如何处理一个库中的变量需要在它外面设置？ [英] How to deal with a variable in a library that needs to be set outside of it?

问题描述

我在几个项目中使用Datomic，现在是时候把所有的公共代码移动到一个小的实用程序库。

一个挑战是处理大多数操作所依赖的共享数据库 uri ，但必须设置该项目使用库。我想知道是否有一个完善的方法来做到这一点。以下是我想到的一些替代方案：

• 删除 uri




• 通过 alter-var->变量修改

  root $ / $>


• 在动态库中保存它 * uri * 并覆盖希望的小适配器层中的值，如

  
  
  

  （def my-url ... bla ...） / p>
  
  

  （defn my-fun [args]
  （with-datomic-uri my-uri
  my-fun args））




• 将 uri / li>
  

解决方案

Stuart Sierra最后一个演示文稿是Clojure / West，名为 Clojure in the Large ，处理较大的Clojure应用程序的设计模式<

其中一个是你描述的问题。

总结关于手头的问题的提示：

1清除构造函数

所以你有一个明确的初始状态。

 （defn make-connection [uri] 
 {：uri uri 
 ...} 
  

2使依赖关系清楚

  update-db [connection] 
 ... 
  

>

 （deftest t-update 
（let [conn（make-connection）] 
 。（update-db conn）））））
  

4更安全重载

 （require ...：reload）
  

在要在以后绑定的变量中保留 uri 是很常见的，但引入了隐藏的依赖关系，也假设 body 一个单线程。

观看演讲，更多设计提示。

I'm using Datomic in several projects and it's time to move all the common code to a small utilities library.

One challenge is to deal with a shared database uri, on which most operations depend, but must be set by the project using the library. I wonder if there is a well-established way to do this. Here are some alternatives I've thought about:

• Dropping the uri symbol in the library and adding the uri as an argument to every function that accesses the database

• Altering it via alter-var-root, or similar mechanism, in an init function

• Keeping it in the library as a dynamic var *uri* and overriding the value in a hopefully small adapter layer like

  (def my-url ...bla...)

  (defn my-fun [args] (with-datomic-uri my-uri (apply library/my-fun args))

• Keeping uri as an atom in the library

解决方案

There was a presentation from Stuart Sierra last Clojure/West, called Clojure in the Large, dealing with design patterns for larger Clojure applications.

One of those was the problem you describe.

To summarize tips regarding the problem at hand:

1 Clear constructor

So you have a well defined initial state.

  (defn make-connection [uri]
      {:uri uri
       ...}

2 Make dependencies clear

  (defn update-db [connection] 
     ...

3 It's easier to test

(deftest t-update
  (let [conn (make-connection)]
    (is (= ... (update-db conn)))))

4 Safer to reload

 (require ... :reload)

Keeping uri in a variable to be bound later is pretty common, but introduces hidden dependencies, also assumes body starts and ends on a single thread.

Watch the talk, many more tips on design.

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