/usr/share/common-lisp/source/alexandria/definitions.lisp is in cl-alexandria 20140826-1.
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(defun %reevaluate-constant (name value test)
(if (not (boundp name))
value
(let ((old (symbol-value name))
(new value))
(if (not (constantp name))
(prog1 new
(cerror "Try to redefine the variable as a constant."
"~@<~S is an already bound non-constant variable ~
whose value is ~S.~:@>" name old))
(if (funcall test old new)
old
(restart-case
(error "~@<~S is an already defined constant whose value ~
~S is not equal to the provided initial value ~S ~
under ~S.~:@>" name old new test)
(ignore ()
:report "Retain the current value."
old)
(continue ()
:report "Try to redefine the constant."
new)))))))
(defmacro define-constant (name initial-value &key (test ''eql) documentation)
"Ensures that the global variable named by NAME is a constant with a value
that is equal under TEST to the result of evaluating INITIAL-VALUE. TEST is a
/function designator/ that defaults to EQL. If DOCUMENTATION is given, it
becomes the documentation string of the constant.
Signals an error if NAME is already a bound non-constant variable.
Signals an error if NAME is already a constant variable whose value is not
equal under TEST to result of evaluating INITIAL-VALUE."
`(defconstant ,name (%reevaluate-constant ',name ,initial-value ,test)
,@(when documentation `(,documentation))))
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