What if we didn’t use GENSYM?

(defmacro if-n-z-p-alt (expr f1 f2 f3)

(let ((val 'x))

`(let ((,val ,expr))

(cond ((minusp ,val) ,f1)

((zerop ,val) ,f2)

((plusp ,val) ,f3) ) ) ) )

> (macroexpand

> '(if-n-z-p-alt (- x 3) (- x 1) x (+ x 1)))

(LET ((X (- X 3)))

(COND ((MINUSP X) (- X 1)) ((ZEROP X) X) ((PLUSP X) (+ X 1))))

T

> (if-n-z-p-alt (- x 3) (- x 1) x (+ x 1)))

-9

Previous slide

Next slide

Back to first slide

View graphic version