in lisp, constants are variables that never change their values during program execution. constants are declared using the defconstant construct.

example

the following example shows declaring a global constant pi and later using this value inside a function named area-circle that calculates the area of a circle.

the defun construct is used for defining a function, we will look into it in the functions chapter.

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

(defconstant pi 3.141592)
(defun area-circle(rad)
   (terpri)
   (format t "radius: ~5f" rad)
   (format t "~%area: ~10f" (* pi rad rad)))
(area-circle 10)

when you click the execute button, or type ctrl+e, lisp executes it immediately and the result returned is.

radius:  10.0
area:   314.1592