Functions in

About Functions

In Common Lisp, global, named functions are defined with defun.

This form takes as its first argument a list of parameters for the function being defined (the list of parameters is also called a 'lambda list'). After that it can, optionally, take a string for use as documentation to that function. Finally there are zero or more expressions which make up the body of the function.

Calling a function is done by using the name of the function as the first element of an expression, followed by any parameters to that function.

(defun add-one (x) (1+ x))

(add-one 3) ;; => 4

Functions can also take optional parameters, keyword parameters, rest parameters. These will be discussed in later concepts.

The name of the function is in scope in the body of the function so that recursion can be performed.

(defun countdown (x)
  (if (zerop x) (print 'BLAST-OFF)
    (progn (print x) (countdown (1- x)))))

(countdown 3) ;; will print "3","2", "1" and "BLAST-OFF" to
              ;; standard output before evalutaing to 'BLAST-OFF