filteredBy cond lens = filtered (cond <<< view lens)
[{value: Just 1}, {value: Nothing}] ^.. traversed <<< filteredBy isJust _value

filteredByLens lens = L.filtered (\x -> isJust (x L.^? lens))
[{value: Just 1}, {value: Nothing}] ^.. traversed <<< filteredByLens (_value <<< _Just)

type Start = Int
type End = Int
data FastArray a = Literally (Array a) | View Start End (Array a)

derive instance genericFastArray :: Generic (FastArray a) _
instance showFastArray :: Show a => Show (FastArray a) where
  show = genericShow

uncns :: forall a. FastArray a -> Maybe { head:: a, tail :: FastArray a }
uncns (Literally arr) =
  map (\head -> { head, tail: View 1 (length arr) arr}) (head arr)
uncns (View start end arr)
  | start < end =
    map (\head -> { head, tail: View (start + 1) end arr }) (arr !! start)
  | otherwise = Nothing