在榆木中什么是不透明类型？为什么它很有价值？ [英] What is an opaque type in Elm and why is it valuable?

问题描述

我以前使用过类型，但不知道什么是不透明类型。我也看到了它的提法。

I've used types before but don't know what an opaque type is. I've seen it mentioned as well. Is it better to expose an opaque type than a type alias?

推荐答案

让我们首先查看类型别名来回答这个问题：

Let’s answer this question by first looking at type aliases:

类型别名是完全透明的。这意味着任何其他导入它的模块都可以完全访问其内部工作方式。假设我们有一个 User 模块公开了 User 类型：

A type alias is fully transparent. This means that any other module importing it will have full access to its inner workings. Let’s say we’ve got a User module exposing a User type:

module User exposing User

type alias User =
    { userName : String
    , age : Int
    }

任何导入 User 的人都可以操纵数据，例如 newUser = {oldUser |年龄= 25} 。或执行 someUser =用户账单 27 。当您可以控制它们存在的上下文时，这些操作就很好。

Anyone importing User can manipulate the data, e.g. newUser = { oldUser | age = 25 }. Or do someUser = User "Bill" 27. These manipulations are fine when you have control over the context that they exist in.

但是，如果 User 是其中的一部分图书馆，那么对 User 类型的每次更改对使用图书馆的人来说都是一个重大更改。例如，如果将 email 字段添加到 User ，则构造函数示例（ someUser =用户 Bill 27 ）将给出编译器错误。

However, if User is part of a library then every change to the User type is a breaking change to people that use the library. For example, if an email field is added to User, then the constructor example (someUser = User "Bill" 27) will give a compiler error.

即使在项目代码库内部，类型别名也可以提供太多信息其他模块，导致代码难以维护和发展。也许 User 在某个时候发生了巨大变化，并具有一组全新的属性。无论代码在何处操纵 User s，都需要进行更改。

Even inside of a project codebase, a type alias can provide too much information to other modules which leads to code that is difficult to maintain and evolve. Perhaps a User changes drastically at some point and has a completely new set of properties. This would require changes wherever the code manipulates Users.

不透明类型很有价值，因为它们避免了这些问题。这是 User 的不透明版本：

Opaque types are valuable because they avoid these issues. Here’s an opaque version of User:

module User exposing User

type User =
    User
        { userName : String
        , age : Int
        }

使用此版本，其他模块无法直接访问或操作数据。通常，这意味着您将制作并公开一些吸气剂和功能：

With this version, other modules cannot access or manipulate the data directly. Often, this means you will make and expose some getter and functions:

initUser : String -> Int -> User
userName : User -> String
age : User -> String
setAge : Int -> User -> User

这是更多工作，但它具有以下优点：

This is more work, but it has advantages:

• 其他模块仅关心 User 函数，不需要知道类型中的数据


• 可以在不破坏包含模块外部代码的情况下更新类型

• Other modules only care about the User functions and don’t need to know what data is in the type
• The type can be updated without breaking code outside the containing module

大部分解释来自 @wintvelt ： elmlang.slack.com

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