为什么 SBCL 抱怨 setf？

Question

在 SBCL 中，这将分配'bar给foo，并带有警告：

* (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
* 

以下来自图雷茨基。我将其输入“7.29.lisp”并保存。

(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)))

于是：

* (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] 

我需要了解什么才能让 SBCL 对这些作业感到满意？

Answer 1

第一个警告对我来说似乎很清楚，tbh。in (setf foo 'bar),foo不知道是指变量。它没有通过像形式被本地绑定作为词法变量lambda或let，并且它没有被全局约束如通过像形式的特殊变量defvar或defparameter。如果你想让它作为顶级形式出现，你应该写(defparameter *foo* 'bar)（注意，常见的 lisp 约定是在耳罩中包围特殊变量的名称）。如果您想在函数体内或有限范围内执行此操作，则应该执行(let ((foo 'bar)) (do-stuff-with foo)).

至于您的第二个问题，除了database未绑定的事实之外，您还有错误的引号字符。’不是 ascii 单引号，因此被解析为符号。编译器将您的表单视为: (setf database |’| ((b1 shape green) ...))，这不是有效的setf表单。要解决此问题，请弄清楚如何键入字符'，和/或不要将格式化文档中的代码复制粘贴到纯文本文件中。