Main.hs

{-# LANGUAGE StandaloneDeriving, FlexibleContexts, UndecidableInstances #-}
-- zippers is probably a better answer
import Control.Lens
import Data.Maybe

-- with index
data WI m = WI (Index m) m
deriving instance (Show m, Show (Index m)) => Show (WI m)

mkwi' :: Ixed m => Index m -> m -> WI m
mkwi' i m  = fromJust (mkwi i m)

mkwi :: Ixed m => Index m -> m -> Maybe (WI m)
mkwi i m | has (ix i) m = Just (WI i m)
mkwi _ _ = Nothing

-- with index element
wie :: Ixed m => Lens' (WI m) (IxValue m)
wie f (WI i m) = WI i <$> unsafeSingular (ix i) f m

-- with index index, though if mkwi
wii f (Just (WI i m)) = flip mkwi m <$> f i

d = mkwi' 1 "abc"


main = do
  d & print
  d & wie .~ 'x' & print

  let Just e = Just d & wii .~ 0

  e & wie .~ 'x' & print
{-
*Main> main
WI 1 "abc"
WI 1 "axc"
WI 0 "xbc"
-}