https://github.com/hannesm/gmap

heterogenous Map over a GADT
https://github.com/hannesm/gmap

data-structure gadt heterogeneous map ocaml

Last synced: 3 months ago
JSON representation

heterogenous Map over a GADT

• Host: GitHub
• URL: https://github.com/hannesm/gmap
• Owner: hannesm
• License: other
• Created: 2018-06-16T18:28:26.000Z (over 6 years ago)
• Default Branch: main
• Last Pushed: 2021-08-04T14:38:29.000Z (over 3 years ago)
• Last Synced: 2024-10-03T12:24:25.359Z (4 months ago)
• Topics: data-structure, gadt, heterogeneous, map, ocaml
• Language: OCaml
• Size: 44.9 KB
• Stars: 42
• Watchers: 4
• Forks: 1
• Open Issues: 4
• Metadata Files:
  • Readme: README.md
  • Changelog: CHANGES.md
  • License: LICENSE.md

Awesome Lists containing this project

README

        
## Gmap - heterogenous maps over a GADT

%%VERSION%%

Gmap exposes the functor `Make` which takes a key type (a

[GADT](https://en.wikipedia.org/wiki/Generalized_algebraic_data_type) '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](http://caml.inria.fr/pub/docs/manual-ocaml/libref/Map.html) data

structure.

```OCaml

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

end

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](http://erratique.ch/software/hmap) is that in gmap the key-value GADT

type must be provided when instantiating the functor.  In hmap, keys are created

dynamically.

## Documentation

[![Build Status](https://travis-ci.org/hannesm/gmap.svg?branch=master)](https://travis-ci.org/hannesm/gmap)

[API documentation](https://hannesm.github.io/gmap/doc/) is available online.

## Installation

You need [opam](https://opam.ocaml.org) installed on your system.  The command

`opam install gmap`

will install this library.