Question

我正在尝试以递归方式浏览文件和文件夹的嵌套列表。虽然我这样做，但我想打印我访问的文件和文件夹的名称，但我做的事情非常错误。这就是我所拥有的：

type 'a fileTree =¬
  | File of 'a¬
  | Folder of 'a * ('a fileTree list)

let printFiles tree =
  let rec visit tree = function
    | [] -> print_string "Done"
    | File f :: t -> visit t
    | Folder (name, contents) :: t -> visit contents
  in
  visit tree

这样做我得到了

Error: This expression has type 'a fileTree list -> unit but an expression was expected of type unit

代码行(File f :: t -> visit t)。

我做错了什么？

编辑1

现在代码

let printFiles tree =
  let rec visit = function
    | File file -> Printf.printf "file: %s\n" file
    | Folder (name, contents) ->
      Printf.printf "folder: %s\n" name;
      List.iter visit contents
  in
  List.iter visit tree

但我仍然在最后一行得到这个错误：

Error: This function has type ('a -> unit) -> 'a list -> unit
   It is applied to too many arguments; maybe you forgot a `;'

Answer 1

你忘了括号：

print_string "File " ^ f

它被评估为(print_string "File ") ^ f，但您期望的是

print_string ("File " ^ f)

与"Folder"案例相同。

更新

在更新的示例中，您有一个应用于一个参数的函数。我想，而不是：

 let rec visit tree = function

你想写

 let rec visit = function

相当于

 let rec visit tree = match tree with

Answer 2

问题在于您已将visit作为函数写入列表，但在文件树上调用它。

让我们将其更改为树上的函数，并使用List.iter来遍历列表：

let print_files tree =
  let rec visit = function
    | File file -> 
      Printf.printf "file: %s\n" file
    | Folder (name, contents) ->
      Printf.printf "folder: %s\n" name;
      List.iter visit contents in
  visit tree

Answer 3

我最终以下列方式解决了这个问题：

let writeFilesFromTree tree =
  let rec visit = function
    | [] -> print_string "-\n"
    | File f :: t -> Printf.printf "file: %s\n" f ; visit t
    | Folder (name, contents) :: t ->
      Printf.printf "name: %s\n" name ;
      visit contents ;
      visit t in
  visit tree;;

它仍然感觉不是最佳解决方案，我感觉我在最后两次连续调用visit时破坏了尾递归。

模式匹配ocaml中的嵌套元组

3 个答案:

更新