我正在将一些Clojure代码移植到OCaml，但遇到了以下问题：
let rotate ll =
let cons x y = x :: y in
let init = List.map (fun _ -> []) (List.hd ll) in
let rres = List.fold_right (List.map2 cons) ll init in
List.rev rres;;
let rec spiral_print matrix acc =
match matrix with
| [] -> acc
| head :: tail -> spiral_print (rotate tail) (acc @ head);;
不幸的是，这导致以下结果：
utop # spiral_print [[1; 2; 3]; [8; 9; 4]; [7; 6; 5]] [];;
Exception: (Failure hd).
我想调试为什么会这样，但是当我打开trace时，这就是我得到的：
#trace spiral_print;;
spiral_print [[1; 2; 3]; [8; 9; 4]; [7; 6; 5]] [];;
spiral_print <-- [[<poly>; <poly>; <poly>]; [<poly>; <poly>; <poly>]; [<poly>; <poly>; <poly>]]
spiral_print --> <fun>
spiral_print* <-- []
spiral_print <-- [[<poly>; <poly>]; [<poly>; <poly>]; [<poly>; <poly>]]
spiral_print --> <fun>
spiral_print* <-- [<poly>; <poly>; <poly>]
spiral_print <-- [[<poly>; <poly>]; [<poly>; <poly>]]
spiral_print --> <fun>
spiral_print* <-- [<poly>; <poly>; <poly>; <poly>; <poly>]
spiral_print <-- [[<poly>]; [<poly>]]
spiral_print --> <fun>
spiral_print* <-- [<poly>; <poly>; <poly>; <poly>; <poly>; <poly>; <poly>]
spiral_print <-- [[<poly>]]
spiral_print --> <fun>
spiral_print* <-- [<poly>; <poly>; <poly>; <poly>; <poly>; <poly>; <poly>; <poly>]
spiral_print* raises (Failure hd)
spiral_print* raises (Failure hd)
spiral_print* raises (Failure hd)
spiral_print* raises (Failure hd)
spiral_print* raises (Failure hd)
Exception: (Failure hd).
有没有办法以某种方式检查单个函数执行的内部状态以捕获错误？
我会使用其他语言的印刷品或精美印刷品。
ocamldebug 除非您限制类型，否则顶层将拒绝打印多态容器的元素：
let int_spiral_print : int list list -> int list -> int list = spiral_print;;
val int_spiral_print : int list list -> int list -> int list = <fun>
# #trace int_spiral_print;;
int_spiral_print is now traced.
# int_spiral_print [[1; 2; 3]; [8; 9; 4]; [7; 6; 5]] [];;
int_spiral_print <-- [[1; 2; 3]; [8; 9; 4]; [7; 6; 5]]
int_spiral_print --> <fun>
...
但是在这里具有正常的回溯将更有生产力。要获取它，请将您的代码放入文件中，例如，
$ cat spiral_print.ml
let rotate ll =
let cons x y = x :: y in
let init = List.map (fun _ -> []) (List.hd ll) in
let rres = List.fold_right (List.map2 cons) ll init in
List.rev rres
let rec spiral_print matrix acc =
match matrix with
| [] -> acc
| head :: tail -> spiral_print (rotate tail) (acc @ head)
let _ =
Printexc.record_backtrace true;
spiral_print [[1; 2; 3]; [8; 9; 4]; [7; 6; 5]] []
请注意，我添加Printexc.record_backtrace true了启用通常被禁用的回溯记录的功能。您也可以使用环境变量启用它OCAMLRUNPARAM=b。然后，您可以编译并运行程序，以获得良好的回溯信息：
ocamlbuild spiral_print.d.byte --
Fatal error: exception Failure("hd")
Raised at file "pervasives.ml", line 30, characters 22-33
Called from file "spiral_print.ml", line 3, characters 36-48
Called from file "spiral_print.ml", line 10, characters 33-46
Called from file "spiral_print.ml", line 14, characters 2-51
适当的文本编辑器（aka emacs）甚至会为您突出显示异常源是(List.hd ll)。
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我来说两句