1 files changed, 37 insertions, 0 deletions
diff --git a/definitions.lisp b/definitions.lisp
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+(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))))