[Haskell-beginners] Parsec newbie question

[Daniel Fischer]

Am Montag 31 August 2009 19:33:48 schrieb Patrick LeBoutillier:
> Hi all,
>
> I'd like to use parsec to parse IP addresses. So far I've written a (tiny)
> parser for bytes:
>
> byte :: GenParser Char st Word8
> byte = do
>          n <- many1 digit
>          return (read n)
>
> The function works fine, but it accepts numbers greater than 255.
> How do I encapsulate this condition in the parser so that it fails (with en
> appropriate error message) in this case?
>

byte :: GenParser Char st Word8
byte = do
    ds <- many1 digit
    let n :: Integer  -- or Int, if you're not too paranoid
        n = read ds
    if n < 256
      then return (fromIntegral n)
      else fail $ "Number " ++ ds ++ " too large for a byte."

>
> Thanks a lot,
>
> Patrick