functional programming - In OCaml, I defined in AST file that stmt can be stmt or stmt List, but when used in function, it reports error -

i defined in ast file stmt=|stmt|stmt list. have function parameter of type stmt, cannot use stmt list? can tell me how make work?

the error when compiling is:

error: expression has type ast.stmt list        expression expected of type ast.stmt 

below code function:

let rec eval_stmt (stmt:stmt) (ps:proc_state) (env:environment) (store:store) : (stmtevalres*proc_state*store) = match stmt | 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 parser *)     (next, ps, store) | list (stmt1::stmts) -> eval_stmt stmt1 ps env store;     eval_stmt stmts ps env store;(next,ps,store) 

below ast file:

type stmt = (* statements in c-flat language *)   empty  | varass of expr * expr  (* expressions can statements in themselves. useful when    expression function call. calls built-in functions    special cases of calls. *) | expr of expr  | printstr of string (* string static! *) | printint of expr (* expr should 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 | of stmt * cond * stmt * stmt  | break (* used in switch, while , *) | continue (* used in while , *)  (* nested statements *) | list of stmt list  ;; 

your define stmt variant type, i.e., finite collection of alternatives. 1 of alternatives list indeed contains stmt list. doesn't mean stmt list same type stmt. means value list ... (with list of statements inside) of type stmt.