Parsec String（）（String，String）的含义是什么？ [英] What is the meaning of Parsec String () (String,String)?

问题描述

我理解了 Parsec 模块的 parse 函数，它带有规则参数，错误消息和输入字符串：

 解析规则文本= Parsec.parse规则（源）文本
  Parsec.Parsec 的含义，或者它是如何不同于 Parsec.ParsecT 。为什么自定义分析器的类型签名使用这个名称？
 
 
 
例如，在下面的代码片段中取自 this blogpost ，
  myParser :: Parsec.Parsec String（）（String，String）
 myParser = do 
 letters<  -  Parsec.many1 Parsec.letter 
 Parsec.spaces 
 digits<  -  Parsec.many1 Parsec.digit 
返回（字母，数字）
  
  Parsec.Parsec 和（）是否在 myParser 的类型签名中？
解决方案 
  ParsecT 和 Parsec  
 
 
 
在 parsec  3， ParsecT 和 Parsec 在  Text.Parsec.Prim 模块： 
 
 
 
 
  data ParsecT suma  
 
 
 
  ParsecT suma 是一个流类型为 s ，用户状态类型 u ，底层monad  m 和返回类型 a 。
 
 
 （流类型的例子是 String ， ByteString 和 Text 。）
 
 
 
  Parsec 只是 ParsecT 专门用于 身份 monad ：
 
 
  type Parsec su = ParsecT su Identity < code $ <
 
 
 
 
解释 myParser 的签名
 
 
 
返回到您的类型签名，位于 
 
  myParser :: Parsec.Parsec String （）（String，String）
  
 
 
 
 
 
流类型是 ST ring ; 
 
 
用户状态只是空元组（也称为单元）;换句话说， myParser 解析了一些内容，但没有记录任何有用的状态; 
 
结果类型是一对 String  s。
 
 
 
 
 
此外，类型签名使用因为在 haskell-parsec-basicsrel =nofollow>您链接的博文， Text.Parsec 被导入  qualified  为 Parsec 。
 
 
 
 解析器类型同义词
 
 
 
如果所有解析器都有流类型 String 并且不跟踪任何状态，您可能想要抽取一些 parsec 复杂度。在这种情况下，应该使用 Parser 类型同义词， Text.Parsec.String 模块定义为
 
 
 
  type Parser = Parsec String（）
  
例如，使用以下导入
  import Text.Parsec.String（Parser）
  
您可以将 myParser 的类型签名简化为
  myParser :: Parser（String，String）
  
 
I understand the Parsec modules' parse function, which takes a rule argument, an error message, and an input string:
parse rule text = Parsec.parse rule "(source)" text
However, I don't understand the meaning of Parsec.Parsec, or how it's different from Parsec.ParsecT. Why do type signatures of custom parsers use this name?

For example, in the following code snippet taken from this blogpost,
myParser :: Parsec.Parsec String () (String,String)
myParser = do
    letters <- Parsec.many1 Parsec.letter
    Parsec.spaces
    digits <- Parsec.many1 Parsec.digit
    return (letters,digits)
what does Parsec.Parsec and () mean in myParser's type signature?
解决方案
ParsecT and Parsec

In parsec 3, ParsecT and Parsec are defined and explained in the Text.Parsec.Prim module:

  
data ParsecT s u m a
  
  
ParsecT s u m a is a parser with stream type s, user state type u, underlying monad m and return type a.
(Examples of stream types are String, ByteString, and Text.)

Parsec is simply  a version of ParsecT specialised to the Identity monad:

  
type Parsec s u = ParsecT s u Identity


The signature of myParser explained

Going back to your type signature, in
myParser :: Parsec.Parsec String () (String,String)



the stream type is String;
the user state is simply the empty tuple (also known as "unit"); in other words, myParser parses something but doesn't keep track of any useful state;
the result type is a pair of Strings.


Moreover, the type signature uses Parsec.Parsec (and not simply Parsec) because, in the blogpost you link to, Text.Parsec is imported qualified as Parsec.

The Parser type synonym

If all your parsers have stream type String and don't keep track of any state, you probably want to abstract some of that parsec complexity away. In that case, you should use the Parser type synonym, which the Text.Parsec.String module defines as
type Parser = Parsec String ()
For instance, with the following import
import Text.Parsec.String ( Parser )
you can simplify myParser's type signature to
myParser :: Parser (String, String)


                        
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