How to parse a line using Parsec?

Question

I'm using the parsing library Parsec to parse some text. I simply need to parse lines, which are strings of arbitrary characters, ending with a '\n' or an eof when its at the end of the string. When calling parseHS' I get the complaint that Exception: Text.ParserCombinators.Parsec.Prim.many: combinator 'many' is applied to a parser that accepts an empty string..

parseHS' :: String -> Either ParseError [String]
parseHS' input = parse hsFile' "(unknown)" input

hsFile' :: GenParser Char st [String]
hsFile' = do
    many1 line

line :: GenParser Char st String
line = do
    result <- many (noneOf "\n")
    optional newline
    return result

How could this be achieved correctly?

Answer 1

Of course, if you only need to split the input by lines, you could use lines.

sepEndBy in Parsec does what you want - splits input into a list of parsed entities separated by a given separator, optionally ending with it or eof.

Your grammar for line permits the parser to produce a never-ending stream of lines for any input. This can be resolved by making the decision about newline externally to line:

hsFile' = do
        x <- line
        xs <- many $ do
                newline
                line
        eof
        return (x:xs)

line = many $ noneOf "\n"

This one will produce a empty line at the end in case the file ends with newline.