module Error = struct
  open Concepts.Condition

  let not_indexed () =
    condition "not-indexed" "A handle was expected to carry the indexed protocol and did not" empty
end

class type implementation = object
  method indexes : string list
  method lookup : string -> Concepts.Value.value -> (Handle.t, Concepts.Condition.condition) result
end

type Handle.protocol += Indexed of implementation
type t = implementation

let make impl = Indexed (impl :> implementation)
let from handle = Handle.into handle (function Indexed impl -> Some impl | _ -> None)
let require handle = from handle |> Option.to_result ~none:(Error.not_indexed ())
let indexes i = Handle.invoke i (fun o -> o#indexes)
let lookup i attribute value = Handle.invoke i (fun o -> o#lookup attribute value)