Put template fields at the bottom of opam file

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

dune.opam

@@ -1,10 +1,3 @@
-build: [
-  # opam 2 sets OPAM_SWITCH_PREFIX, so we don't need a hardcoded path
-  ["ocaml" "configure.ml" "--libdir" lib] {opam-version < "2"}
-  ["ocaml" "bootstrap.ml"]
-  ["./boot.exe" "--release" "--subst"] {pinned}
-  ["./boot.exe" "--release" "-j" jobs]
-]
 opam-version: "2.0"
 maintainer: ["Jane Street Group, LLC <opensource@janestreet.com>"]
 authors: ["Jane Street Group, LLC <opensource@janestreet.com>"]
@@ -41,3 +34,10 @@ conflicts: [
   "jbuilder" {!= "transition"}
   "odoc" {< "1.3.0"}
 ]
+build: [
+  # opam 2 sets OPAM_SWITCH_PREFIX, so we don't need a hardcoded path
+  ["ocaml" "configure.ml" "--libdir" lib] {opam-version < "2"}
+  ["ocaml" "bootstrap.ml"]
+  ["./boot.exe" "--release" "--subst"] {pinned}
+  ["./boot.exe" "--release" "-j" jobs]
+]

src/opam_create.ml

@@ -108,15 +108,19 @@ let add_rule sctx ~project ~pkg =
      | Some p -> Build.contents p
      | None -> Build.return "")
     >>>
-    Build.arr (fun contents ->
-      let opamfile = Opam_file.of_string ~path:opam_path contents in
-      let generated =
+    Build.arr (fun template ->
+      let opamfile = Opam_file.of_string ~path:opam_path template in
+      let existing_vars_template = Opam_file.existing_variables opamfile in
+      let generated_fields =
         let package = Local_package.package pkg in
-        let more_fields = opam_fields project package in
-        Opam_file.Create.add_missing_bindings opamfile more_fields
+        opam_fields project package
+        |> List.filter ~f:(fun (v, _) ->
+          not (String.Set.mem existing_vars_template v))
+        |> Opam_file.Create.of_bindings ~file:opam_path
       in
-      OpamPrinter.Preserved.items contents opamfile.file_contents
-        generated.file_contents)
+      sprintf "%s\n%s"
+        (OpamPrinter.opamfile generated_fields)
+        template)
     >>> Build.write_file_dyn opam_path
   in
   let dir = Local_package.build_dir pkg in

src/opam_file.ml

@@ -80,25 +80,25 @@ let absolutify_positions ~file_contents t =
 
 let nopos : OpamParserTypes.pos = ("",0,0) (* Null position *)
 
+let existing_variables t =
+  List.fold_left ~init:String.Set.empty t.file_contents
+    ~f:(fun acc l ->
+      match l with
+      | Section (_, _) -> acc
+      | Variable (_, var, _) -> String.Set.add acc var)
+
 module Create = struct
   let string s = String (nopos, s)
   let list f xs = List (nopos, List.map ~f xs)
   let string_list xs = list string xs
 
-  let add_missing_bindings (t : t) vars =
-    let existing_variables =
-      List.fold_left ~init:String.Set.empty t.file_contents
-        ~f:(fun acc l ->
-          match l with
-          | Section (_, _) -> acc
-          | Variable (_, var, _) -> String.Set.add acc var)
-    in
-    let vars =
-      List.filter_map vars ~f:(fun (var, value) ->
-        if String.Set.mem existing_variables var then
-          None
-        else
-          Some (Variable (nopos, var, value)))
+  let of_bindings vars ~file =
+    let file_contents =
+      List.map vars ~f:(fun (var, value) ->
+        Variable (nopos, var, value))
     in
-    { t with file_contents = vars @ t.file_contents }
+    let file_name = Path.to_string file in
+    { file_contents
+    ; file_name
+    }
 end

src/opam_file.mli

@@ -24,6 +24,8 @@ val absolutify_positions : file_contents:string -> opamfile -> opamfile
 
 val nopos : OpamParserTypes.pos
 
+val existing_variables : t -> String.Set.t
+
 module Create : sig
   open OpamParserTypes
 
@@ -33,5 +35,5 @@ module Create : sig
 
   val string_list : string list -> value
 
-  val add_missing_bindings : t -> (string * value) list -> t
+  val of_bindings : (string * value) list -> file:Path.t -> t
 end