Type Classes & implicit instantination in OCaml

type_classes.ml

type 'u anySuch = { va: 'u ; to_string : 'u -> string };; (* type interface *)
type someSuch = Such : _ anySuch -> someSuch;; (* existential bounded by interface *)
type (_, 'g) anyInstance = { instance : 'g };; (* typeclass instance *)
type _ someInstance = Instance : (_, 'g) anyInstance -> 'g someInstance;; (* abstract typeclass *)

let anySuchInstance : (_, someSuch) anyInstance = { instance = Such { va = 6 ; to_string = string_of_int } };; (* implementation *)
let someSuchInstance : someSuch someInstance = Instance anySuchInstance;; (* type class existential *)

let f : someSuch someInstance -> string = function Instance i ->
                                            match i.instance with Such e -> e.to_string e.va;; (* generic function *)
let () = f @@ someSuchInstance |> ignore;; (* abstract instantiation *)

(* But how can I instantiate `Such` instance from specific type, e.g. `int`, can type singletones be helpful there? *)