Tutorial by Examples

The special symbol T represents the value true in Common Lisp, while the special symbol NIL represents false: CL-USER> (= 3 3) T CL-USER> (= 3 4) NIL They are called “Constant Variables” (sic!) in the standard, since they are variables whose value cannot be modified. As a consequence, y...

Actually any value different from NIL is considered a true value in Common Lisp. For instance: CL-USER> (let ((a (+ 2 2))) (if a a "Oh my! 2 + 2 is equal to NIL!")) 4 This fact can be combined with the boolean operators to make programs m...