Functional programming 在OCaml中，我在AST文件中定义了stmt可以是stmt或stmt List，但在函数中使用时，它会报告错误

我在AST文件中定义了stmt=|stmt | stmt List。那么我有一个参数类型为stmt的函数，我不能使用stmt List吗？有人能告诉我怎么做吗

编译时的错误是：

Error: This expression has type Ast.stmt list
       but an expression was expected of type Ast.stmt

下面是我的函数代码：

let rec eval_stmt (stmt:stmt) (ps:proc_state) (env:environment) (store:store) : (stmtEvalRes*proc_state*store) = match stmt with
| PrintInt e ->
    let r = eval_expr e ps env store in
    print_int r; (Next, ps, store)
| PrintStr s ->
    print_string (Str.global_replace (Str.regexp "\\\\n") "\n" s); 
    (* Escaping characters here because it's not done in the parser *)
    (Next, ps, store)
| List (stmt1::stmts) -> eval_stmt stmt1 ps env store;
    eval_stmt stmts ps env store;(Next,ps,store)

下面是我的AST文件：

type stmt = (* Statements in C-flat language *)
  Empty 
| VarAss of expr * expr

(* Expressions can be statements in themselves. This is most useful when
   the expression is a function call. Calls to built-in functions are
   just special cases of calls. *)
| Expr of expr 
| PrintStr of string (* string is only static! *)
| PrintInt of expr (* expr should be int *)

(* Control Flow *)
| IfThen of cond * stmt
| IfThenElse of cond * stmt * stmt
| Switch of expr * (int * stmt list) list * stmt list 
  (* Switch (cond, cases, default) *)

| While of cond * stmt
| For of stmt * cond * stmt * stmt

| Break (* used in switch, while and for *)
| Continue (* used in while and for *)

(* nested statements *)
| List of stmt list 
;;

---

您可以将

stmt

定义为变体类型，即有限的备选方案集合。备选方案之一是

List

，它确实包含

stmtlist

。但这并不意味着

stmt列表

与

stmt

的类型相同。这意味着一个值

列表…

（里面有一个语句列表）的类型是

stmt

，您可能希望代码的格式稍微好一点；现在很难读。只需将其缩进4个空格就足够了。