On Sun, Feb 25, 2007 at 11:11:14PM +0000, Neil Mitchell wrote:
Hi Stefan,
Since we don't know in advance the length of the list, we need an existensial type; this immediately causes problems because existential types do not exist in any current Haskell implementation. There are two approaches:
-- Using an existential datatype box data VarList a = forall t. VarList (List a t)
fromListV :: [a] -> VarList a fromListV [] = VarList Nil fromListV (x:xs) = case fromListV xs of VarList xs' -> VarList (Cons x xs')
-- using CPS transform (turns existentials into rank2) -- generally prefered because it avoids naming a one-use data type, -- but YMMV
fromListC :: [a] -> (forall t. List a t -> r) -> r fromListC [] fn = fn Nil fromListC (x:xs) fn = fromListC xs (\sl -> fn (Cons x sl))
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: -- write this as you normally would, no cps at all do_something_with_safe_list :: List t a -> (something -> somethingelse ...) --assoc for clarity do_something_with_unsafe_list :: [a] -> (same as above) do_something_with_unsafe_list ls = fromListC ls do_something_with_safe_list