在haskell中构造 [英] construct in haskell

问题描述

我正在努力学习Haskell，并希望编写一个将文件内容打印到屏幕上的小程序。当我将它加载到GHCi中时，我得到以下错误：

lockquote

'do'结构中的最后一个语句必须是一个表达式

我知道这个问题已经在这里问过了： Haskell - do中的最后一个语句必须是表达式。

即使我的代码非常相似，但我仍然无法弄清楚问题所在。

 模块Main（main）其中
如果有人可以向我指出问题，我会非常感激。 
 import System.IO 
 import System（getArgs）
 $ b $ main :: IO（）
 main = do 
 args<  -  getArgs 
 inh < -  openFile $ ReadMode head args 
 printFile inh 
 hClose inh 
 
 printFile :: Handle  - > IO（）
 printFile handle = do 
 end<  -  hIsEOF handle 
 if end b $ b then return（）
 else do line<  -  hGetLine handle 
 putStrLn line 
 printFile handle 
  

解决方案

您的缩进被打破。这些更好：

  printFile :: Handle  - > IO（）
 printFile handle = do 
 end<  -  hIsEOF handle 
 if end b $ b then return（）
 else do line<  -  hGetLine handle 
 putStrLn行
 printFile句柄
 
 printFile :: Handle  - > IO（）
 printFile handle = do 
 end<  -  hIsEOF handle 
 if end b $ b then return（）
 else do 
 line<  -  hGetLine处理
 putStrLn行
 printFile处理
  

通过使如果进一步缩进< end < - hIsEof句柄，它实际上是一个续行，而不是做。同样， putStrLn行少于行<-hGetLine句柄的事实意味着在 else 内）在那里结束。

I'm trying to learn Haskell and want to write a small program which prints the content of a file to the screen. When I load it into GHCi I get the following error:

The last statement in a 'do' construct must be an expression

I know this question has be asked already here: Haskell — "The last statement in a 'do' construct must be an expression".

Even though my code is very similar I still can't figure out the problem. If anyone could point out the problem to me I'd be very thankful.

module Main (main) where

import System.IO
import System(getArgs)

main :: IO()
main = do
    args <- getArgs
    inh <- openFile $ ReadMode head args
    printFile inh
    hClose inh

printFile :: Handle -> IO ()
printFile handle = do
    end <- hIsEOF handle
        if end
            then return ()
            else do line <- hGetLine handle
                putStrLn line
                printFile handle

解决方案

Your indentation is broken. These are better:

printFile :: Handle -> IO ()
printFile handle = do
    end <- hIsEOF handle
    if end
        then return ()
        else do line <- hGetLine handle
                putStrLn line
                printFile handle

printFile :: Handle -> IO ()
printFile handle = do
    end <- hIsEOF handle
    if end
        then return ()
        else do
            line <- hGetLine handle
            putStrLn line
            printFile handle

By having if further indented than end <- hIsEof handle, it was actually a line continuation, not a subsequent action in the do. Similarly, the fact that you had putStrLn line less indented than line <- hGetLine handle means that the do (inside the else) ended there.

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