在 Common Lisp 中创建方法

Question

你好我正在做一个条件，我只想在条件为真时调用一个方法，问题是我找不到语法如何在 C-Lisp 中创建一个方法我是这种语言的新手这里是代码。

/* I want to create a method here which i can all anytime in my condition but I am having problem with a syntax 

 (void method()
   (print "Invalid")
 )

*/

(print "Enter number") 
(setq number(read())
(cond((< 1 number) (print "Okay"))
     ((> 1 number) /*I want to call a method here (the invalid one)*/ )
)

Answer 1

要在 common lisp 中创建函数，您可以使用 defun 运算符：

(defun signal-error (msg)
   (error msg))

现在你可以这样称呼它了：

(signal-error "This message will be signalled as the error message")

然后你可以像这样将它插入你的代码中：

(print "Enter number") 
(setq number (read)) ;; <- note that you made a syntax error here.
(cond ((< 1 number) (print "Okay"))
      ((> 1 number) (signal-error "Number is smaller than 1."))))

在你的问题中，你问的是 method。方法在 classes 上运行。例如假设你有两个 classes human 和 dog:

(defclass human () ())
(defclass dog () ())

要为每个 class 创建一个特定的方法，您使用 defmethod:

(defmethod greet ((thing human))
  (print "Hi human!"))

(defmethod greet ((thing dog))
  (print "Wolf-wolf dog!"))

让我们为每个创建两个实例 class:

(defparameter Anna (make-instance 'human))
(defparameter Rex (make-instance 'dog))

现在我们可以用同样的方式问候每一个众生了：

(greet Anna) ;; => "Hi human"
(greet Rex)  ;; => "Wolf-wolf dog!"

common lisp知道执行哪个方法的过程叫做"Dynamic dispatch"。基本上它匹配给定参数的 classes 到 defmethod 定义。

但我不知道为什么您的代码示例中需要方法。

如果我是你，我会这样写代码：

;; Let's wrap the code in a function so we can call it
;; as much as we want
(defun get-number-from-user ()
  (print "Enter number: ")
  ;; wrapping the number in a lexical scope is a good
  ;; programming style. The number variable is not
  ;; needed outside the function.
  (let ((number (read)))
    ;; Here we check if the number satisfies our condition and
    ;; call this function again if not.
    (cond ((< number 1) (print "Number is less than 1")
                        (get-number-from-user))
          ((> number 1) (print "Ok.")))))

我建议您阅读 "The Land of Lisp"。这本书适合初学者。

在 Common Lisp 中创建方法

Creating a method in Common Lisp

clisp

common-lisp