On Fri, Mar 14, 2008 at 8:06 AM, Ville Tirronen <aleator@gmail.com> wrote:

Hi all.

I managed to paint myself in a corner where I did this:

...

data ShowDynamic = forall a . (Show a) => SD Dynamic a toShowDyn a = SD (toDyn a) a fromShowDyn (SD d a) = fromDynamic d

...

instance Show ShowDynamic where ...

Then I began to wonder if there was any hack to do:

...

data AnyDynamic X = forall a . (X a) => AD Dynamic a

which could be kinda neat, but I saw no obvious way to do this. Ideas?

You can't abstract over type classes, but you can abstract over a dictionary. data ShowD a = ShowD { showD :: a -> String } data AnyDynamic dict = forall a. AD Dynamic (dict a) On the other hand, if all you need is some fixed intersection of classes, it's better to make your own Dynamic. data ShowableDynamic = forall a. (Show a, Typeable a) => SD a toShowDyn :: (Show a, Typeable a) => a -> ShowableDynamic toShowDyn = SD fromShowDyn :: (Show a, Typeable a) => ShowableDynamic -> Maybe a fromShowDyn (SD a) = cast a Finally, you can make your own dynamic augmented with an arbitrary dictionary: data AnyDynamic dict = forall a. (Typeable a) => AD a (dict a) -- Dave Menendez <dave@zednenem.com> <http://www.eyrie.org/~zednenem/>