package melange

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

Example

The following shows how type identifiers can be used to implement a simple heterogeneous key-value dictionary. In contrast to Stdlib.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