重载结构构造函数 [英] Overloading a struct constructor

问题描述

有没有办法重载 Racket 中结构的构造函数，以便我可以使继承的参数成为可选的?

Is there a way to overload the constructor for a struct in Racket, so I can make the inherited parameters optional ?

就我而言，我想为我的应用定义一些自定义异常.例如:

In my case, I want to define some custom exceptions for my app. For example :

(struct exn:my-app exn ())
(struct exn:my-app:illegal-access exn:my-app ())

但是，要实例化非法访问异常，我必须使用从 exn 继承的 2 个参数(消息和延续标记)调用构造函数，这非常麻烦.

However, to instantiate an illegal-access exception, I have to call the constructor with the 2 arguments inherited from exn (message and continuation-marks), which is quite cumbersome.

是否可以定义(对于 exn:my-app 及其所有后代)一个构造函数，它可以使两个参数都可选?所以我可以打电话给:

Is it possible to define (for exn:my-app and all its descendants) a constructor, which could make both parameters optional ? So I could call either :

(raise (exn:my-app:illegal-access))
(raise (exn:my-app:illegal-access "Message")) ?

谢谢，

推荐答案

这是一种方法:

(struct exn:my-app exn ()
        ;; change the name of the constructor
        #:constructor-name make-exn:my-app)

;; custom constructor
(define (exn:my-app [msg "default msg"]
                    [marks (current-continuation-marks)])
  (make-exn:my-app msg marks))

(exn:my-app) ; this works now

由于您需要为每个结构类型执行此操作，因此您可能需要定义一个宏来抽象它.我敢打赌，有人已经在 Racket 邮件列表上分享了这样的宏，但我想不起来，所以如果找到参考，我会更新这个答案.

Since you need to do this for each structure type, you may want to define a macro that abstracts over this. I bet someone has already shared such a macro on the Racket mailing list, but I don't recall one off the top of my head so I'll update this answer if I find a reference.

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