When a JSON API returns an Array of elements of different types as if it were a tuple, you can use this module to parse each element in a different manner and assert that you didn't miss any. Example:

  let of_json =
    let open Of_json.Let_syntax in
    let%map_open parsed =
      "key" @. tuple Array_as_tuple.(
        [%map
          let a = shift @@ string
          and b = shift @@ number @> Int.of_string
          in (a, b)])
    in parsed

Wrap your normal Of_json combinators with shift to have them operate on the head of the list element, passing the tail on to the next combinator. Build up a t by combining these with apply or bind.