Create a zip-esque running annotation for a list based upon a predicate.

saddle.hs

saddle :: (a->Bool) -> [a] -> [(a,a)]
saddle f [] = []
saddle f (x:xs) = saddle' f x xs

saddle' :: (a->Bool) -> a -> [a] -> [(a,a)]
saddle' f initial [] = []
saddle' f initial (x:xs) = (x, sideValue x) : saddle' f (sideValue x) xs
  where sideValue x = if f x then x else initial