oop - python 的 __repr__ 和 __str__ 的 Racket 等价物？

Question

在 python 对象中，覆盖方法 __repr__和 __str__一个对象的定义允许人们分别提供该对象的“明确的”和“人类可读的”表示。如何在 Racket 中实现类似的行为？

我遇到了 printable<%>界面 here ，看起来它应该可用于此目的，但我无法让它像我期望的那样工作。基于文档中的标准“鱼”示例:

(define fish%
  (class* object% (printable<%>)
    (init size) ; initialization argument
    (super-new) ; superclass initialization
    ;; Field
    (define current-size size)

    ;; Public methods
    (define/public (get-size)
      current-size)
    (define/public (grow amt)
      (set! current-size (+ amt current-size)))
    (define/public (eat other-fish)
      (grow (send other-fish get-size)))

    ;; implement printable interface
    (define/public (custom-print port quoting-depth)
      (print "Print called!"))
    (define/public (custom-write port)
      (print "Write called!"))
    (define/public (custom-display port)
      (print "Display called!"))))

这是我得到的输出:

> (define f (new fish% [size 10]))
> f
"Display called!""Display called!""Print called!"
> (print f)
"Write called!""Print called!"
> (display f)
"Display called!""Display called!"
> (write f)
"Write called!""Write called!"
> 

所以这个问题有三个方面:

1. 为什么它会以这种方式运行，即在对象的明显单一渲染上调用多个方法？

2. custom-print、custom-write 和 custom-display 方法的计算结果应该是什么？他们应该简单地返回一个字符串，还是应该实际承担打印、写入或显示的副作用(视情况而定)？例如。自定义写入方法是否应调用 write内部功能？

3. 这是用于此目的的正确结构吗？如果没有，有没有/它是什么？

Answer 1

至于

1. Why does it behave this way, i.e. with the multiple methods being invoked on an apparently singular rendering of the object?

你不小心在write中使用了print，所以写入值时，会先打印值。

(define/public (custom-write port)
   (print "Write called!"))

display 中也存在类似的问题。

还要记住打印/写入/显示到正确的端口。

尝试

#lang racket
(define fish%
  (class* object% (printable<%>)
    (init size) ; initialization argument
    (super-new) ; superclass initialization
    ;; Field
    (define current-size size)

    ;; Public methods
    (define/public (get-size)
      current-size)
    (define/public (grow amt)
      (set! current-size (+ amt current-size)))
    (define/public (eat other-fish)
      (grow (send other-fish get-size)))

    ;; implement printable interface
    (define/public (custom-print port quoting-depth)
      (print (~a "Print " current-size "\n") port))
    (define/public (custom-write port)
      (write (~a "Write " current-size "\n") port))
    (define/public (custom-display port)
      (display (~a "Display " current-size "\n") port))))

在repl中你会看到:

> (define f (new fish% [size 10]))
> f
"Print 10\n"
> (display f)
Display 10
> (write f)
"Write 10\n"