指定者的性质是什么？

问题描述

Svante只是通过在另一个答案中显示字符串指示符来震撼我的头脑：

 （string =：&& ;）-> T 
  

看CLHS，他们说一个指示符是一个表示另一个的对象很好，但是由于这些是不同的对象，因此某些地方需要进行某种强制。我的意思是，如果以下列表指示符可以由非零原子满足，则存在某些逻辑可以处理此问题。

我认为指示符可能是一个概念，例如，泛型函数。来自CLHS ...

...使得看上去非常具体。

所以我的问题

• 在实现中如何实现指示符的示例是什么？


• 该机制是否可以由用户以任何方式扩展？


• 这种机制在指定人之间是否一致？ （从clhs看，似乎有18种指示符）

干杯

解决方案

一个指示符仅是（或少于）一个指示的对象。关于它们的语言没有什么特别的；名称符的概念只是使某些编程实践更容易的一种。词汇表说：

该部分的链接很有帮助：

能够在词汇表中查找内容会有所帮助。例如，字符串指示符可以代表字符串：

该标准也恰好定义了函数，该函数获取由指定的字符串字符串指示符：

这简化了必须使用字符串和类似字符串的函数。例如，您可以定义 make-person 函数，该函数带有字符串指示符：

 （defun make -person（名称）
返回一个由NAME指定的名称的人。 
（列表：名称（字符串名称）））
 
（取消命名人的名称（人）
返回一个人的名字（字符串）。 
（getf人：名称））
  

指定符的概念不过是使定义灵活的API更加容易的编程约定。 Common Lisp被定义为一种统一一堆现有Lisps的语言，它可能是统一不同实现的行为的更简便方法之一。

case

我不知道返回列表指定者指定的列表的函数，但是编写起来很容易（这不能处理 t 的特殊行为以及大小写所需要的其他，但是它通常处理列表指示符）：

$ c>（defun to-list（x）
返回由x指定的列表。
（if（listp x）x
（list x）））

有时这样的约定在您自己的代码中可能会很有用，尤其是当您在注册表东西。例如，如果您编写了以下任何一个文件：

 （defmacro deftransform（name& rest args）
`（setf （gethash'，name * transforms *）
（make-transform，@ args）））
  

 （defmacro deftransform（name& rest args）
（setf（get'，name'transform） （make-transform，@ args）））
  

然后可以将转换指示符的概念定义为转换对象或符号（在* transforms *表中指定该符号的值，或该符号上的transform属性的值）。例如：

 （defun transform（x）
（if（transformp x）x 
（gethash name） ****）））
  

 （defun transform（x）
（if（transformp x）x 
（get x'transform）））
  

这可能会使部分代码更易于使用。功能指示符相似

Svante just blew my mind by showing string designators in another answer do this:

(string= :& "&") -> T

Looking at CLHS, they say A designator is an object that denotes another object. which is fine but as these are different objects some kind of coercion needs to happen somewhere. By which I mean if the following list designator can be satisfied by a 'non-nil atom' some logic exists somewhere for handling this.

I thought designators just could be a concept resulting from, for example, generic functions.. but the following line from CLHS...

... makes then seem very concrete.

So my questions

• What is an example of how designators could be implemented in an implementation?
• Is this mechanism extensible in any way by users?
• Is this mechanism consistent across designators? (looking in clhs it seems there are 18 kinds of designator)

Cheers

解决方案

A designator is nothing more (or less) than an object that designates another. There's nothing special in the language about them; the concept of designators is just one that makes certain programming practices easier. The glossary says:

The link to that section is helpful:

Being able to look for things in the glossary helps. For instance, a string designator is something that can stand for a string:

The standard also happens to define the function string that gets the string designated by a string designator:

This simplifies the implementation of functions that have to work with strings and string like things. For instance, you can define a make-person function takes a string designator:

(defun make-person (name)
  "Return a person with the name designated by NAME."
  (list :name (string name)))

(defun person-name (person)
  "Return the name of a person (a string)."
  (getf person :name))

The concept of designator isn't anything but a programming convention that makes defining flexible APIs easier. Common Lisp was defined as a language to unite a bunch of existing Lisps, and it may have been one of the easier ways to unify the behavior of different implementations.

There's a concept of list designator that gets used in case

I don't know of a function that returns the list designated by a list designator, but it's easy enough to write (this doesn't handle the special behavior of t and otherwise that case needs, but it handles list designators in general):

(defun to-list (x)
  "Return the list designated by x."
  (if (listp x) x
    (list x)))

Conventions like these can be useful in your own code sometimes, especially if you're defining things where there's a "registry" of things. E.g., if you have written either of:

(defmacro deftransform (name &rest args)
  `(setf (gethash ',name *transforms*)
         (make-transform ,@args)))

(defmacro deftransform (name &rest args)
  (setf (get ',name 'transform) (make-transform ,@args)))

Then you can define the concept of a transform designator as either a transform object, or a symbol (which designates the value for the symbol in the *transforms* table, or the value of transform property on the symbol). E.g.:

(defun transform (x)
  (if (transformp x) x
    (gethash name *transforms*)))

(defun transform (x)
  (if (transformp x) x
    (get x 'transform)))

That might make parts of your code easier to use. Function designators are similar

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