On Thu, Jul 23, 2009 at 4:55 PM, Patrick
LeBoutillier
Hi,
I'm playing around with typeclasses and trying to a feel on how you implement "inheritance" (not sure if that's the good word here) in Haskell.
We don't do "inheritance" in Haskell (though you can simulate it thanks to the power of the type system you generally don't need to as there is often a more natural solution with the other features of the type system). It would be more usefull if you could describe what you want to do in general and why you think you need inheritance for it.
class (Show a) => IPHost a where
class (Show a) => IPMask a where
Those class are basically synonyms of Show, is that intentional (and really useful ?) or do you think you'll add some other functions in there ?
class IPAddr a where host :: (IPHost b) => a -> b mask :: (IPMask b) => a -> b
This definition basically say that for a given instance a of IPAddr host can return any type that is an instance of IPHost, it so happen that with what we saw from your code so far this means that you can't write instances of IPAddr (since there are no function in IPHost and a typeclass is open)... so I somehow doubt that was what you wanted to say. Probably you wanted something like :
class (IPHost (Host a), IPMask (Mask a)) => IPAddr a where type Host a type Mask a host :: a -> Host a mask :: a -> Mask a
Now each instance of IPAddr will associate two type to the instancied type, one that will be an instance of IPHost and one of IPMask. And this function will compile :
showIPAddr :: (IPAddr a) => a -> String showIPAddr a = (show . host $ a) ++ "/" ++ (show . mask $ a)
I'm equally pretty sure that's not really what you want, but without more details I can't help much more. -- Jedaï