a nice Pair,, was: Multiparameter classes in HUGS and GHC

[Lloyd Allison]

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
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