"Dominic Steinitz" <[EMAIL PROTECTED]> wrote:
> I've managed to crack something that always annoyed me when I used to do
> network programming.
[. . .]
>
> Suppose I want to send an ICMP packet. The first byte is the type and the
> second byte is the code. Furthermore, the code depends on the type. Now you
> know at compile time that you can't use codes for one type with a different
> type. However, in Pascal (which is what I used to use) you only seemed to
> be
> able to carry out run time checks.
I'm not sure I understand your problem. I don't see what's
wrong with the following approach, which is Haskell 98. The
type byte is coded as the type of the packet. Excuse the
perhaps ideosyncratic style ... (in particular, I'm
expecting people to use import qualified with this).
module ICMP where
data Type = Redirect RedirectData
| TimeExceeded TimeData
{- so you get an alternative for each of the packet types -}
instance Enum Type where
fromEnum (Redirect _) = 5
fromEnum (TimeExceeded _) = 11
{- we can't derive Enum for ICMP.Type, because it has non-nullary
constructors. That just makes it a bit more tedious
One could provide a class "code" with code:: t -> Int
instead of fromEnum
-}
{- now we define individual record types for each of the different
ICMP types -}
data RedirectData = RedirectData {redirectCode:: RedirectCode,
ip_addr:: Int, -- whatever
redirectData:: [Int]} -- or whatever
data RedirectCode = RedirNet
| RedirHost
| RedirNetToS
| RedirHostToS
deriving Enum
data TimeData = TimeData {timeCode:: TimeExceededCode,
timeData:: [Int]} -- or whatever
data TimeExceededCode = ExcTTL
| ExcFragTime
deriving Enum
{- Since Haskell 98 doesn't have MPTCs, if we want to
encode packets as anything other than [Int] we'd have
to define more classes. Encode serves as an example. -}
class Encode t where
encode:: t -> [Int]
instance Encode Type where
encode p@(Redirect d) = fromEnum p: encode d
encode p@(TimeExceeded d) = fromEnum p: encode d
instance Encode RedirectData where
encode d = fromEnum (redirectCode d): ip_addr d: redirectData d
instance Encode TimeData where
encode d = fromEnum (timeCode d): 0: timeData d
so one can go
encode (Redirect (RedirectData RedirNet 0 [0]))
and get [5,0,0,0], but
encode (TimeExceeded (TimeData RedirNet 0 [0]))
gives an error, as one would hope. What am I missing?
Cheers,
Jón
--
Jón Fairbairn [EMAIL PROTECTED]
31 Chalmers Road [EMAIL PROTECTED]
Cambridge CB1 3SZ +44 1223 570179 (after 14:00 only, please!)
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