diff --git a/lib/lwd/lwd.ml b/lib/lwd/lwd.ml
index 3c77a27..4fb8ad5 100644
--- a/lib/lwd/lwd.ml
+++ b/lib/lwd/lwd.ml
@@ -308,6 +308,8 @@ let peek = function
   | Operator ({desc = Var v; _}) -> v.binding
   | _ -> assert false
 
+let update f v = set v (f (peek v))
+
 (* Primitives *)
 type 'a prim = 'a t
 let prim ~acquire ~release =
diff --git a/lib/lwd/lwd.mli b/lib/lwd/lwd.mli
index 386dde3..e42389f 100644
--- a/lib/lwd/lwd.mli
+++ b/lib/lwd/lwd.mli
@@ -67,6 +67,9 @@ val peek : 'a var -> 'a
 (** Observe the current value of the variable, without any dependency
     tracking. *)
 
+val update : ('a -> 'a) -> 'a var -> unit
+(** Modify a variable based on its currently observed value. *)
+
 type +'a prim
 (** A primitive document. It can correspond, for example, to
     a primitive UI element.