[Haskell-cafe] Re: How to do a special kind of comment with the TokenParser

[Christian Maeder]

TokenParser seems to pose more problems than it solves. I think it is 
usually easier to define your own scanner and avoid the necessary 
Haskell language extensions used there. (Surely parts of the code from
TokenParser can be copied.)

Cheers Christian

John Ky wrote:
> Hi,
> 
> TokenParser supports two kinds of comments, the multi-line comments (ie. 
> {- -}) and the single line comments (ie. -- \n).
> 
> The language I am trying to parse, however, has comments which are 
> neither.  The -- acts like a single line comment which extends to the 
> end of the line usually, but can also be truncated to before the end of 
> the line by another --.  For example:
> 
>   noncomment -- comment comment
>   noncomment -- comment comment -- noncomment noncomment -- comment -- 
> noncomment
>   noncomment
> 
> I haven't been able to get the TokenParser to work with this style of 
> comment.  The best I could do was copy the whole Token module and modify 
> the code:
> 
> data LanguageDef st 
>     = LanguageDef
>     {      {- snip -}
>     *, commentLine    :: String*
>            {- snip -}
>     }
> 
>            {- snip -}
> 
> makeTokenParser languageDef
>     = TokenParser{   {- snip -}   }
>     where
>            {- snip -}
>       whiteSpace
>         | noLine && noMulti  = skipMany (simpleSpace *<|> customComment* 
> <?> "")
>         | noLine             = skipMany (simpleSpace *<|> customComment* 
> <|> multiLineComment <?> "")
>         | noMulti            = skipMany (simpleSpace *<|> customComment* 
> <|> oneLineComment <?> "")
>         | otherwise          = skipMany (simpleSpace *<|> customComment* 
> <|> oneLineComment <|> multiLineComment <?> "")
>         where
>           noLine  = null (commentLine languageDef)
>           noMulti = null (commentStart languageDef)
>       *customComment =
>         do{commentCustom languageDef
>           ;return()
>         }*
> 
> Then I put my specialised comment parser in the customComment field:
> 
> languageDef = TOKEN.LanguageDef
>    {        {- snip -}
>    , TOKEN.commentCustom = customComment
>             {- snip -}
>    }
>    where
>       customComment = do
>          string "--"
>          untilLineCommentEnd
>          return ()
> 
>       untilLineCommentEnd = do
>          c <- manyTill anyChar (string "\n" <|> try (string "--"))
>          return ()
> 
> Anyone know of a way I could reuse the TokenParser code rather than copy 
> and tweaking it?
> 
> Thanks
> 
> -John
> 
> 
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