读取一个字符串输入 [英] Read a string input

问题描述

 1 3  -  $ 
 
我试图编写Read类的实例来读取输入，如下所示： b $ b  -  2 3 
  -   -  5
  

将其转换为 [[Maybe Int]]

- 会被翻译成 Nothing
3将是只是3

  （'a'，3） - （'b'，7）
（'c'，5）（'e'，0） -  
（'d'，9） - （'h' ，8）
  

将其转换为 [[Maybe（Char，Int） ]]

- 将被转换为Nothing
（'a'，3）将会被变为 Just（'a'，3）

我试图通过使用List of Char来编写它们，但它需要很多工作的。你有什么建议吗？对不起，我对Haskell很新，所以我问你这个问题。 ：（

）

解决方案

如果您摆脱了 - 可以很快做到这一点

  Prelude>（map（map read）。map words。lines $（'a'， 3）（'b'，4）\\\
（'c'，5））:: [[（Char，Int）]] 
 [[（'a'，3），'''或者将它定义为一个函数，4）]，[（'c'，5）]] 
  

  genericReadLines :: Read a => String  - > [[a]] 
 genericReadLines = map（map阅读）。map words。lines 
  

你可以这样使用：

  * Main>（genericReadLines（'a'，3）（'b'，4）\\\
（'c'，5））:: [[（Char，Int）]] 
 [[（'a'，3），（'b'，4）]，[（'c'，5）]] 
  

但您可能会发现它更容易做到

  readCharInts :: String  - > [[（Char，Int）]] 
 readCharInts = genericReadLines 
 $ b $ readInts :: String  - > [[Int]] 
 readInts = genericReadLines 
  
 
 $ b $ p 
 $ b pre $ $ $ $ $ $ $ $ *主> readInts1 2 3\\\
4 5 6\\\
7 8 9
 [[1,2,3]，[4,5,6]，[7,8,9]] 
 *主> readCharInts（'a'，3）（'b'，4）\\\
（'c'，5）
 [[（'a'，3），（'b'，4）]， [（'c'，5）]] 
  

---

但是如何保持 - ？您必须使用Maybe数据类型来表示对列表中的某些点没有值;我们可以使用 - 作为 Nothing 和 a 的缩写。 的简写。

 阅读'::阅读a => ;字符串 - >也许一个
读' - =没有
读'xs = Just（读xs）
  

我应该警告你，如果你的数据可能是' - '，那么代码是脆弱的，但也许它不能。

  genericMaybeReadLines :: Read a =>字符串 - > [[也许a]] 
 genericMaybeReadLines =地图（地图阅读'）。地图字。行
  

然后我们可以有



  readMaybeCharInts :: String  - > [[Maybe（Char，Int）]] 
 readMaybeCharInts = genericMaybeReadLines 
 
 readMaybeInts :: String  - > [[Maybe Int]] 
 readMaybeInts = genericMaybeReadLines 
  

现在我们可以做

  * Main> readMaybeCharInts（'a'，3）（'b'，4）\\\
-（'c'，5）
 [[Just（'a'，3），Just（'b'，4 ）]，[Nothing，Just（'c'，5）]] 
 * Main> readMaybeInts2 3 -\
4  -  2
 [[Just 2，Just 3，Nothing]，[Just 4，Nothing，Just 2]] 
  




I am trying to write instance of Read class to read an input as string as follow:

"1 3 -    
 - 2 3      
 - - 5" 

Convert it into [[Maybe Int]]

"-" will be translated into Nothing "3" will be Just 3

"('a',3) - ('b',7)
 ('c',5) ('e',0) -  
 ('d',9) - ('h',8)"

Convert it into [[Maybe (Char,Int)]]

"-" will be translated into Nothing "('a',3)" will be Just ('a',3)

I tried to write them by handling with List of Char, but it takes a lot of work. Do you have any suggestion? Sorry, I am quite new to Haskell, so I ask you that question. :(

解决方案

If you got rid of the - entries, you could do this very quickly as

Prelude> (map (map read) . map words. lines $ "('a',3) ('b',4)\n('c',5)")::[[(Char,Int)]]
[[('a',3),('b',4)],[('c',5)]]

Or define it as a function

genericReadLines :: Read a => String -> [[a]]
genericReadLines = map (map read) . map words. lines

Which you can use thus:

*Main> (genericReadLines "('a',3) ('b',4)\n('c',5)")::[[(Char,Int)]]
[[('a',3),('b',4)],[('c',5)]]

but you may find it easier to do

readCharInts :: String -> [[(Char,Int)]]
readCharInts = genericReadLines

readInts :: String -> [[Int]]
readInts = genericReadLines

So you can just type

*Main> readInts "1 2 3\n4 5 6\n7 8 9"
[[1,2,3],[4,5,6],[7,8,9]]
*Main> readCharInts "('a',3) ('b',4)\n('c',5)"
[[('a',3),('b',4)],[('c',5)]]

---

But what about keeping the -? You'll have to use a Maybe data type, to represent not having a value for certain points in your list; we can use - as shorthand for Nothing and a as shorthand for Just a.

read' :: Read a => String -> Maybe a
read' "-" = Nothing
read' xs = Just (read xs)

I should warn you that that code is fragile if your data could possibly be '-', but perhaps it can't.

genericMaybeReadLines :: Read a => String -> [[Maybe a]]
genericMaybeReadLines = map (map read') . map words. lines

Then we can have

readMaybeCharInts :: String -> [[Maybe (Char,Int)]]
readMaybeCharInts = genericMaybeReadLines

readMaybeInts :: String -> [[Maybe Int]]
readMaybeInts = genericMaybeReadLines

So now we can do

*Main> readMaybeCharInts "('a',3) ('b',4)\n- ('c',5)"
[[Just ('a',3),Just ('b',4)],[Nothing,Just ('c',5)]]
*Main> readMaybeInts "2 3 -\n4 - 2"
[[Just 2,Just 3,Nothing],[Just 4,Nothing,Just 2]]

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