Module Type.Id

module Id: sig .. end

Type identifiers.

A type identifier is a value that denotes a type. Given two type identifiers, they can be tested for equality to prove they denote the same type. Note that:

Unequal identifiers do not imply unequal types: a given type can be denoted by more than one identifier.
Type identifiers can be marshalled, but they get a new, distinct, identity on unmarshalling, so the equalities are lost.

See an example of use.

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