X-Git-Url: https://www.chiark.greenend.org.uk/ucgi/~mdw/git/sod/blobdiff_plain/4b8e5c0347115ff30841f1d1e71afe59ecb6c82c..23e44cba8fe33e8cdde22d1ae14d78d992b9f8bb:/src/utilities.lisp

diff --git a/src/utilities.lisp b/src/utilities.lisp
index 099c4ba..98d314a 100644
--- a/src/utilities.lisp
+++ b/src/utilities.lisp
@@ -713,6 +713,18 @@      (defun ,from (object)
 	    `((defun (setf ,from) (value object)
 		(setf (,to object) value))))))
 
+(export 'define-on-demand-slot)
+(defmacro define-on-demand-slot (class slot (instance) &body body)
+  "Defines a slot which computes its initial value on demand.
+
+   Sets up the named SLOT of CLASS to establish its value as the implicit
+   progn BODY, by defining an appropriate method on `slot-unbound'."
+  (with-gensyms (classvar slotvar)
+    `(defmethod slot-unbound
+	 (,classvar (,instance ,class) (,slotvar (eql ',slot)))
+       (declare (ignore ,classvar))
+       (setf (slot-value ,instance ',slot) (progn ,@body)))))
+
 ;;;--------------------------------------------------------------------------
 ;;; CLOS hacking.