LISP:关键字参数，提供-p [英] LISP: Keyword parameters, supplied-p

问题描述

目前，我正在研究Peter Seibel的"Practical Common Lisp".

在实用:简单数据库"一章中( http://www.gigamonkeys.com/book/practical-a-simple-database.html )Seibel通过以下示例说明了关键字参数和提供的参数的用法:

(defun foo (&key a (b 20) (c 30 c-p)) (list a b c c-p))

结果:

(foo :a 1 :b 2 :c 3)  ==> (1 2 3 T)
(foo :c 3 :b 2 :a 1)  ==> (1 2 3 T)
(foo :a 1 :c 3)       ==> (1 20 3 T)
(foo)                 ==> (NIL 20 30 NIL)

因此，如果我在参数列表的开头使用& key，则可以使用3个参数名称，默认值和第三个参数(如果没有提供)的列表.好的. 但是，请看上面示例中的代码:

(list a b c c-p)

lisp解释器如何知道c-p是我的提供的参数"?

解决方案

让我们重新缩进函数 foo :

(defun foo (&key a
                 (b 20)
                 (c 30 c-p))
   (list a b c c-p))

如果像这样缩进，您将看到该函数具有三个关键字参数:a，b和c.这些在函数主体中可用.

对于关键字参数 c ，有一个声明为 cp 的变量，该变量将为T或NIL，这取决于在获取foo时是否已传递 c 叫.

通常可以将关键字参数声明为以下选项之一:

1. 作为单个变量名称
2. 变量名和默认值的列表
3. 变量名，默认值和变量的列表，这些列表将显示调用函数时是否已传递参数

当要查看值是来自调用还是默认值时，提供的-p尤其有趣:

(defun make-my-array (size &key (init-value nil init-value-supplied-p))
   (if init-value-supplied-p
       (make-array size :initial-element init-value)
       (make-array size)))

现在用户可以将元素初始化为NIL:

(make-my-array 10 :init-value nil)

此处，默认值和提供的值都可以为NIL，但是我们需要有所不同.通过变量 init-value-supplied-p ，可以查看变量 init-value 的NIL值是来自默认值还是来自函数调用. >

At the moment I'm working through "Practical Common Lisp" from Peter Seibel.

In the chapter "Practical: A Simple Database" (http://www.gigamonkeys.com/book/practical-a-simple-database.html) Seibel explains keyword parameters and the usage of a supplied-parameter with the following example:

(defun foo (&key a (b 20) (c 30 c-p)) (list a b c c-p))

Results:

(foo :a 1 :b 2 :c 3)  ==> (1 2 3 T)
(foo :c 3 :b 2 :a 1)  ==> (1 2 3 T)
(foo :a 1 :c 3)       ==> (1 20 3 T)
(foo)                 ==> (NIL 20 30 NIL)

So if I use &key at the beginning of my parameter list, I have the possibility to use a list of 3 parameters name, default value and the third if the parameter as been supplied or not. Ok. But looking at the code in the above example:

(list a b c c-p)

How does the lisp interpreter know that c-p is my "supplied parameter"?

解决方案

Let's reindent the function foo:

(defun foo (&key a
                 (b 20)
                 (c 30 c-p))
   (list a b c c-p))

If you indent it like this you will see that the function has three keyword parameters: a, b and c. These are available in the body of the function.

For the keyword parameter c there is a variable declared c-p that will be T or NIL depending whether c has been passed when foo gets called.

A keyword parameter generally can be declared as one of the following options:

1. as a single variable name
2. a list of a variable name and a default value
3. a list of a variable name, a default value and a variable that will show whether the parameter has been passed or not when the function gets called

The supplied-p is particularly interesting when one wants to see whether the value comes from the call or the default value:

(defun make-my-array (size &key (init-value nil init-value-supplied-p))
   (if init-value-supplied-p
       (make-array size :initial-element init-value)
       (make-array size)))

Now the user can init the elements to NIL:

(make-my-array 10 :init-value nil)

Here the default value and the supplied value can both be NIL, but we need to make a difference. The variable init-value-supplied-p makes it possible to see whether the NIL value of the variable init-value comes from the default or from the function call.

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