怎么设置！在方案中定义? [英] How is set! defined in scheme?

问题描述

您将如何实现自己的 set! 函数? set!函数是一种破坏性过程，它会更改考虑到先前值的已定义值.

How would you implement your own set! function in Scheme? A set! function is a destructive procedure that changes a value that is defined taking into account the previous value.

推荐答案

如注释中所述，set!是Scheme中的原语，必须由实现提供.同样，您不能在大多数编程语言中实现赋值运算符=.在Common Lisp中，可以扩展setf(使用setf -expanders)以允许(setf form value)处理新的形式.

As noted in the comments, set! is a primitive in Scheme that must be provided by the implementation. Similarly, you can't implement the assignment operator, =, in most programming languages. In Common Lisp, setf can be extended (using setf-expanders) to allow (setf form value) to work on new kinds of forms.

因为Scheme的set!仅修改了变量绑定(例如Common Lisp的setq)，所以仍然值得问一问我们如何实现set-car!和其他结构修饰符之类的功能.从某种意义上说，这可以看作是对变量的赋值的概括"，但是由于词汇变量(以及闭包)足以表示任意复杂的结构，因此它也可以看作是更 specialized 案例.在Scheme中(除了像数组这样的内置图元)，对象字段的变异是 specialization ，因为对象可以通过词法闭包来实现，并可以以set!的方式实现.这是一个典型的练习，展示了如何单独使用词汇闭包来实现结构(例如cons单元格).这是一个显示单值可变单元格实现的示例:

Because Scheme's set! only modifies variable bindings (like Common Lisp's setq), it is still worth asking how can we implement functions like set-car! and other structural modifiers. This could be seen, in one sense, as a generalization of assignment to variables, but because lexical variables (along with closures) are sufficient to represent arbitrarily complex structures, it can also be seen as a more specialized case. In Scheme (aside from built in primitives like arrays), mutation of object fields is a specialization, because objects can be implemented by lexical closures, and implemented in terms of set!. This is a typical exercise given when showing how structures, e.g., cons cells, can be implemented using lexical closures alone. Here's an example that shows the implementation of single-value mutable cells::

(define (make-cell value)
  (lambda (op)
    (case op
      ((update)
       (lambda (new-value)
         (set! value new-value)))
      ((retrieve)
       (lambda ()
         value)))))

(define (set-value! cell new-value)
  ((cell 'update) new-value))

(define (get-value cell)
  ((cell 'retrieve)))

基于这些定义，我们可以创建一个以值4开头的单元格，使用我们的set-value!将值更新为8，然后获取新值:

Given these definitions, we can create a cell, that starts with the value 4, update the value to 8 using our set-value!, and retrieve the new value:

(let ((c (make-cell 4)))
  (set-value! c 8)
  (get-value c))
=> 8

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