Question

I wonder, how can I print in LISP each new value from loop “for” in new list, which each time creates by calling the function.

I have created the func:

(defun make (id name surname) (list :id id :name name :surname surname) )

Here I created the global variable:

(defvar *db* nil)

And here I defined the func for adding each new value to store it in db:

(defun add (cd) (push cd *db*))

So, I’m able to add each new data to db, like that:

(add (make 0 "Oleg" "Orlov" ) )

To look the content of my db , I can use:

*db*

So, I wonder how to put each new record-list to db using “for” loop, I print values in “for” loop in lisp like this:

 (loop for i from 1 to 10 do ( ... ))

If , I use:

 (loop for i from 0 to 10 do (add (make i "Oleg" "Orlov") ) )

If you read db using *db* you will see, that all evelen records were added, but after calling the last line, you will get NIL result in return.

Why do I catch NIL result, not T and what does it mean?

Thanks, best regards!

Answer 1

Every form in Lisp evaluates to something.

If a form you type in doesn’t return a value, it will evaluate to NIL by default (otherwise, it evaluates to the value(s) it returns). Your loop doesn’t actually return a value itself; it just performs 10 assignments (each of the intermediate expressions do return a value, but you don’t collect and return them). Therefore, that code will return NIL.

If you haven’t done so already, check out chapter 3 of Practical Common Lisp, in which Peter Seibel goes step-by-step through creating a simple database. It might give you some insights into the basics of how Lisp works. The specific question you ask (why forms return NIL by default, and what it means specifically in the context of Common Lisp) is answered in chapter 2 of the same book

As for how you would explicitly cause your loop to emit the list of items it added to *db*, try the following

(loop for i from 1 to 10
      for elem = (make i "Oleg" "Orlov")
      do (add elem)
      collect elem)