haskell - parsec:意外字符解析嵌套注释

Question

我正在尝试解析嵌套的类 C block 注释

import Text.ParserCombinators.Parsec
import Control.Monad (liftM)

flat :: Monad m => m [[a]] -> m [a]
flat = liftM concat

comment :: Parser String
comment = between (string "/*") (string "*/") (try nested <|> content)
  where
    content = many (try (noneOf "*/")
                   <|> try (char '*' >> notFollowedBy (char '/') >> return '*')
                   <|> try (char '/' >> notFollowedBy (char '*') >> return '/'))
    nested  = flat $ many comment

"1234567890" 解析正常，但是当我尝试时

parse comment "" "/*123/*456*/789*/"

我明白了

Left (line 1, column 3):
unexpected "1"
expecting "/*" or "*/"

我想不通为什么，我能想到的地方都有try。请帮忙。

Answer 1

在类似 a <|> b 的表达式中, 如果 a可以匹配空串然后b永远不会尝试，这发生在try nested <|> content .

您可以通过要求至少一个评论匹配或另一个字符来修正您的方法:

comment :: Parser String
comment = between (string "/*") (string "*/") ( flat $ many $ (try comment <|> liftM toString other ) )
  where
    toString x = [x]
    other = try (noneOf "*/")
            <|> try (char '*' >> notFollowedBy (char '/') >> return '*')
            <|> try (char '/' >> notFollowedBy (char '*') >> return '/')

FWIW，这是如何Text.Parsec.Token这样做:

https://github.com/aslatter/parsec/blob/master/Text/Parsec/Token.hs#L698-714

对于您的特定情况，等效代码是:

import Data.List (nub)

commentStart = "/*"
commentEnd = "*/"

multiLineComment =
    do { try (string commentStart)
       ; inComment
       }

inComment = inCommentMulti

inCommentMulti
    =   do{ try (string commentEnd) ; return () }
    <|> do{ multiLineComment                     ; inCommentMulti }
    <|> do{ skipMany1 (noneOf startEnd)          ; inCommentMulti }
    <|> do{ oneOf startEnd                       ; inCommentMulti }
    <?> "end of comment"
    where
      startEnd   = nub (commentEnd ++ commentStart)