LISP - cannot call function with optional parameter

(defun lastplus (x &optional (n 0)) //default value for n is 0
    ( if (listp x) //if x is a list
        (
            (list (length x) (n)) //return list that contains length(x) and n
        )
        (n) //else return n
    )
)

Your formatting style is not Lispy.

Adapt to Lisp formatting:

(defun lastplus (x &optional (n 0)) ; default value for n is 0
   (if (listp x) ; if x is a list
        ((list (length x) (n))) ; return list that contains length(x) and n
     (n)))

You said: cannot call function with optional parameter.

Sure you can. The error message did say something else. You can call a function with an optional parameter. The error is inside the function.

The error says: Error: Undefined function N called with arguments ().

So you are calling a function called N, which does not exist. With no arguments. Like in (n). Check your code - can you find (n)?

Now ask yourself:

• What does a function call look like?

• Answer: open parenthesis, function, possibly some arguments, closing parenthesis

• What does (n) look like?

• Answer: it looks like a function call.

• Is that what you wanted?

• Certainly not.

• What did you want?

• The variable value.

• What does that look like?

• just n.

• Are there other errors?

• Hmm.

• What about the form on the third line?

• That looks wrong, too.

• It's wrong, too. Same error..