Error with (pop (nthcdr n l))

[Scott Brown]

I am using SBCL 1.0.57.0.debian on Ubuntu. It also fails on SBCL 1.0.55.0-abb03f9 on OS X 10.7.

> (setf x '(1 2 3 4 5 6))
( 1 2 3 4 5 6)
> (cdddr x)
(4 5 6)
> (nthcdr 3 x)
(4 5 6)
> (pop (cdddr x))
4
> (setf x '(1 2 3 4 5 6))
( 1 2 3 4 5 6)
;;; This should work the same as (pop (cdddr x))
> (pop (nthcdr 3 x))

; in: POP (NTHCDR 3 X)
; (FUNCALL #'(SETF NTHCDR) #:NEW1009 3 #:X1010)
; ==>
; (SB-C::%FUNCALL #'(SETF NTHCDR) #:NEW1009 3 #:X1010)
;
; caught WARNING:
; The function (SETF NTHCDR) is undefined, and its name is reserved by ANSI CL so
; that even if it were defined later, the code doing so would not be portable.

; (LET* ((#:X1010 X) (#:G1011 (NTHCDR 3 #:X1010)) (#:NEW1009 (CDR #:G1011)))
; (FUNCALL #'(SETF NTHCDR) #:NEW1009 3 #:X1010)
; (CAR #:G1011))
;
; caught WARNING:
; undefined variable: X
;
; compilation unit finished
; Undefined function:
; (SETF NTHCDR)
; Undefined variable:
; X
; caught 2 WARNING conditions

debugger invoked on a UNDEFINED-FUNCTION in thread
#<THREAD "main thread" RUNNING {10036D8BC3}>:
  The function (COMMON-LISP:SETF COMMON-LISP:NTHCDR) is undefined.

Type HELP for debugger help, or (SB-EXT:QUIT) to exit from SBCL.

restarts (invokable by number or by possibly-abbreviated name):
  0: [ABORT] Exit debugger, returning to top level.

("undefined function")
0]

[Paul Khuong (pvk)]

nthcdr is a function, not an accessor. While an implementation is free to define a SETF expansion for such a function, the standard does not demand it. Moreover, it's not clear to me what would happen in cases like (setf (nthcdr 3 variable-bound-to-nil) 42).

[Stas Boukarev (stassats)]

If you want to use setf on nthcdr, use (setf (car (nthcdr ...))) or (setf (cdr (nthcdr ...))).