[WIP] Add nix shell.

pin.nix

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+let
+  # 2021-08-15
+  nix-rev = "3ab8ce12c2db31268f579c11727d9c63cfee2eee";
+  nix-src = builtins.fetchTarball {
+    url = "https://github.com/NixOS/nixpkgs/archive/${nix-rev}.tar.gz";
+    sha256 = "13fx0yb95gxyk0jyvgrxv2yp4fj54g7nzrlvjfc8slx9ccqd2v86";
+  };
+in import nix-src {
+  overrides = [
+    (self: super: {
+      ocaml = self.ocaml_4_03;
+      ocamlPackages = super.ocamlPackages.overrideAttrs
+        (attrs: attrs // { ocaml = self.ocaml_4_03; });
+    })
+  ];
+}

shell.nix

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+let
+  pkgs = import ./pin.nix;
+  ocaml-xkbcommon = pkgs.ocamlPackages.buildDunePackage {
+    pname = "xkbcommon";
+    version = "0.1";
+    minimumOcamlVersion = "4.04";
+    useDune2 = true;
+    src = pkgs.fetchFromGitHub {
+      owner = "Armael";
+      repo = "ocaml-xkbcommon";
+      rev = "af0cd8c938938db3e67b65ac13b6444102756ba0";
+      sha256 = "0zh80aczdn821xkwjn8m74mjcqb6f1hj0qfh1kxrz2z9h5as2r8k";
+    };
+    buildInputs = [ pkgs.libxkbcommon pkgs.ocamlPackages.dune-configurator ];
+    propagatedBuildInputs = [
+      pkgs.ocamlPackages.ctypes
+      pkgs.ocamlPackages.base
+      pkgs.ocamlPackages.stdio
+      pkgs.libxkbcommon
+    ];
+    meta = {
+      homepage = "https://github.com/Armael/ocaml-xkbcommon";
+      synopsis = "OCaml bindings to xkbcommon";
+      description = "This package provides OCaml bindings to xkbcommon";
+      maintainers = [ ];
+    };
+  };
+  ocaml-unix-time = pkgs.callPackage (import ./unix-time.nix) { };
+in pkgs.mkShell {
+  name = "wlroots-ocaml-shell";
+  propagatedNativeBuildInputs = [
+    pkgs.ocamlPackages.ctypes
+    pkgs.ocamlPackages.dune-configurator
+    pkgs.ocamlPackages.mtime
+    ocaml-unix-time
+    ocaml-xkbcommon
+  ];
+  nativeBuildInputs = [
+    pkgs.dune_2
+    pkgs.ocamlPackages.findlib
+    # Should be put upstream as an input for wlroots
+    pkgs.libudev
+    pkgs.m4
+    pkgs.ocaml
+    pkgs.pixman
+    pkgs.pkg-config
+    pkgs.less
+    pkgs.wayland
+    pkgs.wayland-protocols
+    pkgs.wlroots
+  ];
+}

unix-time.nix

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+{ ocamlPackages, fetchFromGitHub }:
+ocamlPackages.buildOcaml rec {
+  name = "unix-time";
+  version = "0.1.0";
+  minimumOcamlVersion = "4.04";
+  src = fetchFromGitHub {
+    owner = "dsheets";
+    repo = "ocaml-unix-time";
+    rev = version;
+    sha256 = "0000000000000000000000000000000000000000000000000000";
+  };
+  doCheck = true;
+  nativeBuildInputs = [ ocamlPackages.alcotest ocamlPackages.ctypes ];
+  propagatedBuildInputs = [ ocamlPackages.unix-errno ocamlPackages.base-unix ];
+  meta = {
+    homepage = "https://github.com/dsheets/ocaml-unix-time";
+    synopsis = "Unix time.h types, maps, and support";
+    description = ''
+      unix-time provides access to the features exposed in time.h in a way
+      that is not tied to the implementation on the host system.
+      The Time module provides types which are analogous to those defined in
+      time.h such as timespec.
+      The Time_unix module provides a ctypes view for timespec.
+    '';
+    maintainers = [ ];
+  };
+}