escape

escape.ml

# module type S = sig type t val e : t end;;
module type S = sig type t val e : t end
# let f () = (module struct type t = int let e = 14 end : S);;
val f : unit -> (module S) = <fun>
# let module M = (val f () : S) in M.e;;
Characters 33-36:
  let module M = (val f () : S) in M.e;;
                                   ^^^
Error: This expression has type M.t but an expression was expected of type
         M.t
       The type constructor M.t would escape its scope
#