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1opam-version: "2.0"
2maintainer: "Hannes Mehnert <hannes@mehnert.org>"
3authors: ["Hannes Mehnert <hannes@mehnert.org>"]
4homepage: "https://github.com/hannesm/gmap"
5doc: "https://hannesm.github.io/gmap/doc"
6dev-repo: "git+https://github.com/hannesm/gmap.git"
7bug-reports: "https://github.com/hannesm/gmap/issues"
8license: "ISC"
9depends: [
10 "ocaml" {>= "4.04.2"}
11 "ocamlfind" {build}
12 "ocamlbuild" {build}
13 "topkg" {build}
14 "fmt"
15]
16build: [
17 [ "ocaml" "pkg/pkg.ml" "build" "--pinned" "%{pinned}%" "--tests" "false" ]
18]
19synopsis: "Heterogenous maps over a GADT"
20description: """
21Gmap exposes the functor `Make` which takes a key type (a
22[GADT](https://en.wikipedia.org/wiki/Generalized_algebraic_data_type) 'a key)
23and outputs a type-safe Map where each 'a key is associated with a 'a value.
24This removes the need for additional packing. It uses OCaml's stdlib
25[Map](http://caml.inria.fr/pub/docs/manual-ocaml/libref/Map.html) data
26structure.
27
28```OCaml
29type _ k =
30 | A : int k
31 | B : string k
32
33module K = struct
34 type 'a t = 'a k
35
36 let compare : type a b. a t -> b t -> (a, b) Gmap.Order.t = fun t t' ->
37 let open Gmap.Order in
38 match t, t' with
39 | A, A -> Eq | A, _ -> Lt | _, A -> Gt
40 | B, B -> Eq
41
42 let pp : type a. Format.formatter -> a t -> a -> unit = fun ppf t v ->
43 match t, v with
44 | A, x -> Fmt.pf ppf "A %d" x
45 | B, s -> Fmt.pf ppf "B %s" s
46end
47
48module M = Gmap.Make(K)
49
50
51let () =
52 let m = M.empty in
53 ...
54 match M.find A m with
55 | Some x -> Printf.printf "got %d\\n" x
56 | None -> Printf.printf "found nothing\\n"
57```
58
59This is already an exhaustive pattern match: there is no need for another case
60(for the constructor `B`) since the type system knows that looking for `A` will
61result in an `int`.
62
63Motivation came from parsing of protocols which usually specify optional values
64and extensions via a tag-length-value (TLV) mechanism: for a given tag the
65structure of value is different - see for example IP options, TCP options, DNS
66resource records, TLS hello extensions, etc.
67
68Discussing this problem with Justus Matthiesen during summer 2017, we came up
69with this design. Its main difference to Daniel C. Bünzli's
70[hmap](http://erratique.ch/software/hmap) is that in gmap the key-value GADT
71type must be provided when instantiating the functor. In hmap, keys are created
72dynamically."""
73url {
74 src:
75 "https://github.com/hannesm/gmap/releases/download/0.2.0/gmap-0.2.0.tbz"
76 checksum: [
77 "sha256=00c6a2186c0c9908f6057566ac72de062b26ac5b1f5d0d78721c8526a580884d"
78 "md5=fbf9d1d5a38c2c86ef2f122e22359896"
79 ]
80}