Add onix-based flake, associated make task, and .envrc template

.envrc.tmpl

@@ -0,0 +1,13 @@
+# rebuild the environment on `nix-direnv-reload` only
+nix_direnv_manual_reload
+
+# direnv will consider the environment changed if these files change, and
+# nix-direnv will then prompt for the manual reload
+watch_file flake.nix flake.lock onix-lock.json onix-lock-dev.json
+
+# use the flake with the `default` devShell:
+use flake
+
+# To use another devShell, you can use (e.g. for devShell.other:
+# use flake .#other
+

Makefile

@@ -6,3 +6,9 @@ clean:
 
 watch:
 	@dune build @all -w
+
+onix-lock:
+	onix lock ./imandra-document.opam --resolutions="ocaml-system=5.2.0" --lock-file ./onix-lock.json
+	onix lock ./imandra-document.opam --resolutions="ocaml-system=5.2.0,ocaml-lsp-server" --with-test=true --lock-file ./onix-lock-dev.json 
+	git add onix-lock.json onix-lock-dev.json
+

flake.nix

@@ -0,0 +1,56 @@
+{
+  inputs = {
+    nixpkgs.url = "nixpkgs/nixpkgs-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+
+    onix.url = "github:rizo/onix";
+    onix.inputs.nixpkgs.follows = "nixpkgs";
+  };
+
+  outputs = { self, nixpkgs, flake-utils, onix }@inputs:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+        onix' = onix.packages.${system}.latest;
+        opamFiles = [ 
+          ./imandra-document.opam
+        ];
+        onixEnv = onix'.env {
+          path = ./.;
+          roots = opamFiles;
+          lock = ./onix-lock.json;
+          deps = { "ocaml-system" = "*"; };
+        };
+        onixEnvDev = onix'.env {
+          path = ./.;
+          roots = opamFiles;
+          lock = ./onix-lock-dev.json;
+          deps = {
+            "ocaml-system" = "*";
+            "ocaml-lsp-server" = "*";
+          };
+        };
+
+      in rec {
+        formatter = pkgs.nixfmt-rfc-style;
+
+        packages.imandra-document = onixEnv.pkgs.imandra-document;
+
+        devShells.onixLock = pkgs.mkShell { 
+          buildInputs = [
+            onix'
+          ]; 
+        };
+
+        devShells.default = onixEnvDev.shell.overrideAttrs (final: prev: {
+          buildInputs = prev.buildInputs ++ [ 
+            pkgs.ocamlformat_0_22_4 
+            pkgs.dune_3
+            pkgs.ocaml
+            onix' 
+          ];
+        });
+
+        packages.dev-shell = devShells.default.inputDerivation;
+      });
+}