Question

Im learning lisp and im pretty new at this so i was wondering…

if i do this:

(defparameter *list-1* (list 1 2)) (defparameter *list-2* (list 2 3)) (defparameter *list-3* (append *list-1* *list-2*)) 

And then

(setf (first *list-2*) 1) *list-3* 

I will get (1 2 1 4)

I know this is because the append is going to ‘save resources’ and create a new list for the first chunk, but will actually just point to the second chunk, coz if i do:

(setf (first *list-1*) 0) *list-3* 

I will get (1 2 1 4) instade of the more logical (0 2 1 4)

So my question is, what other cases are like this in lisp, how do you black belt lispers know how to deal with this stuff that is not intuitive or consistent?

Answer 1

One defensive tactic is to avoid sharing structure.

(defparameter *list-3* (append *list-1* *list-2* '())) 

or

(defparameter *list-3* (append *list-1* (copy-list *list-2*))) 

Now the structure of the new *list-3* is all new, and modifications to *list-3* won’t affect *list-2* and vice versa.