[Haskell-beginners] Typeclasses and "inheritance"

[Chaddaï Fouché]

On Thu, Jul 23, 2009 at 10:18 PM, Chaddaï
Fouché<chaddai.fouche at gmail.com> 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