在父代签名中进一步完善嵌套模块的签名 [英] Further refining the signature of a nested module in the signature of the parent

问题描述

我有一个包含嵌套模块的ML文件.例如:

I have an ML file which contains a nested module. For example:

let f n = n + 1

module type M_type = sig
  val g : int -> int
  val h : int -> int
end

module M : M_type = struct
  let g n = n + 2
  let h n = n + 3
end

let j n = n |> M.h |> M.g |> f

为此ML编写MLI时，我不希望公开M.h，但希望公开M.g.

When writing an MLI for this ML, I wish not to expose M.h, but I do wish to expose M.g.

如下所示:

module type M_type = sig
  val g : int -> int
end

module M : M_type

val j : int -> int

上面的ML和MLI组合无法编译.

The above combination of ML and MLI does not compile.

这个想法是我的嵌套模块M包含一些我希望公开给父模块中其他函数的功能，而不是公开给父模块用户的一些功能.

The idea is that my nested module M contains some functions that I do wish to expose to other functions in the parent module, but not to a user of the parent module.

有没有合法的方法来实现这一目标?如果不存在，那么实现签名缩小的最佳选择是什么?

Is there a legal way to achieve this? If there is none, what is the best alternative to achieve this kind of narrowing of the signature?

谢谢！

推荐答案

如果您仔细查看编译器错误:

If you look closely at the compiler error:

 Module type declarations do not match:
         module type M_type = sig val g : int -> int val h : int -> int end
       does not match
         module type M_type = sig val g : int -> int end

您将看到编译器没有抱怨模块M，而是抱怨模块类型M_type.

you will see that the compiler does not complain about the module M, but about the module type M_type.

实际上，两个模块类型的定义不匹配:

Indeed, the definition of the two module type does not match:

module type M_type = sig
  val g : int -> int
  val h : int -> int
end

与

module type M_type = sig
  val g : int -> int
end

有很多方法可以解决此问题. 一种可能性是在不必要时不使用模块类型作为签名约束:

There are many ways to fix this problem. One possibility is to not use module type as signature constraint when unneccessary:

(* a.ml *)
module M = struct
  let g n = n + 2
  let h n = n + 3
end

类似地，mli文件可以写为

Similarly, the mli file can be written as

(*a.mli*)    
module M : sig val g: int -> int end

另一种可能性是仅定义受约束的模块类型

Another possibility is to only define the constrained module type

(* a.ml *)
module type M_type = sig val g: int -> int end
module M: sig include M_type val h: int -> int end =
(* Note that the signature constraint above is not necessary *)
struct
  let g n = n + 2
  let h n = n + 3
end

以及相关的mli文件:

with the associated mli file:

(*a.mli*)
module type M_type = sig val h: int -> int end    
module M : sig val g: int -> int end

还可以为.ml文件定义扩展模块类型EXT:

It is also possible to define an extended module type EXT for the .ml file:

module type M_type = sig val g: int -> int end
module type EXT = sig include M_type val h: int -> int end
module M: EXT = struct
  let g n = n + 2
  let h n = n + 3
end

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