alice-teacher Alice 2.3 bug with deleting an object on a list

[Don Slater]

This is a known problem in Alice and there is no easy fix on our end, as it is more of a programmer error… Before deleting an object from the Alice world, a student would go through the code and delete all commands that are related to that object. This is also true for lists. The list element should be removed or set to a different object value, because once an object is deleted, Alice has no easy way to backtrack through all the code and find every place that object was moved. In fact, Alice has no way of knowing which element points to which object. 

The pointer from the list position to the object is a one-way path. The list knows that its 3rd element may be pointing to the dragon, but the dragon does not know that it is being pointed to by the 3rd element in the list. And so when the dragon is deleted, it cannot tell the list that its 3rd element should not point to it anymore.

Normally, when an object is deleted from an Alice world that is still being used in some way, a dialog box appears. listing all the places that object is being used. But the user is given the option to have all those references set to none, with the understanding that the user is taking responsibility for finding those <none> references, which will appear in red, and deleting the command, or changing the object using the command, from dragon to knight, for example.

I may be wrong, but I do not know that Alice can go back and set the list reference to <none> in red. And from your description, it sounds like Alice freezes up even before the user has a chance to go back to the list and reset the element. That is a problem that we can look at.

But students should be warned to be very careful about deleting objects from a world, without first being sure of all the possible side-effects.

All the best,
Don Slater

On Sep 26, 2012, at 2:50 PM, Steven Huss-Lederman <huss at beloit.edu> wrote:

> Hi,
> 
> Just in case others run into the same issue and to make sure it isn't something I'm doing wrong, I'll report a problem with Alice 2.3 that has caused issues for several students.  If you have an object on a list and you delete the object then Alice has an unknown error that won't go away (even after you say ok).  The only way we have found to get rid of it is to terminate the Alice process (force kill) in which you lose any unsaved work.  This happens on Mac OS 10.6.  I have not taken the time to test on Windows 7 or Alice 2.2 (to see if it is an old issue).  I can send in a world to demonstrate but simply put an object in a world, create a world Object list, add the object, save the list, then delete the item.
> 
> Steve
> 
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