Heterogenous maps over a GADT

Gmap exposes the functor Make which takes a key type (a GADT 'a key) and outputs a type-safe Map where each 'a key is associated with a 'a value. This removes the need for additional packing. It uses OCaml's stdlib Map data structure.

type _ k =
  | A : int k
  | B : string k

module K = struct
  type 'a t = 'a k

  let compare : type a b. a t -> b t -> (a, b) Gmap.Order.t = fun t t' ->
    let open Gmap.Order in
    match t, t' with
    | A, A -> Eq | A, _ -> Lt | _, A -> Gt
    | B, B -> Eq

  let pp : type a. Format.formatter -> a t -> a -> unit = fun ppf t v ->
    match t, v with
    | A, x -> Fmt.pf ppf "A %d" x
    | B, s -> Fmt.pf ppf "B %s" s
end

module M = Gmap.Make(K)


let () =
  let m = M.empty in
  ...
  match M.find A m with
  | Some x -> Printf.printf "got %d\n" x
  | None -> Printf.printf "found nothing\n"

This is already an exhaustive pattern match: there is no need for another case (for the constructor B) since the type system knows that looking for A will result in an int.

Motivation came from parsing of protocols which usually specify optional values and extensions via a tag-length-value (TLV) mechanism: for a given tag the structure of value is different - see for example IP options, TCP options, DNS resource records, TLS hello extensions, etc.

Discussing this problem with Justus Matthiesen during summer 2017, we came up with this design. Its main difference to Daniel C. Bünzli's hmap is that in gmap the key-value GADT type must be provided when instantiating the functor. In hmap, keys are created dynamically.