parser.hs

-- | [33] IntConstant     ::=  ('+' | '-')? Digit+
-- >>> parse intConstant "fail" "+123"
-- Right "+123"
--
-- >>> parse intConstant "fail" "0003"
-- Right "0003"
--
-- >>> parse intConstant "fail" "-321"
-- Right "-321"
intConstant :: GenParser Char st String
intConstant = do
    chr <- optionMaybe (char '+' <|> char '-')
    str <- many digit
    return $ case chr of Just a -> a : str
                         Nothing -> str