test.ml

effect Id : 'a -> 'a;;

let handler th =
  match th () with
  | x -> x
  | effect (Id v) k -> continue k v;;

let handler' th =
  match th () with
  | x -> x
  | effect (Id (v' : int)) k -> continue k (v' + 3)
  | effect (Id v) k -> continue k v;;
(*
File "test.ml", line 11, characters 17-27:
Error: This pattern matches values of type int
       but a pattern was expected which matches values of type effect
*)

let header''  th =
  match th () with
  | x -> x
  | effect (Id v) k -> v
(*
File "test.ml", line 22, characters 23-24:
Error: This expression has type effect but an expression was expected of type
         'a
       The type constructor effect would escape its scope
*)