在OCaml中输入共享 - typechecker错误 [英] Type sharing in OCaml - typechecker error

问题描述

在编译这个程序时：

 模块类型Inc = 
 sig 
类型t 
 val inc：t  - > t 
结束
 
模块类型Dec = 
 sig 
类型t 
 val dec：t  - > t 
 end 
 
模块合并（I：Inc）（D：Dec类型为t = I.t）= 
 struct 
 let merge x = x |> I.inc |> D.dec 
 end 
 
 module IntInc：Inc = 
 struct 
 type t = int 
 let inc x = x + 10 
 end 
 
模块IntDec：Dec = 
 struct 
 type t = int 
 let dec x = x  -  8 
 end 
 
模块Combiner =合并（IntInc）（IntDec）
  

我得到以下错误：

 文件simple.ml，第30行，字符35-41：
错误：签名不匹配：
模块不match：
 sig type t = IntDec.t val dec：t  - > t end 
不包含在
中sig type t = IntInc.t val dec：t  - > t结束
类型声明不匹配：
类型t = IntDec.t 
不包含在
类型中t = IntInc.t 
文件simple.ml ，第13行，字符38-50：预期声明
文件simple.ml，第9行，字符13-14：实际声明
  

我认为 D：Dec类型为t = It 约束将确保 Dt = It 。为什么不是这样？

更有趣的是，当我删除 module Combiner = Merger（IntInc）（IntDec）行编译没有错误。

我的问题是：我做错了什么？

解决方案

您对 Merger 的约束定义是完全正确的，这就是为什么该部分无误地编译。

说，唯一不编译的部分是 module Combiner = Merger（IntInc）（IntDec）。这是因为，据OCaml所知，约束 IntInt.t = IntDec.t 未被满足。原因是OCaml不知道 IntInt.t 和 IntDec.t 是 INT 。它只知道 IntInt：Inc 和 IntDec：Dec - 其他所有内容都是模块的私有细节。 / p>

若要解决此问题，可以将模块的标题更改为 IntInt：（Inc类型为t = int）和 IntDec :( Dec类型为t = int），使类型 t 成为模块公共接口的一部分，而不是一个私人的细节，允许OCaml在解决合并函子的约束时使用这些信息。

When compiling this program:

module type Inc =
    sig
        type t
        val inc : t -> t
    end

module type Dec = 
    sig
        type t
        val dec : t -> t
    end

module Merger (I : Inc) (D : Dec with type t = I.t) =
    struct
        let merge x = x |> I.inc |> D.dec
    end

module IntInc : Inc = 
    struct
        type t = int
        let inc x = x + 10
    end

module IntDec : Dec = 
    struct 
        type t = int
        let dec x = x - 8
    end

module Combiner = Merger (IntInc) (IntDec)

I get the following error:

File "simple.ml", line 30, characters 35-41:
Error: Signature mismatch:
       Modules do not match:
         sig type t = IntDec.t val dec : t -> t end
       is not included in
         sig type t = IntInc.t val dec : t -> t end
       Type declarations do not match:
         type t = IntDec.t
       is not included in
         type t = IntInc.t
       File "simple.ml", line 13, characters 38-50: Expected declaration
       File "simple.ml", line 9, characters 13-14: Actual declaration

I thought that D : Dec with type t = I.t constraint will ensure that D.t = I.t. Why it is not so?

What is more interesting, when I remove module Combiner = Merger (IntInc) (IntDec) line it compiles without errors.

My question is : What am I doing wrong?

解决方案

Your definition of Merger with the constraint is perfectly correct, which is why that part compiles without error.

As you said, the only part that doesn't compile is module Combiner = Merger (IntInc) (IntDec). This is because, as far as OCaml knows, the constraint IntInt.t = IntDec.t is not met. The reason for that is that OCaml does not know that IntInt.t and IntDec.t are int. All it knows is that IntInt : Inc and IntDec : Dec - everything else is a private detail of the module.

To fix this problem you can change the module's headers to IntInt : (Inc with type t = int) and IntDec : (Dec with type t = int), making the type t a part of the modules' public interface rather than a private detail, allowing OCaml to use that information when resolving the constraint of the Merger functor.

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