parsing - 如果解析器失败，请尝试从下一个特殊符号出现处进行解析

Question

假设有一些解析器：

valid :: Parser String
valid = string "valid" <* skipWhile (/= '\n')

它可用于"valid"从多行文本中获取字符串：

> parseOnly (many $ valid <* optional endOfLine) "valid\nvalid\nvalid"
Right ["valid","valid","valid"]

如果有一行valid解析器失败，则根本不会解析进一步的文本：

> parseOnly (many $ valid <* optional endOfLine) "valid\ninvalid\nvalid"
Right ["valid"]

怎么Rigth["valid", "valid"]取而代之？我认为这try可能会有所帮助，但不确定如何从下一行继续解析。

Answer 1

使用秒差：

-- parser for the rest of the line
rest = manyTill anyChar (eof <|> char '\n' *> return ()) <* optional (char '\n')

-- change this to accept lines, but Just the valid ones
valid :: Parser (Maybe String)
valid = (Just <$> string "valid" <|> const Nothing <$> anyChar) <* rest

-- filter out Nothing
valids = catMaybes <$> many valid

-- Run
*Foo> runParser valids () "input" "valid1\ninvvalid2\nvalid3"
Right ["valid","valid"]
*Foo> runParser valids () "input" "valid1\nvalid2\nvalid3"
Right ["valid","valid","valid"]

在这里我必须做一个错误的破解：const Nothing <$> anyChar所以valid总是至少消耗一些东西，否则我不能把它交给many. 但是，Maybe您可以根据需要重写解析器（例如强制换行）

非常相似的方法适用于 attoparsec，抱歉破坏了自己制作的乐趣。

{-# LANGUAGE OverloadedStrings #-}
import Data.Attoparsec.Text
import Control.Applicative
import Data.Maybe
import Data.Text

-- parser for the rest of the line
rest = skipWhile (/= '\n') <* optional endOfLine

-- change this to accept lines, but Just the valid ones
valid :: Parser (Maybe Text)
valid = (Just <$> string "valid" <|> const Nothing <$> anyChar) <* rest

-- filter out Nothing
valids = catMaybes <$> many valid

*Main> parseOnly valids "valid1\nvalid2\nvalid3"
Right ["valid","valid","valid"]
*Main> parseOnly valids "valid1\ninvalid2\nvalid3"
Right ["valid","valid"]