Lisp - keyword Parameters

Keyword parameters allow you to specify which values go with which particular parameter.

It is indicated using the &key symbol.

When you send the values to the function, you must precede the values with :parameter-name.

The following example illustrates the concept.

Example

Create a new source code file named main.lisp and type the following code in it.

main.lisp

; define a function show-members to print list of arguments passed
(defun show-members (&key a b c d ) (write (list a b c d)))
; call function with three parameters
(show-members :a 1 :c 2 :d 3)
; terminate printing
(terpri)
; call function with two parameters
(show-members :a 1 :b 2)

Output

When you execute the code, it returns the following result −

(1 NIL 2 3)
(1 2 NIL NIL)

Example

Update the source code file named main.lisp and type the following code in it.

main.lisp

; define a function show-members to print list of arguments passed
(defun show-members (&key a b c d ) (write (list a b c d)))
; call function with three parameters
(show-members :a 'p :b 'q :c 'r :d 's)
; terminate printing
(terpri)
; call function with two parameters
(show-members :a 'p :d 'q)

Output

When you execute the code, it returns the following result −

(P Q R S)
(P NIL NIL Q)

Example

Update the source code file named main.lisp and type the following code in it.

main.lisp

; define a function show-members to print list of arguments passed
(defun show-members (&key a b c d ) (write (list a b c d)))
; call function with three parameters
(show-members :a 1.0 :c 2.0 :d 3.0)
; terminate printing
(terpri)
; call function with two parameters
(show-members :a 1.0 :b 2.0)

Output

When you execute the code, it returns the following result −

(1.0 NIL 2.0 3.0)
(1.0 2.0 NIL NIL)