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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
