Type identifiers

type !'a t

The type for identifiers for type 'a .

val make : unit -> 'a t

make () is a new type identifier.

val uid : 'a t -> int

uid id is a runtime unique identifier for id .

val provably_equal : 'a t -> 'b t -> ('a, 'b) Type.eq option

provably_equal i0 i1 is Some Equal if identifier i0 is equal to i1 and None otherwise.

Example

The following shows how type identifiers can be used to implement a simple heterogeneous key-value dictionary. In contrast to Map values whose keys map to a single, homogeneous type of values, this dictionary can associate a different type of value to each key.

(** Heterogeneous dictionaries. *)
module Dict : sig
  type t
  (** The type for dictionaries. *)
  type 'a key
  (** The type for keys binding values of type ['a]. *)
  val key : unit -> 'a key
  (** [key ()] is a new dictionary key. *)
  val empty : t
  (** [empty] is the empty dictionary. *)
  val add : 'a key -> 'a -> t -> t
  (** [add k v d] is [d] with [k] bound to [v]. *)
  val remove : 'a key -> t -> t
  (** [remove k d] is [d] with the last binding of [k] removed. *)
  val find : 'a key -> t -> 'a option
  (** [find k d] is the binding of [k] in [d], if any. *)
end = struct
  type 'a key = 'a Type.Id.t
  type binding = B : 'a key * 'a -> binding
  type t = (int * binding) list
  let key () = Type.Id.make ()
  let empty = []
  let add k v d = (Type.Id.uid k, B (k, v)) :: d
  let remove k d = List.remove_assoc (Type.Id.uid k) d
  let find : type a. a key -> t -> a option = fun k d ->
    match List.assoc_opt (Type.Id.uid k) d with
    | None -> None
    | Some (B (k', v)) ->
        match Type.Id.provably_equal k k' with
        | Some Type.Equal -> Some v
        | None -> assert false
end