Haskell的`reads`在哪些场景下返回多个列表项？

问题描述

我可以这样做：

  Prelude>读取1 2 3:: [（Int，String）] 
 [（1，2 3）] 
 Prelude> （1,2）（3,4）:: [（（Int，Int），String）] 
 [（（1,2），（3,4））] 
前奏> （1,2）（3,4）:: [（（Int，Int），String）] 
 [（（1,2），（3,4））] 
前奏>读取（1,2）\\\
（3,4）:: [（（Int，Int），String）] 
 [（（1,2），\\\
（3,4） ）] 
前奏> （1,2）（3,4）:: [（（Int，Int），String）] 
 [（（1,2），（3,4））] 
  

我可以派生读取并读取以读取它们。但是我从来没有得到过读取来返回列表中的多个元组。为什么读取返回一个列表？

解决方案

没有标准实例这样做，但它的目的是模糊的分析;因为这不是非常有用，并且使用此功能的解析器效率非常低，所以的读取的返回值实际上是用于 Maybe 伪装成 [] 。

的定义揭示了多个解析的预期含义：

  read ::（阅读a）=>字符串 - > 
读取s = case [x | （x，t）<  - 读取
 [x]  - >的s，（，） -  lex t] x 
 []  - >错误PreludeText.read：no parse
 _  - >错误PreludeText.read：ambiguous parse
  

因此：历史原因基本上。 b $ b

I can do the following:

Prelude> reads "1 2 3" :: [(Int, String)]
[(1," 2 3")]
Prelude> reads "(1,2) (3,4)" :: [((Int, Int), String)]
[((1,2)," (3,4)")]
Prelude> reads "(1,2)(3,4)" :: [((Int, Int), String)]
[((1,2),"(3,4)")]
Prelude> reads "(1,2)\n(3,4)" :: [((Int, Int), String)]
[((1,2),"\n(3,4)")]
Prelude> reads "(1,2)    (3,4)" :: [((Int, Int), String)]
[((1,2),"    (3,4)")]

I can derive Read and get reads to read those too. But I've never gotten reads to return more than one tuple in the list. Why does reads return a list?

解决方案

None of the standard instances do so, but it's intended for ambiguous parses; since this is not really very useful, and parsers that use this functionality would be very inefficient, reads's return value is for all practical purposes a Maybe masquerading as a [].

The Report's definition of read reveals the intended meaning of multiple parses:

read    :: (Read a) => String -> a  
read s  =  case [x | (x,t) <- reads s, ("","") <- lex t] of  
              [x] -> x  
              []  -> error "PreludeText.read: no parse"  
              _   -> error "PreludeText.read: ambiguous parse"

So: historical reasons, basically.

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