haskell type family

class ListLike l where
  type Elem l
  indexl :: l -> Int -> Elem l

instance ListLike (V.Vector a) where
  type Elem (V.Vector a) = a
  indexl = (V.!)

instance ListLike [a] where
  type Elem [a] = a
  indexl = (!!)

cutrod ::
  forall l.
  ListLike l =>
  l ->
  Int ->
  Int
cutrod v n = go 1 0
  where
    go l result | l == n + 1 = result
    go l result | otherwise = go (l + 1) result'
      where
        result' = maximum [result + (indexl v 0), (indexl v (l - 1))]


src/MyLib.hs:50:38-47: error: …
    • Couldn't match expected type ‘Int’ with actual type ‘Elem l’
    • In the second argument of ‘(+)’, namely ‘(indexl v 0)’
      In the expression: result + (indexl v 0)
      In the first argument of ‘maximum’, namely
        ‘[result + (indexl v 0), (indexl v (l - 1))]’