Common Lisp provides an IF statement:
(IF < test> < then-form> ) (IF < test> < then-form> < else-form> )The < test> is evaluated first. If it returns a non- NIL value, the < then-form> is evaluated and its value is the value of the IF; otherwise, the < else-form> is evaluated and its value is the value of the IF.
Since NIL is false and anything else is treated as true, a common convention is for a function to return NIL if it did not work, or an answer if it did work.
(SETQ Y (IF (< X 0.0)
(* X X)
(SQRT X)))
Note that the < then-form> and < else-form> are single forms; if multiple things need to be done, they must be enclosed in a PROGN.