diff --git a/build_system/clerk_driver.ml b/build_system/clerk_driver.ml
index 1302c7b7..afc0ab3b 100644
--- a/build_system/clerk_driver.ml
+++ b/build_system/clerk_driver.ml
@@ -240,7 +240,7 @@ let get_lang file =
   Option.bind (Re.exec_opt catala_suffix_regex file) @@ fun g ->
   List.assoc_opt (Re.Group.get g 1) Cli.languages
 
-let scan_tree (dir : File.t) : catala_build_item list =
+let scan_tree (dir : File.t) : catala_build_item Seq.t =
   File.scan_tree
     (fun f ->
       match get_lang f with
@@ -602,7 +602,7 @@ let collect_in_folder
       | Some (test_file_name, ninja) ->
         ninja, test_file_names ^ " $\n  " ^ test_file_name
     in
-    List.fold_left
+    Seq.fold_left
       (fun acc (file, lang) ->
         let acc = collect collect_all_ninja_build acc file lang in
         collect collect_inline_ninja_builds acc file lang)
diff --git a/compiler/catala_utils/file.ml b/compiler/catala_utils/file.ml
index f350824c..b7743ee5 100644
--- a/compiler/catala_utils/file.ml
+++ b/compiler/catala_utils/file.ml
@@ -132,17 +132,20 @@ let scan_tree f t =
       false
   in
   let not_hidden t = match t.[0] with '.' | '_' -> false | _ -> true in
-  let rec do_dir acc d =
-    let items = Sys.readdir d |> Array.map (fun t -> d / t) |> Array.to_list in
-    do_files acc items
-  and do_files acc flist =
+  let rec do_dir d =
+    Sys.readdir d
+    |> Array.to_list
+    |> List.filter not_hidden
+    |> List.map (fun t -> d / t)
+    |> do_files
+  and do_files flist =
     let dirs, files =
       flist
-      |> List.filter not_hidden
       |> List.sort (fun a b -> -compare a b)
       |> List.partition is_dir
     in
-    let acc = List.rev_append (List.filter_map f files) acc in
-    List.fold_left do_dir acc dirs
+    Seq.append
+      (Seq.concat (Seq.map do_dir (List.to_seq dirs)))
+      (Seq.filter_map f (List.to_seq files))
   in
-  do_dir [] t
+  do_dir t
diff --git a/compiler/catala_utils/file.mli b/compiler/catala_utils/file.mli
index ab3b0e03..a1ddfd49 100644
--- a/compiler/catala_utils/file.mli
+++ b/compiler/catala_utils/file.mli
@@ -105,7 +105,7 @@ val format : Format.formatter -> t -> unit
 (** Formats a filename in a consistent style, with double-quotes and color (when
     the output supports) *)
 
-val scan_tree : (t -> 'a option) -> t -> 'a list
+val scan_tree : (t -> 'a option) -> t -> 'a Seq.t
 (** Recursively scans a directory for files. Directories or files matching ".*"
     or "_*" are ignored. Unreadable files or subdirectories are ignored with a
     debug message. *)