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Remark on closing the lernel

• To: mathgroup at smc.vnet.net
• Subject: [mg122127] Remark on closing the lernel
• From: Fred Simons <f.h.simons at tue.nl>
• Date: Sun, 16 Oct 2011 07:04:37 -0400 (EDT)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com

The following is a highly irrelevant and completely unimportant report 
on a difference between closing the kernel by evaluation of the command 
Quit[] or by the frontend using the evaluation menu. The reason for 
nevertheless publishing it, is that there might be others like me, who 
consider such differences as interesting.

Evaluation of a cell with the next two lines

Dynamic[b]
Dynamic[b=RandomReal[], UpdateInterval->1]

results in two dynamic output cells, both showing the current value of 
the kernel variable b. Every second the displayed value changes.

When we quit the kernel by evaluating a cell with the command Quit[], 
then both output cells still show the same value. But when we quit the 
kernel by pressing alt-v-q, the two output cells display differently: 
the first one shows the name of the variable, b, and the second one a 
number.

The following is maybe an incorrect, but anyway incomplete explanation. 
Any comment is welcome.

Roughly, the updating mechanism is as follows. When the kernel changes 
the value of the variable b, it sends a message to the frontend that the 
value changed. Having received this message, the frontend will update 
the corresponding cells by asking the kernel for the current value of b 
and then adapting the display of these cells.

When we close the kernel by the sending the command Quit[] through the 
frontend to the kernel, then the kernel will close, but the frontend is 
unaware of that. The frontend does not receive any message on the 
variable b, so the display is unchanged.

It seems that for identification of the kernel the frontend uses 
something like $ProcessID, for when we restart the kernel e.g. by giving 
the command b=13, the new kernel has a different $ProcessID and nothing 
happens with the two dynamic output cells above.

Now evaluate the above input cell once again, and then close the kernel 
with alt-v-q or similarly. This is a different situation. The frontend 
now knows that the kernel will close. So it not only closes the kernel, 
but also removes the connection between the frontend variable 
$CellContext`b and the kernel variable b. Therefore, in the first output 
cell the name b turns up. In the second output cell the value for that 
$CellContext`b is maintained.

When we restart the kernel by giving the command b=13, then in the first 
output cell the frontend restores the connection between the variable b 
and the new kernel; the value 13 is displayed. But for some or other 
reason this does not happen in the second output cell. It might be that 
the Set command in that cell refers to the function RandomReal in the 
previous kernel, that cannot be called any more, so that a new 
assignment is impossible.

Fred Simons
Eindhoven University of Technology