Topic: Nil

[David Bethel]

I always thought they bound nil to an empty list is so that it would not error out on certain calls

You would think that this would produce an error:

Code: [Select]

(setq ed nil)
(cdr (assoc 10 ed))

But it doesn't because (assoc) is searching a list.  There are several other examples of this behavior.  -David

[gile]

I always thought they bound nil to an empty list is so that it would not error out on certain calls

As LISP in a dynamic typed language, nil type checking is performed at run-time and I think it is bound according to the context.

[Lee Mac]

Some good explanations and examples gile. 

[Jeff H]

Thanks gile!!

[AWW]

I wanted to re-visit this NIL thread again while analyzing this code here:

Code: [Select]

(DleaderWhile
;DleaderWhile subfunction call
   nil
;empty list
   (vlax-ename->vla-object (car DQobject))
;(retrieve first element of DQobject *list*) (transform DQobject entity to vla-object)
   nil
;empty list
)

Notice I've added some comments for what I think is going on. So is this just setting the list empty? To be filled later within the program? Or to preserve memory?

[Lee Mac]

Perhaps those arguments are boolean flags requiring a T/nil value, it would depend how those arguments are used in the function.