-;;; Some symbols are defined by ANSI to be self-evaluating. Return
-;;; non-NIL for such symbols (and make the non-NIL value a traditional
-;;; message, for use in contexts where the user asks us to change such
-;;; a symbol).
-(defun symbol-self-evaluating-p (symbol)
- (declare (type symbol symbol))
- (cond ((eq symbol t)
- "Veritas aeterna. (can't change T)")
- ((eq symbol nil)
- "Nihil ex nihil. (can't change NIL)")
- ((keywordp symbol)
- "Keyword values can't be changed.")
- (t
- nil)))
-
-;;; This function is to be called just before a change which would
-;;; affect the symbol value. (We don't absolutely have to call this
-;;; function before such changes, since such changes are given as
-;;; undefined behavior. In particular, we don't if the runtime cost
-;;; would be annoying. But otherwise it's nice to do so.)
-(defun about-to-modify-symbol-value (symbol)
- (declare (type symbol symbol))
- (let ((reason (symbol-self-evaluating-p symbol)))
- (when reason
- (error reason)))
- ;; (Note: Just because a value is CONSTANTP is not a good enough
- ;; reason to complain here, because we want DEFCONSTANT to be able
- ;; to use this function, and it's legal to DEFCONSTANT a constant as
- ;; long as the new value is EQL to the old value.)
+;;; This function is to be called just before a change which would affect the
+;;; symbol value. We don't absolutely have to call this function before such
+;;; changes, since such changes to constants are given as undefined behavior,
+;;; it's nice to do so. To circumvent this you need code like this:
+;;;
+;;; (defvar foo)
+;;; (defun set-foo (x) (setq foo x))
+;;; (defconstant foo 42)
+;;; (set-foo 13)
+;;; foo => 13, (constantp 'foo) => t
+;;;
+;;; ...in which case you frankly deserve to lose.
+(defun about-to-modify-symbol-value (symbol action)
+ (declare (symbol symbol))
+ (multiple-value-bind (what continue)
+ (when (eq :constant (info :variable :kind symbol))
+ (cond ((eq symbol t)
+ (values "Veritas aeterna. (can't ~@?)" nil))
+ ((eq symbol nil)
+ (values "Nihil ex nihil. (can't ~@?)" nil))
+ ((keywordp symbol)
+ (values "Can't ~@?." nil))
+ (t
+ (values "Constant modification: attempt to ~@?." t))))
+ (when what
+ (if continue
+ (cerror "Modify the constant." what action symbol)
+ (error what action symbol))))