On Thu, Mar 16, 2006 at 12:57:54PM +0100, Matthias Fischmann wrote:
i have a type class that looks something like this:
class Resource a where resourceName :: a -> String resourceAdvance :: a -> a resourceStarved :: a -> Bool resourceSpend :: a -> Int -> a resourceEarn :: a -> Int -> a
resource types are rice, crude oil, pizza, software code, and so on. they all have a different internal structure and the same abstract interface, that's why i have defined this type class.
now i want to create a list of a type similar to
[r1, r2, r3] :: (Resource a) => [a]
but with r1 being pizza, r2 being crude oil, and so on.
The existential is equivalent to a recursive type, as in this solution (in Haskell 98): data ResourceProperties a = ResourceProperties { resourceName :: String, resourceAdvance :: a, resourceStarved :: Bool, resourceSpend :: Int -> a, resourceEarn :: Int -> a } instance Functor ResourceProperties where fmap f p = ResourceProperties { resourceName = resourceName p, resourceAdvance = f (resourceAdvance p), resourceStarved = resourceStarved p, resourceSpend = f . resourceSpend p, resourceEarn = f . resourceEarn p } class Resource a where resourceProperties :: a -> ResourceProperties a newtype Rs = Rs (ResourceProperties Rs) instance Resource Rs where resourceProperties (Rs r) = r mkRs :: Resource a => a -> Rs mkRs = Rs . fmap mkRs . resourceProperties There won't be an unRs.