Type.Id(3)

NAME
Type.Id - Type identifiers.

Module
Module Type.Id

Documentation

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 Type.Id.provably_equal 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 Type.Id.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