lisp:if语句的格式正确 [英] lisp: properly form if statement

问题描述

作为我自己学习普通口语的一项任务，我正在尝试重新创建 lodash

As a task for myself to learn common lisp, I'm trying to recreate lodash.

在重新创建_.chunk的过程中，我编写了以下代码来测试可选参数

En route to recreating _.chunk, I've written the following to test for an optional argument

(defun _.chunk (array &optional size)
    (if (size)
        (write ("there") )
        (write ("not") )
    ) 
)  

设置(setf x #('a 'b 'c 'd))然后运行(_.chunk x)，我得到一个错误:

Setting (setf x #('a 'b 'c 'd)) and then running (_.chunk x), I get an error:

; caught ERROR:
;   illegal function call

;     (SB-INT:NAMED-LAMBDA _.CHUNK
;         (ARRAY &OPTIONAL SIZE)
;       (BLOCK _.CHUNK
;         (IF (SIZE)
;             (WRITE ("there"))
;             (WRITE ("not"))))) 

测试可选功能参数的正确方法是什么?

What's the correct way to test for optional function parameters?

推荐答案

size-p ，可以在关键字或可选参数的默认值之后指定的可选变量的名称如果参数作为参数传递给函数调用，则为true.

size-p, the name of the optional variable which may be specified after the default value of a keyword or optional argument, will be true if the parameter was passed as an argument to a function call.

因此您可以执行以下操作:

so you could do something such as:

(defun _.chuck (array &optional (size 0 size-p))
  (if size-p
      (rest of your form...)))

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