Re: [Haskell-beginners] Typeclasses and "inheritance"

24 Jul 2009

      Hi,

On Thu, Jul 23, 2009 at 4:26 PM, Chaddaï Fouché wrote:
...
On Thu, Jul 23, 2009 at 10:18 PM, Chaddaï
Fouché wrote:
...
Your instance would look like that :
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
This is great stuff and exactly what I wanted. Thank you very much guys!

I kept on hacking at it and so far I have this code:
http://hpaste.org/fastcgi/hpaste.fcgi/view?id=7428
which unfortunately doesn't  compile:

Net/IP.hs:44:39:
    Couldn't match expected type `Word (Host a)'
           against inferred type `Word (Mask a)'
    In the second argument of `($)', namely `(bits h) .&. (bits m)'
    In the first argument of `makeIPAddr', namely
        `(fromBits $ (bits h) .&. (bits m))'
    In the first argument of `($)', namely
        `makeIPAddr (fromBits $ (bits h) .&. (bits m))'

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.

Note: I'm not sure how practical all this is going to be in the end (perhaps
a bit to convoluted), but it's an excellent learning exercise for me.

Thanks a lot,

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