Always getting "RETURN-FROM: no block named NIL is currently visible"

Question

(defun take-n (lst i)
  (setf newlst '())    
  (dotimes (n i)
    (setf newlst (cons (car lst) newlst))
    (print (cons (car lst) newlst))
    (setf lst (cdr lst)))
  (return newlst))
(print (take-n '(1 2 3) 2))

This gives me an error RETURN-FROM: no block named NIL is currently visible. I have tried moving the return statement around but I am not sure what it means.

Answer 1

1. Please indent your code in a Lisp way (this can be done automatically in many editors, like Emacs):

   (defun take-n (lst i)
     (setf newlst '())
     (dotimes (n i)
       (setf newlst (cons (car lst) newlst))
       (print (cons (car lst) newlst))
       (setf lst (cdr lst)))
     (return newlst))
   
   (print (take-n '(1 2 3) 2))
   

2. Do not setf a variable that was not declared beforehand: (setf newlst '()) here refers to a hypothetical global variable newlst, but in a function you should strive to only have local state. You introduce variables in a block using let, as follows:

   (let ((new-list ()))
     ...
     ;; here you can setf new-list if you need
     ...)
   

3. (setf newlst (cons (car lst) newlst)) can also be written (push (car lst) newlst), but please do not use overly abbreviated names; you can use list and new-list, for example.

4. return makes a return to the enclosing block named nil, but here you have no such block. Instead, defun introduces an implicit block named like the function you define, i.e. you have implicitly:

   (block take-n
     ...)
   

   So if you wanted to return from it, you would need to do (return-from take-n newlst).

5. BUT, you do not need to return since the last form being evaluated in the function is the value associated with the function call anyway.