读取一个字符串输入

问题描述

  
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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