How to implement a short-circuited "and" macro in Common Lisp?

Question

Assume that the macro would take the boolean types a and b . If a is nil, then the macro should return nil (without ever evaluating b), otherwise it returns b. How do you do this?

Answer 1

This really depends on what you can use. E.g., is or available? if? cond?

Here is one example:

(defmacro and (a b)
  `(if ,a ,b nil)

EDIT. In response to a comment, or is more complicated because we have to avoid double evaluation:

(defmacro or (a b)
  (let ((v (gensym "OR")))
    `(let ((,v ,a))
       (if ,v ,v ,b))))