On Thu, Nov 03, 2005 at 08:35:26PM -0800, Andrew Pimlott wrote:
I want a function
newtype Empty = Empty (forall a. [a]) listToEmpty :: [v] -> IO Empty listToEmpty l = liftM Empty (mapM no l) where no :: a -> IO a' no x = fail "element found" ... I could probably define a generic family of wrappers
newtype Forall = Forall (forall a. a) newtype Forall1 c1 = Forall1 (forall a. c1 a)
This worked out ok for me: newtype Forall = Forall { unForall :: forall a. a } newtype Forall1 c1 = Forall1 { unForall1 :: forall a. c1 a } liftForall :: Functor f => f Forall -> Forall1 f liftForall f = Forall1 (fmap unForall f) type Empty = Forall1 [] listToEmptyForall :: [v] -> IO Empty listToEmptyForall l = liftM liftForall (mapM no l) where no :: a -> IO Forall no x = fail "element found" I can live with that. Hmm... it occurs to me I could instead write type Empty = [Forall] listToEmptyForall :: [v] -> IO Empty listToEmptyForall l = mapM no l where no :: a -> IO Forall no x = fail "element found" Followup question: unForall and unForall1 are no-ops at run-time. Is "map unForall" also a no-op? That seems too much to ask for; I assume the spine of the list would be copied, right? Essentially, I'm thinking about how expensive a "cast" "[Forall] -> [Int]" is, compared to "Forall1 [] -> [Int]", which should be free. Andrew