On Wed, Apr 30, 2003 at 07:20:06PM +0100, Graham Klyne wrote:
I've trying to understand better how to use multiparameter classes, and in particular the things that can be declared as instances. I've consulted the following: ...
[[ class (Eq k, Show k) => Pair a k v where newPair :: (k,v) -> a k v getPair :: a k v -> (k,v)
newtype MyPair1 k v = P1 (Int,String)
instance Pair MyPair1 Int String where newPair (x,y) = P1 (x,y) getPair (P1 (x,y)) = (x,y) ...
--The nicest Pair that I've ever managed is not: class Pair p a b where -- if we use fst' then b is "loose" and v.v. fst' :: p->a -- seems to be :: forall a.(p->a), regardless class Pair p. snd' :: p->b -- can't seem to get a and b from the instances. -- Seems to be a problem overloading both fst and snd in one class -- but OK if one operator per class ... --But rather: -- ... so we can split Pair into two classes, "left" and "right"! class PairL p a where fst'' :: p->a -- OK but a needs to be implied by use class PairR p b where snd'' :: p->b -- ditto instance PairL (((,) a b)) a where fst'' (x,y) = x instance PairR (((,) a b)) b where snd'' (x,y) = y -- could make (a trivial) Pair' a subclass of PairL and PairR --which incidentally will combine with class Function fnType t u where ($) :: fnType -> t -> u -- NB hid std $ instance Function (t->u) t u where f $ x = f x -- Hask' list 29 Oct 2002 --allowing instance Function ((t->u),(v->w)) (t,v) (u,w) where (f, g) $ (x, y) = (f x, g y) --e.g. f2b = ( (\ch->if ch=='a' then 'z' else ch), (not) ) ... ( (f2b $ ('a', False)) :: (Char,Bool) ) -- need the type ... Lloyd -- Lloyd ALLISON, CSSE, Monash University, Victoria, Australia 3800. web: http://www.csse.monash.edu.au/~lloyd/ tel: +61 3 9905 5205 use: http://www.linux.org/ OpenOffice: http://www.openoffice.org/