Add Ordered_set_lang.Unexpanded.fold_strings

Signed-off-by: Jeremie Dimino <jeremie@dimino.org>

src/ordered_set_lang.ml

@@ -271,6 +271,26 @@ module Unexpanded = struct
     in
     loop t.ast
 
+  type position = Pos | Neg
+
+  let fold_strings t ~init ~f =
+    let rec loop (t : ast) pos acc =
+      let open Ast in
+      match t with
+      | Standard | Include _ -> acc
+      | Element x -> f pos x acc
+      | Union l -> List.fold_left l ~init:acc ~f:(fun acc x -> loop x pos acc)
+      | Diff (l, r) ->
+        let acc = loop l pos acc in
+        let pos =
+          match pos with
+          | Pos -> Neg
+          | Neg -> Pos
+        in
+        loop r pos acc
+    in
+    loop t.ast Pos init
+
   let expand t ~files_contents ~f  =
     let context = t.context in
     let rec expand (t : ast) : ast_expanded =

src/ordered_set_lang.mli

@@ -76,6 +76,17 @@ module Unexpanded : sig
     -> files_contents:Sexp.Ast.t String.Map.t
     -> f:(String_with_vars.t -> string)
     -> expanded
+
+  type position = Pos | Neg
+
+  (** Fold a function over all strings in a set. The callback receive
+      whether the string is in position or negative position, i.e. on
+      the left or right of a [\] operator. *)
+  val fold_strings
+    :  t
+    -> init:'a
+    -> f:(position -> String_with_vars.t -> 'a -> 'a)
+    -> 'a
 end with type expanded := t
 
 module String : S with type value = string and type 'a map = 'a String.Map.t