在新行上打印列表元素 [英] Printing elements of a list on new lines

问题描述

  printElements 
 
 :: [String]  - > IO（）
 printElements（x：xs）= print x（某种新行，然后循环？）printElements xs 
  

所以：

  [1，2，2，4 ] 
  

会给：

  1 
 2 
 3 
 4 
  

在大多数情况下，您不需要在列表上编写循环，它已经完成了。要使用monadic函数遍历列表，您可以使用 mapM （及其

如果您使用 print ， [1，2 ，3，4] 你会得到：

  Prelude> mapM_ print [1，2，3，4] 
1
2
3
4
前奏> 
  

  print :: Show a => a  - > IO（）
 print x = putStrLn（show x）
  

show 函数会导致字符串1转换为\1 \， putStrLn 会打印该文件和一个换行符。

如果您替换打印 putStrLn ，您将删除转换步骤并直接打印字符串：

  Prelude> mapM_ putStrLn [1，2，3，4] 
 1 
 2 
 3 
 4 
 Prelude> 
  

现在我想提供另一种解决方案。 Haskell做事的方式尽可能以纯粹的方式进行，只在需要时才使用IO。

所以在这种情况下，我们可以加入所有要用 \\\
打印字符串，并一次打印所有字符串。

要加入所有字符串有一个方便的功能： unlines

  Prelude> unlines [1，2，3，4] 
1 \\\
2\\\
3\\\
4\\\

 Prelude> 
  

现在你只需要打印它;注意 unlines 放置了在列表的最后一项之后的新行，所以我们将使用 putStr 而不是 putStrLn

  Prelude> putStr（unlines [1，2，3，4]）
 1 
 2 
 3 
 4 
 Prelude> 
  

I am trying to print the elements of my list onto new lines, but i cant get it to work;

printElements :: [String] -> IO()
printElements (x:xs) =  print x (some kind of newline, then loop?) printElements xs

So this:

["1","2","2","4"]

would give:

1
2
3
4

解决方案

In most cases you don't need to program a loop over a list, it's been done already. To loop over a list with a monadic function, you would use mapM (and its mapM_ variant if you don't care about the result.)

If you use print, for ["1","2","3","4"] you'd get :

Prelude> mapM_ print ["1","2","3","4"]
"1"
"2"
"3"
"4"
Prelude> 

print is actually :

print :: Show a => a -> IO ()
print x = putStrLn (show x)

the show function causes the string "1" to be converted to "\"1\"", putStrLn prints that and a newline.

If you replace the print by putStrLn, you remove the conversion step and print directly the string:

Prelude> mapM_ putStrLn ["1","2","3","4"]
1
2
3
4
Prelude> 

Now I would like to offer another solution. The Haskell way of doing things is doing as much as you can in a pure way, and only use IO when you need it.

So in this case we can join all strings to be printed with a \n, and print all the strings at once.

To join all the strings there's a handy function : unlines

Prelude> unlines ["1","2","3","4"]
"1\n2\n3\n4\n"
Prelude> 

Now you just have to print that; notice that unlines put a newline after the last item of the list, so we'll use putStr instead of putStrLn

Prelude> putStr ( unlines ["1","2","3","4"] )
1
2
3
4
Prelude> 

这篇关于在新行上打印列表元素的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！