[Haskell-cafe] Parsec question

[Michael Mossey]

Jason Dusek wrote:
> 2009/04/17 minh thu <noteed at gmail.com>:
>> 2009/04/17 Michael Mossey <mpm at alumni.caltech.edu>:
>>> I wonder how I can get the manyTill to be happy with eof
>>> before finding the //? I tried
>>>
>>> parseText = manyTill anyChar (try (string "//") <|> eof)
>>>
>>> but got a type error.
>> You can use 'notFollowedBy' [...]
> 
>   You get a type error because `string "//"` parses to a
>   `String` while `eof` parses to a `()`. Instead you might use:
> 
>     parseText = manyTill anyChar (try (string "//" >> return ()) <|> eof)
> 
> --
> Jason Dusek

Ah.. I think I get it... in the function manyTill, the second argument type doesn't 
matter.. doesn't have to match the first argument type.

Here's what I have so far. It works, but it's a bit weird to consume the // as part 
of the text rather than the keyword. That happens because the try( string "//" ), 
which is part of the end arg to manyTill, consumes the // when it succeeds. But maybe 
it is the most natural way to express the problem.

parseKeyword :: Parser String
parseKeyword = many1 (alphaNum <|> char '_')

parseText :: Parser String
parseText = manyTill anyChar ((try (string "//") >> return ())
                               <|> eof)

parsePair :: Parser (String,String)
parsePair = do k <- parseKeyword
                t <- parseText
                return (k,t)

parseFile :: Parser [(String,String)]
parseFile = do _ <- parseText   -- to skip any text at beginning and 'sync up'
                p <- many parsePair
                return p