Bulat, Dmitri is not necessarily "mixing [...] class and its (default) instance". Bulat, one could suspect a reasonable use of default class methods: http://www.haskell.org/onlinereport/decls.html#overloading Indeed, Dmitri's declarations make sense as default class methods. ... except for Jared's observation: the attempt to rely on Ord. If %< in place of < was used on the RHSs, then this would really look like default methods.
Ralf PS: We don't want to send people using overlapping instances more often than necessary :-) > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:haskell-cafe- > [EMAIL PROTECTED] On Behalf Of Bulat Ziganshin > Sent: Tuesday, July 25, 2006 9:46 AM > To: Dmitri O.Kondratiev > Cc: haskell-cafe@haskell.org > Subject: Re: [Haskell-cafe] Newbie Q: Deriving MyOrd from Eq problem > > Hello Dmitri, > > Tuesday, July 25, 2006, 8:15:41 PM, you wrote: > > > class Eq a => MyOrd a where > > (%<=), (%>), (%>=) :: a -> a -> Bool > > x %<= y = (x < y || x == y) > > x %> y = y < x > > x %>= y = (y < x || x == y) > > you are mixing definition of class and its (default) instance. try the > following: > > class Eq a => MyOrd a where > (%<=), (%>), (%>=) :: a -> a -> Bool > > instance Ord a => MyOrd a where > x %<= y = (x < y || x == y) > x %> y = y < x > x %>= y = (y < x || x == y) > > although i don't think it is what you want. actually Haskell don't > have a good way to define default instance for some subclass, although > there are some proposals how this can be addressed. you should > either define exactly one instance for all "Ord"ed types or duplicate > this trivial definition in every instance. in GHC you also has an > option to use "overlapping" instances but this can work only if all > your other "instance" definitions don't use deriving from typeclasses. > i.e. the following is prohibited: > > class Eq a => MyOrd a where ... > instance Ord a => MyOrd a where ... > instance SomeClass a => MyOrd a where ... > > and even if you will be accurate, i'm not sure that "overlapping" will > work properly > > ps: you can ask me in Russian via private mail > > -- > Best regards, > Bulat mailto:[EMAIL PROTECTED] > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
