Why is SBCL complaining about setf?

Question

In SBCL, this will assign 'bar to foo, with warnings:

* (setf foo 'bar)
; in: SETF FOO
;     (SETF FOO 'BAR)
; ==>
;   (SETQ FOO 'BAR)
; 
; caught WARNING:
;   undefined variable: COMMON-LISP-USER::FOO
; 
; compilation unit finished
;   Undefined variable:
;     FOO
;   caught 1 WARNING condition
BAR
*

The following is from Touretzky. I typed this into "7.29.lisp" and saved.

(setf database
’((b1 shape brick)
(b1 color green)
(b1 size small)
(b1 supported-by b2)
(b1 supported-by b3)
(b2 shape brick)
(b2 color red)
(b2 size small)
(b2 supports b1)
(b2 left-of b3)
(b3 shape brick)
(b3 color red)
(b3 size small)
(b3 supports b1)
(b3 right-of b2)
(b4 shape pyramid)
(b4 color blue)
(b4 size large)
(b4 supported-by b5)
(b5 shape cube)
(b5 color green)
(b5 size large)
(b5 supports b4)
(b6 shape brick)
(b6 color purple)
(b6 size large)))

Whereupon:

* (load "7.29.lisp")
While evaluating the form starting at line 1, column 0
  of #P"/home/redacted/7.29.lisp":

debugger invoked on a SIMPLE-ERROR in thread
#<THREAD "main thread" RUNNING {10005D05B3}>:
  odd number of args to SETF: (SETF DATABASE ’
                                    ((B1 SHAPE BRICK) (B1 COLOR GREEN)
                                     (B1 SIZE SMALL) (B1 SUPPORTED-BY B2)
                                     (B1 SUPPORTED-BY B3) (B2 SHAPE BRICK)
                                     (B2 COLOR RED) (B2 SIZE SMALL)
                                     (B2 SUPPORTS B1) (B2 LEFT-OF B3)
                                     (B3 SHAPE BRICK) (B3 COLOR RED) ...))

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

restarts (invokable by number or by possibly-abbreviated name):
  0: [RETRY   ] Retry EVAL of current toplevel form.
  1: [CONTINUE] Ignore error and continue loading file "/home/redacted/7.29.lisp".
  2: [ABORT   ] Abort loading file "/home/redacted/7.29.lisp".
  3:            Exit debugger, returning to top level.

(SB-C::EXPLODE-SETQ (SETF DATABASE ’ ((B1 SHAPE BRICK) (B1 COLOR GREEN) (B1 SIZE SMALL) (B1 SUPPORTED-BY B2) (B1 SUPPORTED-BY B3) (B2 SHAPE BRICK) (B2 COLOR RED) (B2 SIZE SMALL) (B2 SUPPORTS B1) (B2 LEFT-OF B3) (B3 SHAPE BRICK) (B3 COLOR RED) ...)) ERROR)
0]

What do I need to understand to make SBCL happy with these assignments?

Phoebe Goldman Phoebe Goldman · Accepted Answer · 2020-04-29T00:20:13

that first warning seems pretty clear to me, tbh. in (setf foo 'bar), foo is not known to refer to a variable. it hasn't been locally bound as a lexical variable by a form like lambda or let, and it hasn't been globally bound as a special variable by a form like defvar or defparameter. if you want this to appear as a top-level form, you should write (defparameter *foo* 'bar) (note that common lisp convention is to surround the names of special variables in earmuffs). if you want to do this within the body of a function or for a finite scope, you should do (let ((foo 'bar)) (do-stuff-with foo)).

as to your second problem, in addition to the fact that database is unbound, you have the wrong quote character. ’ is not an ascii single-quote, and as such, is being parsed as a symbol. the compiler sees your form as: (setf database |’| ((b1 shape green) ...)), which is not a valid setf form. to fix this, figure out how to type the character ', and/or don't copy-paste code from formatted documents into plain text files.

Why is SBCL complaining about setf?

2 Answers

defvar, defparameter

let, let*

setf