在Haskell中，如何从字符串的开始和结尾修剪空白字符？ [英] In Haskell, how do you trim whitespace from the beginning and end of a string?

问题描述

  trimabc
 
 => 
 
abc
  

编辑：

好吧，让我更清楚一点。我不明白字符串文字与字符串的处理方式差别很大。

我想这样做：

输入限定的Data.Text作为T 
在T.strip中输入s :: String =abc
 
  

Haskell可能吗？我使用的是-XOverloadedStrings，但只出现在文字上。

解决方案

如果您有严重的文本处理需求， code> text hackage包：

 > ：set -XOverloadedStrings 
>导入Data.Text 
> stripabc
abc
  

如果你太固执，不能使用 text 并且不喜欢反向方法的低效率，那么或许（并且我的意思是MAYBE）如下所示会更有效率：

  import Data.Char 
 
 trim xs = dropSpaceTail$ dropWhile isSpace xs 
 
 dropSpaceTail maybeStuff= 
 dropSpaceTail maybeStuff（x：xs）
 | isSpace x = dropSpaceTail（x：maybeStuff）xs 
 | null maybeStuff = x：dropSpaceTailxs 
 |否则=反向MaybeStuff ++ x：dropSpaceTailxs 
 
 
>修剪你好这\\ \\ t应该修剪好..我想.. \ t
你好这\ t应该修剪好..我想..
  
我写了这个假设，空间的长度是最小的，所以你的O（n） + + 和 reverse 是很不重要的。但是我再次觉得有必要说，如果你真的关心性能，那么你根本不应该使用 String   - 移动到文本。
 
 
 
编辑让我的观点，快速的Criterion基准测试告诉我（对于一个特别长的字符串，和空格）我的修剪需要1.6毫秒，使用反向的修剪需要3.5ms，并且 Data.Text.strip 需要0.0016毫秒... 
 
How do you trim whitespace from the start and end of a string?
trim "  abc " 

=>

"abc"
Edit:

Ok, let me be a little clearer. I did not understand that string literals were treated so differently from Strings.

I would like to do this:
import qualified Data.Text as T
let s :: String = "  abc  "
in T.strip s
Is this possible in Haskell? I am using -XOverloadedStrings but that appears only to work for literals.
解决方案
If you have serious text processing needs then use the text package from hackage:
> :set -XOverloadedStrings
> import Data.Text
> strip "  abc   "
"abc"
If you're too stubborn to use text and don't like the inefficiency of the reverse method then perhaps (and I mean MAYBE) something like the below will be more efficient:
import Data.Char

trim xs = dropSpaceTail "" $ dropWhile isSpace xs

dropSpaceTail maybeStuff "" = ""
dropSpaceTail maybeStuff (x:xs)
        | isSpace x = dropSpaceTail (x:maybeStuff) xs
        | null maybeStuff = x : dropSpaceTail "" xs
        | otherwise       = reverse maybeStuff ++ x : dropSpaceTail "" xs


> trim "  hello this \t should trim ok.. .I  think  ..  \t "
"hello this \t should trim ok.. .I  think  .."
I wrote this on the assumption that the length of spaces would be minimal, so your O(n) of ++ and reverse is of little concern. But once again I feel the need to say that if you actually are concerned about the performance then you shouldn't be using String at all - move to Text.

EDIT making my point, a quick Criterion benchmark tells me that (for a particularly long string of words with spaces and ~200 pre and post spaces) my trim takes 1.6 ms, the trim using reverse takes 3.5ms, and Data.Text.strip takes 0.0016 ms...

                        
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