = do 
    case unpack $ rawPathInfo req of 
    "/items" -> itemsJSON 
    "/" -> indexPage 
    _ -> return $ responseLBS status404 [("Content-Type", "text/plain")] 
      "Not found" 

indexPage = do 
    page <- liftIO $ L.readFile "templates/index.html" 
    return $ responseLBS statusOK [("Content-Type", "text/html; charset=utf-8")] page 

itemsJSON = 
    return $ responseLBS statusOK 
    [("Content-Type", "application/json; charset=utf-8")] "hi" 

main = do 
    run 3000 $ debug $ application 

UPD： 取代片段与完整的计划，并

$ ghc -V 
The Glorious Glasgow Haskell Compilation System, version 6.12.1 

Answer 1

这是一个bug，并在更新版本的GHC中修复。