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?
Thanks
Neil
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