Though I haven't tried it, the explicit 'Sat' dictionary representation would probably work here, something like:
data ShowD a = ShowD { showD :: a -> String } -- our explicit dictionary for show, would need one of -- these for each class we care about
-- the satisfaction class: class Sat t where dict :: t
-- an instance for show: instance Show a => Sat (ShowD a) where dict = ShowD { showD = show } instance Sat (ShowD a) => Show a where show = showD dict
manually generating datatypes and instances is tedious, but could easily be automated. you should be able to use this to write:
satFold :: forall c b . Sat c b => (forall a . Sat (c a) => a -> i -> i) -> b -> r -> b
or something similar. probably worth a shot. On Tue, 9 Mar 2004, MR K P SCHUPKE wrote:
I have written a first attempt at a fold function for the heterogenious list:
class RFold i r where rFold :: (forall a . a -> i -> i) -> i -> r -> i instance RFold i RNil where rFold f i RNil = i instance RFold i r => RFold i (a `RCons` r) where rFold f i (x `RCons` xs) = f x (rFold f i xs)
This works providing the folded 'op' has the type: forall a . a -> i -> i which means it does not work for functions like show :: forall a . Show a => a -> i -> i as the types are different. I have not figured out a way to make it accept a constraint like Show for example. Here is an example:
length = rFold (\_ -> (+1)) 0 relation
The use of such a function seems limited, if constraints like Show cannot be used, as most useful applications of fold would require some kind of class membership for example:
string = rFold shows "" relation
This fails to compile because shows has type:
shows :: forall a . Show a => a -> i -> i
but fold expects
op :: forall a . a -> i -> i
Regards, Keean Schupke. _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
-- Hal Daume III | hdaume@isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume