practical-language-learning

exercise-10

1. If x is a, y is b, and z is (c d),…
2. Define if in terms of cond.
3. Define a macro that takes…
4. Define ntimes (page 167) to…
5. Define a macro n-of that takes…
6. Define a macro that takes…
7. What’s wrong with the following definition of push?
8. Define a macro that doubles its argument:

If x is a, y is b, and z is (c d),…

1.If x is a, y is b, and z is (c d), write backquoted expressions containing only variables that yield each of the following:

1. ((C D) A Z)

   `(,z ,x z)
   

2. (X B C D)

   `(x ,y ,@z)
   

3. ((C D A) Z)

   `((,@z ,x) z)
   

Define if in terms of cond.

(defun new-if (test then else)
  (cond (test then)
        (t else)))

Define a macro that takes…

3.Define a macro that takes a number n followed by one or more expressions, and returns the value of the nth expression:

;;> (let ((n 2))
;;     (nth-expr n (/ 1 0) (+ 1 2) (/ 1 0)))
;;===> 3

(defmacro nth-expr (n &rest body)
  `(case ,n
     ,@(let ((key -1))
         (mapcar #'(lambda (obj)
                     `(,(incf key) ,obj))
                 body))))

Define ntimes (page 167) to…

4.Define ntimes (page 167) to expand into a (local) recursive function instead of a do.

(defmacro ntimes (n &rest body)
  (let ((g (gensym))
        (h (gensym)))
    `(let ((,h ,n))
       (do ((,g 0 (+ ,g 1)))
           ((>= ,g ,h))
         ,@body))))

(defmacro new-ntimes (n &rest body)
  (let ((g (gensym)))
    `(let ((,g ,n))
       (labels ((nt (i)
                  (if (<= i 0)
                      nil
                      (progn
                        ,@body
                        (nt (decf i))))))
         (nt ,g)))))

Define a macro n-of that takes…

5.Define a macro n-of that takes a number n and an expression, and returns a list of n successive values returned by the expression:

;; > (let ((i 0) (n 4))
;;     (n-of n (incf i)))
;; ==>( 1 2 3 4)

(defun fun-n-of (n expr)
  (if (<= n 0)
      nil
      (cons (+ n expr) (fun-n-of (decf n) expr))))

(defmacro n-of (n expr)
  (let ((c (gensym))
        (e (gensym)))
    `(let ((,c ,n)
           (,e ,expr))
       (labels ((nof (i ex)
                  (if (<= i 0)
                      nil
                      (cons (+ i ,e)
                            (nof (decf i) ex)))))
         (nof ,c ,e)))))

TODO Define a macro that takes…

6.Define a macro that takes a list of variables and a body of code, and ensures that the variables revert to their original values after the body of code is evaluated.

(defmacro rev-ori (lst body)
  ())

What’s wrong with the following definition of push?

(defmacro push (obj lst)
  `(setf ,lst (cons ,obj ,lst)))

Give an example of a call where it would not do the same thing as the real push.

(defmacro new-push (obj lst)
  (let ((o (gensym)))
    `(let ((,o ,obj))
       (setf ,lst (cons ,o ,lst)))))

Define a macro that doubles its argument:

> (let ((x 1)))
    (double x)
    x)
2

(defmacro double (x)
  (let ((n (gensym)))
    `(let ((,n ,x))
       (* ,n 2))))

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