[example] adde note about dropping ghouls while iterating over them

1 files changed, 8 insertions, 1 deletions

diff --git a/examples/09-ghoulspree.lisp b/examples/09-ghoulspree.lisp
index 03552f0..a11e6a6 100644
--- a/examples/09-ghoulspree.lisp
+++ b/examples/09-ghoulspree.lisp
@@ -102,7 +102,14 @@ on which boundary VAL is outside of."
               when gravity
                 do (apply-gravity-to ghoul accelleration)
               when (out-of-bounds-p ghoul)
-                do (ww:drop-unit ghoul)))))
+                ;; note, it is generally a BadIdea™ to delete
+                ;; something from an array that you are iterating
+                ;; over. It only works as expected here b/c of
+                ;; implicit knowledge about the imlementation of the
+                ;; scene container. A safer way in most cases would be
+                ;; to `collect ghoul into drop-list` and then deleted
+                ;; all the collected ghouls in a `finally` clause.
+                do (ww:drop-unit ghoul))))) 
 
 (defun random-sign ()
   (if (zerop (random 2)) -1 1))