Hi Brent,

Think about what (show . host $ a) does. It takes 'a', converts it to... any type which is an instance of IPHost, and then shows that, turning it into a String. But there can be multiple types which are instances of IPHost, and each of them could have a *different* Show instance. So the exact String you get out depends on what type you pick... but there's no way for Haskell to infer which type to use. This is why it suggests to add a type signature. But I think there is probably some confusion that needs to be cleared up, the code you have doesn't quite make sense. Here are some questions to ask yourself:

1. Do you really want to be able to have multiple instances for IPHost and IPMask? (The answer might legitimately be 'yes', I'm just making sure.) What sorts of types do you have in mind that will be instances of these classes?

Yes. Basically my goal was to have something like: data IPv4Host = IPv4Host Word32 deriving (Show) instance IPHost IPv4Host where data IPv4Mask = IPv4Mask Word32 deriving (Show) instance IPMask IPv4Mask where data IPv4Addr = IPv4Addr IPv4Host IPv4Mask instance IPAddr IPv4Addr where and the equivalent for IPv6 (to be implemented later when I fugre out how to do it...)

2. Do you really intend for the 'host' and 'mask' methods of the IPAddr class to be able to return *any* types which are instances of IPHost and IPMask, respectively? (This is what the code says right now.) This is actually impossible given that IPHost and IPMask have no methods. Or do you mean that for a given instance of IPAddr, the 'host' method (say) will return some *particular* type which happens to be an instance of IPHost, but which type is returned may differ between instances of IPAddr?

Yes! That's exactly what I want (and yes there will be methods eventually in those classes)

If that's really what you mean, I would suggest either using a multi-parameter type class, like so:

class (IPHost h, IPMask m) => IPAddr a h m where host :: a -> h mask :: a -> m

Ok... So I guess the instance declaration for IPv4Addr will then become this: instance IPAddr IPv4Addr IPv4Host IPv4Mask where but how do I adjust the showIPAddr function to deal with that? I tried this: showIPAddr :: (IPAddr a h m) => a -> String but something is still missing, I get: Could not deduce (IPAddr a b m) from the context (IPAddr a h1 m1) arising from a use of `host' at IP.hs:23:23-26 Possible fix: add (IPAddr a b m) to the context of the type signature for `showIPAddr' In the second argument of `(.)', namely `host' In the first argument of `($)', namely `show . host' In the first argument of `(++)', namely `(show . host $ a)' Thanks a lot, Patrick

OR using existential quantification to hide the particular types returned by 'host' and 'mask'.

-Brent _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

-- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

On Thu, Jul 23, 2009 at 10:18 PM, Chaddaï Fouché wrote:

Your instance would look like that :

...

instance IPAddr IPv4Addr IPv4Host IPv4Mask where   type Host IPv4Addr = IPv4Host   type Mask IPv4Addr = IPv4Mask   host (IPv4Addr h _) = h   mask (IPv4Addr _ m) = m

Oops... I forgot to trim the extra parameters !

instance IPAddr IPv4Addr where type Host IPv4Addr = IPv4Host type Mask IPv4Addr = IPv4Mask host (IPv4Addr h _) = h mask (IPv4Addr _ m) = m

One advantage compared to the multiparameter + functional dependencies solution is that you can write : (IPAddr a) => ... In your context rather than introducing h and m when they're not needed. If you need to write "Host a" several time in a function, you can put the following in your context : (IPAdrr a, Host a ~ h) => ... and use h for Host a thereafter. -- Jedaï

On Fri, Jul 24, 2009 at 6:16 PM, Patrick LeBoutillier wrote:

In the declaration of the class IPAddr, is there any way to force that the IPHost and IPMask types are made up from the same IPBits type? Basically I would like the compiler to enforce that Word (Host a) and Word (Mask a) be the same type for a specific instance of IPAddr.

Unfortunately, this has not been implemented yet (in 6.10), though it should be in a future version of GHC (pretty soon probably), you'll then be able to write :

class (IPHost (Host a), IPMask (Mask a), Word (Host a) ~ Word (Mask a)) => IPAddr a where

but for now you must content yourself with adding it to the function context :

-- Takes an IPAddr and returns another one describing the network subnet :: (Word (Host a) ~ Word (Mask a)) => a -> a subnet a = let m = mask a h = host a in makeIPAddr (fromBits $ (bits h) .&. (bits m)) $ m

Note that I didn't put a IPAddr context since in your code subnet is a method of this class (with a default implementation), if this wasn't your intention you should correct the indentation. -- Jedaï

Patrick LeBoutillier wrote:

BTW: Does what I'm trying to do make any sense at all? Does anyone know of a better/simpler way to do this (i.e making most of these computations independant of the exact underlying type)?

Actually, I would try to mostly avoid typeclasses. If you want to change the representation sometime, that's still possible if you have abstraction! maybe like data IPAddr n = IPAddr n n where typically n would be Word32 or Word128(does that exist?), and a polymorphic function doing math on it might use a (Num n) => context. Actually you might sometimes need to know what kind of address it is. Then I guess we could have data IPVersion = V4 | V6 class IPAddrRepr n where ipVersion :: n -> IPVersion instance IPAddrRepr Word32 where ipVersion _ = V4 instance IPAddrRepr Word128 where ipVersion _ = V6 -- and for convenience, maybe, instance (IPAddrRepr n) => IPAddrRepr (IPAddr n) where ipVersion (IPAddr n _) = ipVersion n Alternatively I have an inkling that you could play with GADTs. But the basic Haskell type system is surprisingly powerful itself! There might be a way to write what you want without extensions. It depends on exactly what you're trying to do. For some purposes we might need to change/add to have a data IPVersioned = V4 (IPAddr Word32) | V6 (IPAddr Word128) -Isaac