OCaml评估函数中的语法错误

Question

我发现此代码用于评估互联网上OCaml中的表达式，并希望尝试了解其工作原理，但当我在编辑器中输入并运行它时，我收到以下错误：

type t =
| Integer of int
| Binary of binary * t * t
and binary =
| Add
| Sub
| Mult
| Div

type token =
| INTEGER of int
| ADD
| SUB
| MULT
| DIV
let rec eval = function
| Integer(k) -> k
| Binary(op, a, b) ->
(match op with
| Add -> ( + )
| Sub -> ( - ) 
| Mult -> ( * )
| Div -> ( / )) (eval a) (eval b)

let lexer s =
let open Str in
let split s =
let rec splitchar x l =
if x< 0 then l else splitchar (x-1) ( s.[x]:: l ) in
splitchar ( String.length s -1) [] 
|> List.map
(function
| "+" -> ADD
| "-" -> SUB
| "*" -> MULT
| "/" -> DIV
| _ ->     failwith "lexer: Invalid token: %s" );;

人物280-282： | _ - ＆gt; failwith＆＃34; lexer：无效的令牌：％s＆＃34; ）;;                                             ^^ 错误：语法错误

错误消息并没有多大帮助，我尝试了一些只会让事情变得更糟的变化。任何人都可以通过弄清楚语法错误是什么来帮助我开始吗？