key-if macro

I stumbled on this page and started to solve the problems, Here is solution to the ex2.

Define the macro key-if to have the form

(KEY-IF test
:THEN exp1 exp2 ...
:ELSE exp3 exp4 ...)

This does about the same thing as:

(COND (test exp exp ...)
   (T else-exp else-exp ...))

Almost everything is optional in key-if except the test. Here are some legal forms and their results:

> (key-if (> 3 1) :then 'ok)
OK
> (key-if (<> (key-if (> 3 1) :else 'oops)
NIL
> (key-if (> 3 1) :then)
NIL
> (key-if (> 3 1) :else 'oops :then 'ok)
OK
> (key-if (> 3 1) :else 'oops :then (print 'hi) 'ok)
HI
OK

Solution:

(defmacro key-if (test &rest args)
 (defun extract-then-else (exp)
   (defun iter (exp then-list else-list flag)
     (cond
       ((null exp) (values (nreverse then-list) (nreverse else-list)))
       ((eq :then (car exp)) (iter (cdr exp)
                                   then-list
                                   else-list t))
       ((eq :else (car exp)) (iter (cdr exp)
                                   then-list
                                   else-list nil))
       (t (if (null flag)
              (iter (cdr exp) then-list (cons (car exp)
                                              else-list) flag)
              (iter (cdr exp) (cons (car exp) then-list)
                    else-list flag)))))
   (iter exp nil nil nil))
 (multiple-value-bind (then else) (extract-then-else args)
   `(cond (,test ,(cons 'progn then))
          (t ,(cons 'progn else)))))