In LISP, constants are variables that never change their values during program execution. Constants are declared using the defconstant construct.
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