这种模式匹配在OCaml中并不详尽。

Question

我是OCaml的新手，我编写了一些代码来获取列表的n元素

let rec n_elem l n = match n with
| 0 -> match l with
    | h::_ -> h
    | _ -> failwith "erorr with empty list"
| _ -> match l with
    | h::t -> n_elem t (n-1)
    | _ -> failwith "erorr with empty list"
;;

当我使用ocaml解释器运行它时，一个警告生成如下：

Warning 8: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:
1
Warning 11: this match case is unused.

当我用：

Printf.printf "%s\n" (n_elem ["a";"b";"c";"d"] 1);;

它产生match_failure..。

有人能帮我一下吗？

Answer 1

这基本上是一个优先问题。第二个_匹配情况是第二个match表达式的一部分。您可以使用begin/end将它们分开：

let rec n_elem l n = match n with
| 0 -> 
    begin
    match l with
    | h::_ -> h
    | _ -> failwith "erorr with empty list"
    end
| _ ->
    begin
     match l with
    | h::t -> n_elem t (n-1)
    | _ -> failwith "erorr with empty list"
    end