gmapversion

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.

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, 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.

AuthorHannes Mehnert <hannes@mehnert.org>
LicenseISC
Published
Homepagehttps://github.com/hannesm/gmap
Issue Trackerhttps://github.com/hannesm/gmap/issues
MaintainerHannes Mehnert <hannes@mehnert.org>
Dependencies
Source [http] https://github.com/hannesm/gmap/releases/download/0.1.0/gmap-0.1.0.tbz
sha256=971dd65451472fbde6a621ac530f6eb7d6a3f28f983551a63eddd72e163c93ea
md5=aeea8ba33dc24e13d34d49ba2bd6e55a
Edithttps://github.com/ocaml/opam-repository/tree/master/packages/gmap/gmap.0.1.0/opam
Required by