feat(Ref): add protect function

Author: c-cube

src/core/CCRef.ml

@@ -32,6 +32,17 @@ let swap a b =
   a := !b;
   b := x
 
+let protect r x f =
+  let old = !r in
+  r := x;
+  try
+    let res = f () in
+    r := old;
+    res
+  with e ->
+    r := old;
+    raise e
+
 let to_list r = [ !r ]
 let to_iter r yield = yield !r
 let pp pp_x out r = pp_x out !r

src/core/CCRef.mli

@@ -33,6 +33,11 @@ val swap : 'a t -> 'a t -> unit
 (** [swap t1 t2] puts [!t2] in [t1] and [!t1] in [t2].
     @since 1.4 *)
 
+val protect : 'a t -> 'a -> (unit -> 'b) -> 'b
+(** [protect r x f] sets [r := x]; calls [f()]; restores [r] to its old value;
+    and returns the result of [f()].
+    @since NEXT_RELEASE *)
+
 val compare : 'a ord -> 'a t ord
 val equal : 'a eq -> 'a t eq
 val to_list : 'a t -> 'a list