Lecture 6: In Class Activity

Problem 1

(defmacro first-item (lst)
 (list 'car lst)
)

(defmacro rest-of-items (lst)
 (list 'cdr lst)
)

Problem 2

(defmacro first-item2 (lst)
 `(car ,lst)
)

(defmacro rest-of-items2 (lst)
 `(cdr ,lst)
)

Problem 3

(defmacro for (var init final &rest body)
 (let ((start-var (gensym))     ; Gensym's prevent variable capture
       (count (gensym))
       (fin (gensym))
      )
  `(do ((,start-var ,var)       ; Bound only on first iteration
        (b (boundp ,var))       ;  -- Prevents multiple evaluation
        (,fin ,final)
        (,count ,init (+ ,count 1))
       )
       ((= ,fin ,count) (if b
                         (setf ,var ,start-var)
                         (makunbound ,var)
                        )
       )
       (setf ,var ,count)
       ,@body
   )
 )
)