[Sbcl-devel] result of setf on (car (nth ( with make-list unexpected for newbie

[Oliver K. <ng...@gm...>]

Hello Lispers,

day 3 of my selflearning Lisp course I've started some coding
and BANG - I'm lost.

Changing an element of a sublist as expected:

* (let ((l1 (list (list 1 1) (list 1 1))))
		(setf (car (nth 1 l1)) 2)
		l1)

=> ((1 1) (2 1))

Changing an element of a sublist created with make-list is unexpected
to me:

* (let ((l1 (make-list 2 :initial-element (list 1 1))))
	 (setf (car (nth 1 l1)) 2)
	 l1)

=> ((2 1) (2 1))

Neither in any tutorial nor in Hyperspec nor in newsgroups I could
find an explanation for this different behavior.

As told in HyperSpec make-list[1] "Returns a list of length given by
size, *each* of the elements of which is initial-element." it seems
that the initial-element is referenced by all list entries. But that's
not true as seen by changing a level 1 element:

* (let ((l1 (make-list 2 :initial-element (list 1 1))))
	 (setf (nth 1 l1) '(1 2))
	 l1)

=> ((1 1) (1 2))

Next I thought setf is not capable of using nested structure
addressing. But that's not true either as seen in the first example.

I've read that make-list uses Cdr-Coding[2], but that only seems to be
for storage efficiency and should not change any functional behavior.

BTW: Everything seems to be Common-Lisp conform as clisp shows the same
results. It's just me not knowing why.

Could please anybody enlight me about what's causing the different
resluts.

  Thanks and Regards,

  Oliver

[1] http://www.lispworks.com/documentation/HyperSpec/Body/f_mk_lis.htm

[2] http://common-lisp.net/project/bknr/static/lmman/fd-con.xml#cdr-code