LISP dynamic functions?

Question

I'm trying to teach myself some LISP and while I understand most of it, I have trouble grasping the eval function. I know that it does it for us already and that it's not good to use (so I hear), but how would I make a function that just adds?

So far I was attempting/thinking

(setf input-prompt "Enter addition epression: ")
(setf output-prompt "The value is: ")

(defun prompt-for-input (msg)
  (format t msg))


(defun sum (expression)
  (format t "Summing ~d and ~d.~%" x y)
  (+ x y))


(defun add ()
  (prompt-for-input input-prompt)
  (let ((expression (read)))
       ((sum (expression)))
  (add)))

Not really sure where to go on this, any help is appreciated.

What are you trying to do? It sounds like you want to call different functions based on the expression you read, without using eval. This is basically writing a small interpreter, which is something covered in most Lisp books (Little Schemer, etc.). But it would help if you could describe your problem more clearly. — user725091
– user725091, Commented Apr 15, 2013 at 23:34

Rainer Joswig · Accepted Answer · 2013-04-15 18:46:23Z

2

(setf input-prompt "Enter addition expression: ")
(setf output-prompt "The value is: ")

(defun prompt-for-input (msg)
  (format t msg)
  (finish-output))

(defun sum (expression)
  (let ((x (second expression))
        (y (third expression)))
    (format t "~%Summing ~d and ~d.~%" x y)
    (+ x y)))

(defun add ()
  (prompt-for-input input-prompt)
  (sum (read)))

Run it:

CL-USER > (add)
Enter addition expression: (+ 1 2)
Summing 1 and 2.
3

edited Apr 15, 2013 at 18:46

answered Apr 15, 2013 at 16:28

Rainer Joswig

140k11 gold badges230 silver badges357 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Nogg Over a year ago

How does LISP go about figuring out what the 'second' and 'third' expression is?

Svante Over a year ago

(Second expression) is the second element of the list expression.

Nogg Over a year ago

Your code doesn't compile, but I got the gist of it. So if I wanted to create other functions like sum and change 'add' to be a general expression holder, would I check which function to send it to with subseq?

Rainer Joswig Over a year ago

@Nogg: why doesn't it compile? It compiles for me.

Collectives™ on Stack Overflow

LISP dynamic functions?

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related