chore: set up CI

.github/workflows/nix.yml

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+name: "Nix Build"
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        ocamlVersion: [5_1]
+    steps:
+    - uses: actions/checkout@v2
+    - uses: cachix/install-nix-action@v21
+      with:
+        extra_nix_config: |
+          extra-substituters = https://anmonteiro.nix-cache.workers.dev
+          extra-trusted-public-keys = ocaml.nix-cache.com-1:/xI2h2+56rwFfKyyFVbkJSeGqSIYMC/Je+7XXqGKDIY=
+    - name: "Run Nix tests"
+      run: nix build .#
+

flake.nix

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+{
+  description = "melange-json Nix Flake";
+
+  inputs.nix-filter.url = "github:numtide/nix-filter";
+  inputs.flake-utils.url = "github:numtide/flake-utils";
+  inputs.nixpkgs = {
+    url = "github:nix-ocaml/nix-overlays";
+    inputs.flake-utils.follows = "flake-utils";
+  };
+  inputs.melange-src = {
+    url = "github:melange-re/melange";
+    inputs.nix-filter.follows = "nix-filter";
+    inputs.flake-utils.follows = "flake-utils";
+    inputs.nixpkgs.follows = "nixpkgs";
+  };
+
+  outputs = { self, nixpkgs, flake-utils, nix-filter, melange-src }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages."${system}".appendOverlays [
+          (self: super: {
+            ocamlPackages = super.ocaml-ng.ocamlPackages_5_1;
+          })
+          melange-src.overlays.default
+        ];
+        inherit (pkgs) nodejs_latest lib stdenv darwin;
+
+        melange-json = with pkgs.ocamlPackages; buildDunePackage {
+          pname = "melange-json";
+          version = "dev";
+
+          src = ./.;
+          nativeBuildInputs = with pkgs.ocamlPackages; [ melange ];
+          propagatedBuildInputs = with pkgs.ocamlPackages; [ melange ];
+        };
+
+        mkShell = { buildInputs ? [ ] }: pkgs.mkShell {
+          inputsFrom = [ melange-json ];
+          nativeBuildInputs = with pkgs; [
+            yarn
+            nodejs_latest
+          ] ++ (with pkgs.ocamlPackages; [
+            ocamlformat
+            merlin
+            melange-jest
+            reason
+          ]);
+          inherit buildInputs;
+        };
+      in
+      rec {
+        packages.default = melange-json;
+        devShells = {
+          default = mkShell { };
+          release = mkShell {
+            buildInputs = with pkgs; [ cacert curl ocamlPackages.dune-release git ];
+          };
+        };
+      });
+}