[Solved] Creating a method in Common Lisp

Question

To create a function in common lisp you can use the defun operator:

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

Now you can call it like so:

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

Then you can insert it in your code like this:

(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."))))

In your question you are asking about a method. Methods operate on classes. For example imagine you have two classes human and dog:

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

To create a method specific for each class you use defmethod:

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

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

Let’s create two instances for each class:

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

Now we can greet each living being with the same method:

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

The process of common lisp knowing which method to execute is called “Dynamic dispatch”. Basically it matches the given argument’s classes to the defmethod definitions.

But I have no idea why you need methods in your code example.

Here is how I would write the code if I was you:

;; 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.")))))

I would suggest you read “The Land of Lisp”. It is great book for beginners.

Accepted Answer