不在范围内:`catch' [英] Not in scope: `catch'

问题描述

我正在阅读，以了解您对haskell的极大兴趣，但是当我阅读输入&的异常部分时，我遇到了Not in scope:`catch'的错误.输出章节

I am reading learn you a haskell for great good but i faced an error of Not in scope: `catch' when I read the exception section in input & output chapter

这是我的代码:

import System.Environment  
import System.IO  
import System.IO.Error  

main = toTry `catch` handler  

toTry :: IO ()  
toTry = do (fileName:_) <- getArgs  
           contents <- readFile fileName  
           putStrLn $ "The file has " ++ show (length (lines contents)) ++ " lines!"  

handler :: IOError -> IO ()  
handler e = putStrLn "Whoops, had some trouble!"  

我收到此错误消息:

Not in scope: `catch'

推荐答案

catch 由 Control.Exception 模块导出.

import Control.Exception

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