Variadic via GADT

data Pill a where
 Stop :: Pill Integer
 O :: Pill (Pill a -> a)

collect :: Integer -> Pill a -> a
collect i Stop = i
collect i O = collect (i+1)

start = collect 0

-- > start Stop
-- 0
-- > start O Stop
-- 1
-- > start O O Stop
-- 2
-- > start O O O Stop
-- 3
-- > start O O O O Stop
-- 4
-- > start O O O O O Stop
-- 5