(in-package :alexandria)

(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))))