ocamlPackages.buildTopkgPackage: Added

This function helps building an OCaml package that builds with topkg.
There are currently many such packages in nixpkgs and this function
would greatly simplify adding more.

This is heavily inspired by `ocamlPackages.buildDunePackage`.

pkgs/build-support/ocaml/topkg.nix

@@ -0,0 +1,28 @@
+{ lib, stdenv, fetchurl, ocaml, findlib, topkg, ocamlbuild, cmdliner, odoc, b0
+}:
+
+{ pname, version, nativeBuildInputs ? [ ], buildInputs ? [ ], ... }@args:
+
+lib.throwIf (args ? minimalOCamlVersion
+  && lib.versionOlder ocaml.version args.minimalOCamlVersion)
+"${pname}-${version} is not available for OCaml ${ocaml.version}"
+
+stdenv.mkDerivation ({
+
+  dontAddStaticConfigureFlags = true;
+  configurePlatforms = [ ];
+  strictDeps = true;
+  inherit (topkg) buildPhase installPhase;
+
+} // (builtins.removeAttrs args [ "minimalOCamlVersion" ]) // {
+
+  name = "ocaml${ocaml.version}-${pname}-${version}";
+
+  nativeBuildInputs = [ ocaml findlib ocamlbuild topkg ] ++ nativeBuildInputs;
+  buildInputs = [ topkg ] ++ buildInputs;
+
+  meta = (args.meta or { }) // {
+    platforms = args.meta.platforms or ocaml.meta.platforms;
+  };
+
+})

pkgs/top-level/ocaml-packages.nix

@@ -1890,6 +1890,8 @@ let
 
     buildOasisPackage = callPackage ../build-support/ocaml/oasis.nix { };
 
+    buildTopkgPackage = callPackage ../build-support/ocaml/topkg.nix { };
+
     # Apps from all-packages, to be eventually removed
 
     google-drive-ocamlfuse = callPackage ../applications/networking/google-drive-ocamlfuse { };