Elisp: Function Parameters (optional, rest)

Optional Parameters

To have optional parameters, add

&optional name1 name2 etc

at the end of parameter list.

• When optional parameter is not given in a function call, its value is nil.
• When an optional parameter has value of nil, there is no way to know if it is given in function call or omitted.

;; defining a function with 2 optional params named cc and dd

(defun ff (aa bb &optional cc dd)
  "test optional params"
  (interactive)
  (message "%s %s %s %s" aa bb cc dd))

;; call it
(ff 1 2)
;; "1 2 nil nil"

(ff 1 2 3)
;; "1 2 3 nil"

(ff 1 2 3 4)
;; "1 2 3 4"

Rest Parameters (Unspecified Number of Parameters)

To specify unspecified number of parameters, add

&rest name

as the last parameter.

In the function, value of name is a list, or nil if omitted.

You can have both &optional and &rest, in that order.

• Argument List (GNU Emacs Lisp Reference Manual)

;; defining a function with rest parameters

(defun ff (aa bb &rest cc)
  "test rest arguments"
  (message "%s" cc) ; cc is a list
  )

;; test
(ff "1" "2" "3" "4")
;; ("3" "4")

Example of both optional and rest parameter

(defun xx-test (aa bb &optional cc dd &rest others)
  "test optional and rest parameters"
  (message
   "aa is %s
bb is %s
cc is %s
dd is %s
rest is %s"
   aa bb cc dd others))

;; test
(xx-test 1 2)
"aa is 1
bb is 2
cc is nil
dd is nil
rest is nil"

(xx-test 1 2 3)
"aa is 1
bb is 2
cc is 3
dd is nil
rest is nil"

(xx-test 1 2 3 4)
"aa is 1
bb is 2
cc is 3
dd is 4
rest is nil"

(xx-test 1 2 3 4 5)
"aa is 1
bb is 2
cc is 3
dd is 4
rest is (5)"

(xx-test 1 2 3 4 5 6)
"aa is 1
bb is 2
cc is 3
dd is 4
rest is (5 6)"

No Default Values

• Emacs lisp does not support parameter default value.
• Emacs lisp does not support parameter type checking.

Function Keyword Parameters (Named Parameters)

• Elisp: Function Keyword Parameters (Named Parameters)