使用OCaml Menhir，有没有一种方法可以在处理之前访问它？

%start <expr> expr

%%

expr:
| INT of int
| PLUS of expr * expr
| TIMES of expr * expr
;

open Example

module type VISITOR = sig
  val visit_expr : expr -> unit
end

module MakeVisitor (V : VISITOR) = struct
  let rec visit = function
    | INT _ -> V.visit_expr (INT 0)
    | PLUS (e1, e2) ->
        V.visit_expr (PLUS (visit e1, visit e2))
    | TIMES (e1, e2) ->
        V.visit_expr (TIMES (visit e1, visit e2))
  ;;
end

open Example

module ExampleVisitor : VISITOR = struct
  let visit_expr expr =
    Printf.printf "Visiting expression: %s\n" (string_of_expr expr)
  ;;

  let string_of_expr = function
    | INT n -> Printf.sprintf "INT %d" n
    | PLUS (e1, e2) -> Printf.sprintf "PLUS (%s, %s)" (string_of_expr e1) (string_of解释器模式在处理之前访问解析树中的元素
    | TIMES (e1, e2) -> Printf.sprintf "TIMES (%s, %se2)" (string_of_expr e1) (string_of_expr e2)
  ;;
end

open Example
open Visitor
open ExampleVisitor

let () =
  let lexbuf = Lexing.from_string "1 + 2 * 3" in
  let parser = Parser.create () in
  let ast = Parser.parse_expr lexbuf parser in
  let visitor = MakeVisitor (ExampleVisitor) in
  visitor.visit ast

Visiting expression: INT 1
Visiting expression: INT 2
Visitée表达式：TIMES (INT 2, INT 3)
Visiting expression: PLUS (INT 1, TIMES (INT 2, INT 3))