"Dominic Steinitz" <dominic.j.steinitz@britishairways.com> 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 Jon.Fairbairn@cl.cam.ac.uk 31 Chalmers Road jf@cl.cam.ac.uk Cambridge CB1 3SZ +44 1223 570179 (after 14:00 only, please!)