add library as an overlay

Author: d4hines

flake.nix

@@ -11,6 +11,7 @@
 
   outputs = { self, nixpkgs, flake-utils, ocaml-overlay, nix-filter }:
     let
+      supported_ocaml_versions = [ "ocamlPackages_4_12" "ocamlPackages_4_13" "ocamlPackages_5_00" ];
       out = system:
         let
           pkgs = import nixpkgs {
@@ -39,11 +40,9 @@
           });
 
           defaultPackage = myPkgs.camlboy;
-
-          defaultApp =
-            flake-utils.lib.mkApp { drv = self.defaultPackage."${system}"; };
-
         };
     in
-    with flake-utils.lib; eachSystem defaultSystems out;
+    with flake-utils.lib; eachSystem defaultSystems out // {
+      overlays = { default = import ./nix/overlay.nix supported_ocaml_versions nix-filter; };
+    };
 }

nix/overlay.nix

@@ -0,0 +1,16 @@
+supported_ocaml_versions: nix-filter:
+
+final: prev: {
+  ocaml-ng = builtins.mapAttrs (name: ocamlVersion:
+    # If the current ocamlVersion exists in supported versions
+    (if (builtins.any (x: x == name) supported_ocaml_versions) then
+      ocamlVersion.overrideScope' (oself: osuper: {
+        camlboy = (final.callPackage ./camlboy.nix {
+          inherit nix-filter;
+          ocamlPackages = oself;
+          doCheck = true;
+        });
+      })
+    else
+      ocamlVersion)) prev.ocaml-ng;
+}