Ocaml 具有Janestreet Core_内核的映射

我正在用OCAML4.06、emacs/merlin制作真实世界的Ocaml。 请参阅下面的代码片段。我有两个问题：

open Core_kernel

let () =
  let digit_alist = [ 0, "zero"; 1, "one"; 2, "two"  ; 3, "three"; 4, "four";
                      5, "five"; 6, "six"; 7, "seven"; 8, "eight"; 9, "nine" ] in
  let _ = Map.of_alist_exn digit_alist ~comparator:Int.comparator in
  ()

从merlin进行计算时，它显示两个错误。看起来像是

Map.of\u alist\u exn

不接受带标签的参数

~comparator

：

This expression has type (int * string) list
       but an expression was expected of type
         ('a, 'b) Core_kernel.Map.comparator =
           (module Core_kernel__.Comparator.S with type comparator_witness = 'b and type t = 'a)

The function applied to this argument has type
         ('a * 'b) Core_kernel__.Import.list -> ('a, 'b, 'c) Base__Map.t
This argument cannot be applied with label ~comparator

Q1）列表exn的映射功能类型是否已更改

我认为函数类型已经改变了。所以我修改了源代码如下：

open Core_kernel

let () =
  let digit_alist = [ 0, "zero"; 1, "one"; 2, "two"  ; 3, "three"; 4, "four";
                      5, "five"; 6, "six"; 7, "seven"; 8, "eight"; 9, "nine" ] in
  let _ = Map.of_alist_exn Int.comparator digit_alist in
  ()

This expression has type
         (Core_kernel.Int.t, Core_kernel.Int.comparator_witness)
         Core_kernel__.Comparator.comparator =
           (Core_kernel.Int.t, Core_kernel.Int.comparator_witness)
           Base__Comparator.t
       but an expression was expected of type
         ('a, 'b) Core_kernel.Map.comparator =
           (module Core_kernel__.Comparator.S with type comparator_witness = 'b and type t = 'a)

此时，梅林抱怨如下：

open Core_kernel

let () =
  let digit_alist = [ 0, "zero"; 1, "one"; 2, "two"  ; 3, "three"; 4, "four";
                      5, "five"; 6, "six"; 7, "seven"; 8, "eight"; 9, "nine" ] in
  let _ = Map.of_alist_exn Int.comparator digit_alist in
  ()

This expression has type
         (Core_kernel.Int.t, Core_kernel.Int.comparator_witness)
         Core_kernel__.Comparator.comparator =
           (Core_kernel.Int.t, Core_kernel.Int.comparator_witness)
           Base__Comparator.t
       but an expression was expected of type
         ('a, 'b) Core_kernel.Map.comparator =
           (module Core_kernel__.Comparator.S with type comparator_witness = 'b and type t = 'a)

我希望

Int.comparator

将扮演有效比较器的角色，但ocaml认为它无效

Q2）我应该给比较器什么

---

@一个挣扎的noob查看比较器的定义，我们可以看到它的定义如下：

type ('k, 'cmp) comparator = (module Comparator.S with type comparator_witness = 'cmp and type t = 'k)

在ocaml中，模块可以用作第一类值。该特性的一个有趣结果是，模块现在可以定义为类型，并作为函数参数/参数传递/使用。Janestreet

core\u kernel

和/或

base

大量使用一流模块，正如您所发现的那样就是这样一个用例

Comparator.S

的定义如下：

module type Core_kernel.Comparator.S
type t
type comparator_witness
val comparator : (t, comparator_witness) comparator

如果查看

utop

中

module Int

-

show Core\u kernel.Int

的模块签名，我们可以看到它定义了

Core\u kernel.Comparator.S

模块类型中规定的类型和函数

因此，

Map.of\u alist\u exn

接受

（模块Int）

作为有效参数

（module Int）

是ocaml语法，表示值作为一级模块传递

参考文献：

---

对于Q2）

Map.of_-alist_-exn（模块Int）数字_-alist

？真实世界Ocaml的开发版本不是用来介绍Janestreet核心库的吗？我不明白为什么

（module Int）

可以是

Map.of\u alist\u exn

的有效参数，它的类型是

（'a，'cmp）core\u kernel.Map.comparator->（'a*'b）core\u kernel\u.Import.list->（'a，'b，'cmp）core\u kernel.Map.t