find_default function for Red-Black trees

vendors/Red-Black_Trees/PolyMap.ml

@@ -31,6 +31,10 @@ let find key map =
 let find_opt key map =
   try Some (find key map) with Not_found -> None
 
+let find_default key make_default_v map =
+  try find key map, map with
+    Not_found -> let v = make_default_v () in v, add key v map
+
 let has_key key map =
   match find_opt key map with
     Some _ -> true

vendors/Red-Black_Trees/PolyMap.mli

@@ -57,6 +57,13 @@ val find : 'key -> ('key, 'value) t -> 'value
 
 val find_opt : 'key -> ('key, 'value) t -> 'value option
 
+(* The value of the call [find_default key make_default_v map] is
+   [value] if the key [key] is bound to [value] in the map [map], and
+   [make_default_v ()] otherwise. In the first case, the
+   [make_default_v] function is not executed *)
+
+val find_default : 'key -> (unit -> 'value) -> ('key, 'value) map -> 'value * ('key, 'value) map
+
 (* The value of the call [find_opt key map] is [true] if the key
    [key] is bound to some value in the map [map], and [None]
    otherwise. *)