Stefan O'Rear wrote:
How do I get my original function back which just turns a standard list to one of the funky lists, or is that just impossible with GADT's? Do I now have to "wrap" all the fuctions I use, i.e. pass safeMap in CPS?
AFAIK you can't. Fortunately the CPS transform can be very local:
Indeed, you can't. I mean, given fromList [] = Nil fromList (a:as) = Cons a (fromList as) what type would it have? One of fromList :: forall t . [a] -> List a t fromList :: [a] -> List a NilT fromList :: forall t . [a] -> List a (ConsT t) ? No, because the second argument depends on the input list. The only thing we know is that there is a type t but we don't know which one. Thus fromList :: [a] -> (exists t . List a t) is the correct type. As Haskell currently doesn't have "first class existentials", we have to either pack it into a data type or appeal to the equivalence exists a . f a ~= forall r . (forall a . f a -> r) -> r Both have of course the problem that we cannot simply write safeMap (fromList [1,2,3]) :: exists t . List a t Regards, apfelmus Exercise: Why is fromList :: exists t . [a] -> List a t wrong as well?