diff --git a/src/env.ml b/src/env.ml
index 69a9ab75..4546584f 100644
--- a/src/env.ml
+++ b/src/env.ml
@@ -77,3 +77,6 @@ let diff x y =
     | Some _ -> None
     | None -> vx)
   |> make
+
+let update t ~var ~f =
+  make (Map.update t.vars var ~f)
diff --git a/src/env.mli b/src/env.mli
index 37cd5fe1..904a8d15 100644
--- a/src/env.mli
+++ b/src/env.mli
@@ -23,4 +23,7 @@ val add : t -> var:Var.t -> value:string -> t
 
 val diff : t -> t -> t
 
+val update : t -> var:string -> f:(string option -> string option) -> t
+
 val sexp_of_t : t -> Sexp.t
+