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heterogenous Map over a GADT
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Gmap - 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 _ key =
  | I : int key
  | S : string key

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

  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
    | I, I -> Eq | I, _ -> Lt | _, I -> Gt
    | S, S -> Eq

module M = Gmap.Make(K)

let () =
  let m = M.empty in
  match M.find I 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 S) since the type system knows that looking for I 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.


Build Status

API documentation is available online.


You need opam installed on your system. The command

opam install gmap

will install this library.

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