Haskell编译器错误：不在范围内 [英] Haskell compiler error: not in scope

问题描述

  main = do 
 putStr我试图通过编写简单的文件来学习haskell util：源：
 srcPath<  -  getLine 
 putStr目的地：
 destPath<  -  getLine 
 putStrLn（从++ srcPath ++复制到++ destPath ++...）
内容<  -  readFile srcPath 
 writeFile destPath内容
 putStrLn完成
  

这会得到我

  GHCi，版本6.10.4：http：/ /www.haskell.org/ghc/：？寻求帮助
加载包ghc-prim ...链接...完成。 
加载软件包整数...链接...完成。 
加载程序包库...链接...完成。 
 [1的1]编译主（D：\Test.hs，解释）
 
 D：\Test.hs：8：22：不在范围内：`contents'
失败，模块加载：无。 
前奏> 
  

我不明白编译器错误，因为这个变量好像没问题。有什么不对？

以下是一个repro文件： at rapidshare

解决方案

看起来你混合了制表符和空格（看看你的问题在编辑视图中查看问题）。当您的编辑器均匀缩进查看代码时，编译器似乎对选项卡的宽度应有不同的解释，导致 writeFile destPath内容行被额外缩进。所以源代码就是这样解释的：

  ... 
 putStrLn（Copying from++ srcPath ++ to++ destPath ++...）
 contents<  -  readFile srcPath writeFile destPath contents 
 putStrLnFinished
  

在创建源代码 contents 之前，会先使用它来解释编译器错误。 / p>

为避免这类错误，最好不要使用标签，或者至少要格外小心，让它们始终如此。

I am trying to learn haskell by writing a simple file copy util:

main = do
         putStr "Source: "
         srcPath <- getLine
         putStr "Destination: "
         destPath <- getLine
         putStrLn ("Copying from " ++ srcPath ++ " to " ++ destPath ++ "...")
         contents <- readFile srcPath
         writeFile destPath contents
         putStrLn "Finished"

This gets me

GHCi, version 6.10.4: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done. 
[1 of 1] Compiling Main             ( D:\Test.hs, interpreted )

D:\Test.hs:8:22: Not in scope: `contents' 
Failed, modules loaded: none.
Prelude>

I don't understand that compiler error because the variable seems to be ok. What is wrong?

Here is a repro file: at rapidshare

解决方案

It looks like you mixed tabs and spaces (just look at your question in "edit" view to see the issue). While your editor views the code evenly indented, the Compiler seems to have a different interpretation how wide a tab should be, resulting in the writeFile destPath contents line being additionally indented. So the source is interpreted like this:

  ...
  putStrLn ("Copying from " ++ srcPath ++ " to " ++ destPath ++ "...")
  contents <- readFile srcPath writeFile destPath contents
  putStrLn "Finished"

In this interpretation of the source code contents is used before it is created, so you get a compiler error.

To avoid these kind of errors best don't use tabs, or at least take additional care that you use them consistently.

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