T7915: add util function value_exists

Author: jestabro

src/config_tree.ml

@@ -146,6 +146,15 @@ let get_value node path =
     | [] -> raise Node_has_no_value
     | x :: _ -> x
 
+let value_exists node path value =
+    (* alert exn Vytree.get:
+        [Vytree.Empty_path] allow raise
+        [Vytree.Nonexistent_path] allow raise
+     *)
+    let node' = (Vytree.get[@alert "-exn"]) node path in
+    let data = Vytree.data_of_node node' in
+    Vylist.in_list data.values value
+
 let delete node path value =
     (* raises:
         [Vytree.Nonexistent_path] from get_values; delete_value

src/config_tree.mli

@@ -52,6 +52,10 @@ val get_value : t -> string list -> string
 [@@alert exn "Vytree.Nonexistent_path"]
 [@@alert exn "Config_tree.Node_has_no_value"]
 
+val value_exists : t -> string list -> string -> bool
+[@@alert exn "Vytree.Empty_path"]
+[@@alert exn "Vytree.Nonexistent_path"]
+
 val set_comment : t -> string list -> string option -> t
 [@@alert exn "Vytree.Empty_path"]
 [@@alert exn "Vytree.Nonexistent_path"]