you need to consume all the whitespace after the match

parsinghask.hs

-- Notes https://www.cs.utoronto.ca/~trebla/CSCC24-2023-Summer/08-parsing.html#token


import Text.Parsec
import Text.Parsec.String (Parser)

-- Parsec parser for a single integer
integerParser :: Parser Int
integerParser = (read <$> many1 digit) <* many space

-- Parsec parser for a list of integers separated by spaces
integerListParser :: Parser [Int]
integerListParser = integerParser `sepBy` spaces

-- Parsec parser for two lists separated by '|'
listsParser :: Parser ([Int], [Int])
listsParser = do
  list1 <- integerListParser 
  char '|' <* many space
  list2 <- integerListParser
  return (list1, list2)

-- Parse function
parseLists :: String -> Either ParseError ([Int], [Int])
parseLists input = parse listsParser "" input

main :: IO ()
main = do
    let input = "1 2 3 4 | 9 0 9 8"
        result = parseLists input

    case result of
        Left err -> print err
        Right (list1, list2) -> do
            putStrLn "List 1:"
            print list1

            putStrLn "List 2:"
            print list2