readBin :: String -> Integer
readBin []  = 0
readBin str = (readBin $ init str) * 2 + (read [last str])

parseBinNumber :: Parser LispVal
parseBinNumber = do char '#'
                    char 'b'
                    input <- many1 $ oneOf "01"
                    return $ Number $ readBin input

parseOctNumber :: Parser LispVal
parseOctNumber = do char '#'
                    char 'o'
                    input <- many1 $ oneOf "01234567"
                    return $ Number $ fst $ head $ readOct input

parseDecNumber :: Parser LispVal
parseDecNumber = do char '#'
                    char 'd'
                    input <- many1 digit
                    return $ Number $ read input

parseHexNumber :: Parser LispVal
parseHexNumber = do char '#'
                    char 'x'
                    input <- many1 $ oneOf "0123456789aAbBcCdDeEfF"
                    return $ Number $ fst $ head $ readHex input

parseDefaultNumber :: Parser LispVal
parseDefaultNumber = do input <- many1 digit
                        return $ Number $ read input

parseNumber :: Parser LispVal
parseNumber = parseBinNumber
          <|> parseOctNumber
          <|> parseDecNumber
          <|> parseHexNumber
          <|> parseDefaultNumber