diff --git a/src/dune_rules/merlin/merlin.ml b/src/dune_rules/merlin/merlin.ml
index e0d074029958..80b5ce7ae4ea 100644
--- a/src/dune_rules/merlin/merlin.ml
+++ b/src/dune_rules/merlin/merlin.ml
@@ -596,6 +596,21 @@ module Unprocessed = struct
       ~f:(pp_flags ctx ~expander t.config.libname)
   ;;
 
+  let add_lib_dirs sctx mode ~init libs =
+    Action_builder.of_memo
+      (let open Memo.O in
+       Memo.parallel_map (Lib.Set.to_list libs) ~f:(fun lib ->
+         let+ dirs = src_dirs sctx lib in
+         lib, dirs)
+       >>| List.fold_left ~init ~f:(fun (src_dirs, obj_dirs) (lib, more_src_dirs) ->
+         ( Path.Set.union src_dirs more_src_dirs
+         , let public_cmi_dir =
+             let info = Lib.info lib in
+             obj_dir_of_lib `Public mode (Lib_info.obj_dir info)
+           in
+           Path.Set.add obj_dirs public_cmi_dir )))
+  ;;
+
   let process
     ({ modules
      ; ident = _
@@ -658,35 +673,10 @@ module Unprocessed = struct
       in
       let+ flags = flags
       and+ src_dirs, obj_dirs =
-        Action_builder.of_memo
-          (let open Memo.O in
-           Memo.parallel_map (Lib.Set.to_list requires) ~f:(fun lib ->
-             let+ dirs = src_dirs sctx lib in
-             lib, dirs)
-           >>| List.fold_left
-                 ~init:(Path.set_of_source_paths source_dirs, objs_dirs)
-                 ~f:(fun (src_dirs, obj_dirs) (lib, more_src_dirs) ->
-                   ( Path.Set.union src_dirs more_src_dirs
-                   , let public_cmi_dir =
-                       let info = Lib.info lib in
-                       obj_dir_of_lib `Public mode (Lib_info.obj_dir info)
-                     in
-                     Path.Set.add obj_dirs public_cmi_dir )))
+        let init = Path.set_of_source_paths source_dirs, objs_dirs in
+        add_lib_dirs sctx mode ~init requires
       and+ hidden_src_dirs, hidden_obj_dirs =
-        Action_builder.of_memo
-          (let open Memo.O in
-           Memo.parallel_map (Lib.Set.to_list requires_hidden) ~f:(fun lib ->
-             let+ dirs = src_dirs sctx lib in
-             lib, dirs)
-           >>| List.fold_left
-                 ~init:(Path.Set.empty, Path.Set.empty)
-                 ~f:(fun (src_dirs, obj_dirs) (lib, more_src_dirs) ->
-                   ( Path.Set.union src_dirs more_src_dirs
-                   , let public_cmi_dir =
-                       let info = Lib.info lib in
-                       obj_dir_of_lib `Public mode (Lib_info.obj_dir info)
-                     in
-                     Path.Set.add obj_dirs public_cmi_dir )))
+        add_lib_dirs sctx mode ~init:(Path.Set.empty, Path.Set.empty) requires_hidden
       in
       let src_dirs =
         Path.Set.union src_dirs (Path.Set.of_list_map ~f:Path.source more_src_dirs)