How do I make named arguments in ELisp?

Question

I figured that since Emacs Lisp and Common Lisp seemed so closely related syntax wise, I could just follow the example code I found on RosettaCode, but it turns out that I was wrong.

The code in question looks like this:

(defun print-name (&key first (last "?"))
  (princ last)
  (when first
    (princ ", ")
    (princ first))
  (values))

And according to RosettaCode it should do the following:

> (print-name)
  ?
> (print-name :first "John")
  ?, John
> (print-name :last "Doe")
  Doe
> (print-name :first "John" :last "Doe")
  Doe, John

Now, here's the thing; whenever I try to run that function in my ELisp interpreter, I get the following error:

*** Eval error ***  Wrong number of arguments: (lambda (&key first (last "?")) (princ la\
st) (if first (progn (princ ", ") (princ first))) (values)), 0

I'm not routined enough in lisp to know what that's supposed to mean, and no amount of Googling has lead me any closer to an answer.

So what's the correct way of doing this in Emacs Lisp?

Answer 1

Since Emacs Lisp does not support keyword arguments directly, you'll need to emulate these, either with cl-defun as in the other answer, or by parsing the arguments as plist:

(defun print-name (&rest args)
  (let ((first (plist-get args :first))
        (last (or (plist-get args :last) "?")))
    (princ last)
    (when first
      (princ ", ")
      (princ first))))

&rest args tells Emacs to put all function arguments into a single list. plist-get extracts a value from a property list, that is, a list of the format (key1 value1 key2 value2 …). Effectively, a plist is a flattened alist.

Together this lets you call print-name just like in your question:

> (print-name)
  ?
> (print-name :first "John")
  ?, John
> (print-name :last "Doe")
  Doe
> (print-name :first "John" :last "Doe")
  Doe, John