wip: use Stag in Format

src/core/CCFormat.ml

@@ -336,15 +336,23 @@ module ANSI_codes = struct
     | _ -> raise No_such_style
 end
 
+type stag +=
+  | Style of ANSI_codes.style list
+
 let color_enabled = ref false
 
+let mark_open_style st style =
+  Stack.push style st;
+  if !color_enabled then string_of_style_list style else ""
+
+let mark_close_style st style =
+
 (* either prints the tag of [s] or delegate to [or_else] *)
 let mark_open_tag st ~or_else s =
   let open ANSI_codes in
   try
     let style = style_of_tag_ s in
-    Stack.push style st;
+    mark_open_style st style
-    if !color_enabled then string_of_style_list style else ""
   with No_such_style -> or_else s
 
 let mark_close_tag st ~or_else s =
@@ -378,6 +386,11 @@ let update_tag_funs_ funs f1 f2 =
     mark_close_tag = f2 ~or_else:funs.mark_close_tag;
   }
 
+let styling stl pp out x =
+  pp_open_stag out (Style stl);
+  try pp out x; pp_close_stag out ()
+  with e -> pp_close_stag out (); raise e
+
 [@@@ocaml.warning "+3"]
 
 [@@@else_]

src/core/CCFormat.mli

@@ -324,6 +324,17 @@ module ANSI_codes : sig
       is a very shiny style. *)
 end
 
+[@@@ifge 4.8]
+
+val styling : ANSI_codes.style list -> 'a printer -> 'a printer
+(** [styling st p] is the same printer as [p], except it locally sets
+    the style [st].
+
+    Available only on OCaml >= 4.08.
+    @since NEXT_RELEASE *)
+
+[@@@endif]
+
 (** {2 IO} *)
 
 val output : t -> 'a printer -> 'a -> unit