On Wed, 26 Mar 2008, Ganesh Sittampalam wrote:
On Wed, 26 Mar 2008, Ross Paterson wrote:
On Wed, Mar 26, 2008 at 08:52:43PM +0000, Ganesh Sittampalam wrote:
I'm a bit confused about why the following program doesn't compile (in
any of 6.6.1, 6.8.1 and 6.9.20080316). Shouldn't the Ord (a, b) context
be reduced?
To use bar, you need (Ord a, Ord b). You're assuming that Ord (a, b)
implies that, but it's the other way round.
Logically, the implication holds. There's an equivalence:
Ord a /\ Ord b <=> Ord (a,b)
Why it does or doesn't work in a Haskell impelmentation iss an
implementation issue / language design issue.. There's a paper by Martin
Sulzmann about extensible superclasses, which shows how this can be
implemented if you don't use dictionaries for your typeclasses, but the
type-passing scheme.
The problem with dictionaries is that you have to store the superclass
dictionaries, here Ord a and Ord b, in the dictionary, here Ord (a,b).
However, what superclass dictionaries you have to store depends on the
instance, e.g. Ord Int doesn't have any superclass and Ord [a] has
superclass Ord a.
There maybe wasn't an easy solution, but here is my idea of how to
to it with data or type families. The dictionary type would be:
data OrdDict a =
{ (<) :: a -> a -> Bool
, ...
, super :: Super a
}
type family Super a :: *
type instance Super Int = ()
type instance Super [a] = OrdDict a
type instance Super (a,b) = (OrdDict a, OrdDict b)
A similar solution is possible with a data family Super (but obviously I'm
in favor of type families :) The openness of the family matches the
openness of the type classes.
There is a little overhead in carrying around the superclasses.
Now ask your favorite Haskell system to implement this feature :)
Tom
--
Tom Schrijvers
Department of Computer Science
K.U. Leuven
Celestijnenlaan 200A
B-3001 Heverlee
Belgium
tel: +32 16 327544
e-mail: [EMAIL PROTECTED]
url: http://www.cs.kuleuven.be/~toms/
_______________________________________________
Glasgow-haskell-users mailing list
Glasgow-haskell-users@haskell.org
http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
