Re: setq failure following list manipulation

[Kevin Rodgers]

Bill Lytton wrote:
> bug or feature?:
>
> quoted arg for setq in function call no longer works after 
maninpulating list
> using global variable
>
> (defun ae ()  (setq nums '(1 2 3)))
> (nconc nums (list 4))
> (ae) ;; returns (1 2 3 4)
> nums ;; returns (1 2 3 4)

I think you meant to call (ae) before (nconc nums (list 4)), since
otherwise nums would not be bound.  So, after (ae) nums is bound to
this cons cell structure:

+-+-+  +-+-+  +-+-+
| | +->| | +->| |*|
+++-+  +++-+  +++-+
 |      |      |
 v      v      v
 1      2      3

and after (nconc nums (list 4)) the last cons cell is modified so that
the structure (to which nums is still bound) is this:

+-+-+  +-+-+  +-+-+  +-+-+
| | +->| | +->| | +->| |*|
+++-+  +++-+  +++-+  +++-+
 |      |      |      |
 v      v      v      v
 1      2      3      4

--
Kevin Rodgers
Sr. Software Engineer, IHS