读实例导致解析错误 [英] read instance causes parse error

问题描述

我想实现一个读取实例，使我能够读取一个字符串（例如： - - 8 - 3 - ）并构建一个包含这些值的列表。

 数据值a = Nul | Val a 
 
 showsValue ::（Show a）=>值a  - > ShowS 
 showsValue（Val x）=（Value++）。显示x 
 showsValue（Nul）=（Nothing 0++）
 
实例显示a =>显示（值a）其中
显示Prec _ x =显示值x 
 
实例读取a =>读取（值a）其中
 readsPrec_m = readsMatrix m 
 
 readsMatrix ::（读取a）=> ReadS（Value a）
 readsMatrix（' - '：s）= [（Nul，rest）| （''，rest）<  - 读取s] 
 readsMatrix s = [（Val x，rest）| （x，''：rest）<  - 读取s] 
  

执行此测试后： p>

 读取 -  8  -   -  3:: Value Int 
  

我得到错误 *** Exception：Prelude.read：no parse

我在这里做错了什么？

更新

  readsMatrix（' - '：s）= [（Nul，dropWhile isSpace s）] 
 readsMatrix s = [（Val x，dropWhile isSpace rest） （x，rest）<  -  reads s] 
  

解决方案

改正值的读取a

首先，不要打扰删除空格，这一切都可以通过 reads 处理：

  readsMatrix ::（阅读a）=> ReadS（Value a）
 readsMatrix（' - '：s）= [（Nul，dropWhile（==''）s）] 
 readsMatrix s = [（Val x，rest）| （x，rest）<  - 读取s] 
  

  *主要>阅读4:: Value Int 
 Value4 
 * Main> read - :: Value Int 
 Nothing 0 
  

正确读取[Value a]

但是你想读取由空格分隔的列表，因为这是非标准行为，所以你需要编写一个自定义的 readList： ：:: ReadS [Value a]

  instance读取a =>读取（值a）其中
 readsPrec _ m = readsMatrix m 
 readList s = [（map read.words $ s，）] 
  
  * Main> pre 
 
 
阅读 -  4 2  -  5:: [Value Int] 
 [Nothing 0，Value4，Value2，Nothing 0，Value5] 
  
但不幸的是，
 
 
  * Main>阅读 -  4 2 \ n  -  5 4:: [Value Int] 
 [Nothing 0，Value4，Value2，Nothing 0，Value5，Value4] 
  pre> 
 
 
甚至更糟，
 
 
  *主要>阅读 -  4 2 \ n  -  5 4:: [[Value Int]] 
 ***例外：Prelude.read：不分析
  
 
 
 
阅读一个矩阵 
 
 
没有简单的方法可以看到，因为没有 readListOfLists 在 Read 类中，所以为什么不做一个独立的函数
  matrix :: Read a =>字符串 - > [[Value a]] 
 matrix = map read.lines 
  
这样
  * Main>矩阵3 4  -  \\ n3  -  6 \ n4 5  - :: [[Value Int]] 
 [[Value3，Value4，Nothing 0]，[Value3，Nothing 0，Value6]，[Value4， Value5，Nothing 0]] 
  
 
 
 
显示不是我想要的选项
 
 
 
我认为您的显示实例 Value 有点误导（ Nul 没有零， Val 没有被写入值）。要么写 
 
 
  data数值a = Nul | Val a派生显示
  
以便它看起来像它或将其定义为匹配 Read  instance 
  instance显示a =>显示（值a）其中
显示Nul = - 
显示（Val a）=显示
  
 
I want to implement an instance of read that enables me to read a string (ex: "- - 8 - 3 -") and construct a list containing those values.
data Value a = Nul | Val a

showsValue :: (Show a) => Value a -> ShowS
showsValue (Val x) =  ("Value" ++) . shows x
showsValue (Nul)   =  ("Nothing 0" ++)

instance Show a => Show (Value a) where
    showsPrec _ x = showsValue x

instance Read a => Read (Value a) where
    readsPrec _ m = readsMatrix m

readsMatrix :: (Read a) => ReadS (Value a)
readsMatrix ('-':s) = [(Nul, rest) | (' ',rest) <- reads s]
readsMatrix s       = [(Val x,rest)| (x,' ':rest) <- reads s]
After performing this test:
read "- 8 - - 3" :: Value Int
I get the error *** Exception: Prelude.read: no parse


What am I doing wrong here?

update
readsMatrix ('-':s) = [(Nul, dropWhile isSpace s)]
readsMatrix s = [(Val x, dropWhile isSpace rest) | (x,rest) <- reads s]

解决方案
Correcting read for Value a

First, don't bother removing spaces, that's all handled fine by reads:
readsMatrix :: (Read a) => ReadS (Value a)
readsMatrix ('-':s) = [(Nul, dropWhile (==' ') s)]
readsMatrix s       = [(Val x,rest)| (x,rest) <- reads s] 
Your read instance is now fine for single Values: 
*Main> read "4" :: Value Int
Value4
*Main> read "-" :: Value Int
Nothing 0


Correcting read for [Value a]

But you want to read lists as separated by spaces, so since that's non-standard behaviour, you'll need to write a custom readList :: :: ReadS [Value a]
instance Read a => Read (Value a) where
    readsPrec _ m = readsMatrix m
    readList s = [(map read.words $ s,"")]
So now
*Main> read "- 4 2 - 5" :: [Value Int]
[Nothing 0,Value4,Value2,Nothing 0,Value5]
but unfortunately, 
*Main> read "- 4 2 \n- 5 4" :: [Value Int]
[Nothing 0,Value4,Value2,Nothing 0,Value5,Value4]
and even worse, 
*Main> read "- 4 2 \n- 5 4" :: [[Value Int]]
*** Exception: Prelude.read: no parse


Reading a matrix

There's no straightforward way round this that I can see, because there's no readListOfLists in the Read class, so why not make a standalone function 
matrix :: Read a => String -> [[Value a]]
matrix = map read.lines
so that
*Main> matrix "3 4 -\n3 - 6\n4 5 -" :: [[Value Int]]
[[Value3,Value4,Nothing 0],[Value3,Nothing 0,Value6],[Value4,Value5,Nothing 0]]


Show isn't what I would choose

I think your Show instance for Value is a little misleading (Nul doesn't have a zero on it, Val isn't written Value). Either write 
data Value a = Nul | Val a  deriving Show
so that it looks as it is or define it to match the Read instance
instance Show a => Show (Value a) where
  show Nul = "-"
  show (Val a) = show a


                        
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