Lisp，说明在defun中不起作用 [英] Lisp, instructions not working in defun

问题描述

我正在尝试制作一个将中缀输入更改为前缀的函数，例如:(x + 1)作为以(+ x 1)输出的输入.

I'm trying to make a function that changes infix input to prefix eg : (x + 1) as input outputted as (+ x 1).

所以这是我目前的代码:

So here is my code for the moment :

(setq x '(Y + 1))
(if (listp x ) (list (second x) (first x) (first (last x))) x)

因此，如果我输入列表，则返回(+ Y 1)；如果不是列表，则返回用户输入.

so it returns (+ Y 1) if I input a list and the user input if it's not a list.

但是，问题是我无法在函数中使用此代码:

However, the problem is that I can't get this code working in a function :

(defun prefixToInfix (x)(
   (if (listp x ) (list (second x) (first x) (first (last x))) x)
   )
 )

该函数确实已创建，但是当我调用它时

the function is indeed created but when I call it

(prefixtoinfix '(Y + 1))

我收到错误

Error: Illegal function object: (IF (LISTP X) (LIST # # #) X).
[condition type: TYPE-ERROR]

我不知道为什么if语句在主程序中起作用，但是当我从函数中运行它时却不起作用.

I don't know why my if statement works in the main program but doesn't when I run it from my function.

推荐答案

您所缺少的是Lisp括号中的有意义.

What you are missing is that in Lisp parentheses are meaningful.

在C/Java/Python& c中，以下表达式相同:

In C/Java/Python &c, the following expressions are the same:

a+b
(a+b)
(a)+(b)
(((a)+(b)))
(((((a)+(b)))))

在Lisp中，以下表达式非常不同:

In Lisp, the following expressions are very different:

• a ---一个符号
• (a) ---具有单个元素的列表，该元素是符号a
• (1 (2)) ---两个元素的列表:

• a --- a symbol
• (a) --- a list with a single element, which is the symbol a
• (1 (2)) --- a list of two elements:

1. 编号1
2. 长度为1的列表，包含数字2

1. number 1
2. list of of length 1, containing number 2

以您的情况为例，功能(请注意缩进和括号的位置！)

In your case, function (note indentation and paren placement!)

(defun prefixToInfix (x)
  ((if (listp x) (list (second x) (first x) (first (last x))) x)))

在if周围有多余的括号(这会导致整个if形式被解释为一个函数，导致灾难性的结果)，应该是(请注意换行符和缩进-lispers不计算括号)，它们看起来缩进以了解代码，请参见 http://lisp-lang.org/style-guide/)

has extra parens around if (and this causes the whole if form to be interpreted as a function, with disastrous results), and should be (note line breaks and indentation - lispers do not count parens, they look at indentation to understand the code, see http://lisp-lang.org/style-guide/)

(defun prefix-to-infix (x)
  (if (listp x)
      (list (second x)
            (first x)
            (first (last x)))
      x))

PS.另请参见想要学习常见的口吻的建议.

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