atIdx ::
  forall fname a arrlen. (
    HasField fname a,
    IsFieldArray fname a 'True,
    arrlen ~ FieldArrayLength fname a,
    KnownNat arrlen
  ) =>
  Integer ->
  (forall idx. CanReadFieldArray fname idx a => Proxy.Proxy idx -> FieldType fname a) ->
  Maybe (FieldType fname a)
atIdx i f =
  case someNatVal i of
    Just (SomeNat iproxy) ->
      case cmpNat iproxy (Proxy.Proxy :: Proxy.Proxy arrlen) of
        CLT Refl -> Just (f iproxy)
        _ -> Nothing
    Nothing -> Nothing

{-
src/Main.hs:685:27-34: error:
    • Overlapping instances for CanReadFieldArray fname n a
        arising from a use of ‘f’
      Matching instances:
        instance [overlappable] (HasField fname a,
                                 IsFieldArray fname a 'True, IndexInBounds fname idx a,
                                 (TypeError ...)) =>
                                CanReadFieldArray fname idx a
          -- Defined in ‘Graphics.Vulkan.Marshal’
        instance [overlapping] (KnownNat idx,
                                IndexInBounds "float32" idx VkClearColorValue) =>
                               CanReadFieldArray "float32" idx VkClearColorValue
          -- Defined in ‘vulkan-api-1.1.3.0:Graphics.Vulkan.Types.Struct.Clear’
        instance [overlapping] (KnownNat idx,
                                IndexInBounds "int32" idx VkClearColorValue) =>
                               CanReadFieldArray "int32" idx VkClearColorValue
          -- Defined in ‘vulkan-api-1.1.3.0:Graphics.Vulkan.Types.Struct.Clear’
        ...plus 25 others
        ...plus one instance involving out-of-scope types
        (use -fprint-potential-instances to see them all)
      (The choice depends on the instantiation of ‘fname, n, a’
       To pick the first instance above, use IncoherentInstances
       when compiling the other instance declarations)
    • In the first argument of ‘Just’, namely ‘(f iproxy)’
      In the expression: Just (f iproxy)
      In a case alternative: CLT Refl -> Just (f iproxy)
    |
685 |         CLT Refl -> Just (f iproxy)
    |                           ^^^^^^^^
-}