Re: LOGO-L> UNDO -- third try

[Michael Tempel <michaelt@media.mit.edu>]

>Date: Thu, 16 Apr 1998 12:08:20 +0100
>To: bh@anarres.CS.Berkeley.EDU, logo-list@gsn.org
>From: Michael Tempel <michaelt@media.mit.edu>
>Subject: Re: LOGO-L> UNDO -- third try
>Cc:
>Bcc:
>X-Attachments:
>
>At 4:17 AM -0700 4/16/98, bh@anarres.CS.Berkeley.EDU (Brian Harvey) wrote:
>>
>>"Charles T. Smith" <mrchuc@math.fan.org> writes:
>>>I discussed this feature a couple of years ago (before I had had a chance
>>>to use it much) with Michael Tempel. He pointed out then that it could be
>>>implemented in Logo easily by keeping all the commands in a Logo list and
>>>then butlast-ing the list and running it again.
>>
>>No, I don't think that's true.  It's true if all you do is draw lines
>>on the screen, but some things change the state of the world in a way
>>that's not so easy to back up.
>>
>
>Even drawing lines is problematic. A long time ago I wrote something like
>this in Apple Logo called TEACH.  There were all kinds of ways in which
>people got themselves into trouble.  What if you gave a PENUP command
>followed by UNDO? The PENUP command is removed from the instruction list,
>but the pen is still up.  When the figure is redrawn nothing gets drawn.
>Changing pen colors didn't undo either.
>
>The most interesting mess I remember was when people tried to define a
>procedure while TEACH was running.  Typing something like TO SQUARE
>appeared to work, but that instruction ended up as part of another
>procedure since TEACH was using DEFINE and TEXT to create a procedure as
>it went along. Using UNDO later on got the message SQUARE IS ALREADY
>DEFINED IN FOO.  Hmmm....
>
>Which is why I tend to agree with Brian more than Chuck:
>
>>Brian Harvey wrote:
>>
>>>  I'll think about a history recording feature; that sounds doable.
>>>  But of course if the recorded history does include state changes,
>>>  we run the risk of a new mystery for the user not to understand,
>>>  when the effect of the replaying is not the same as the effect of
>>>  the original run.
>>
>>I'm not sure why you include "risk" and "mystery" in the same sentence.
>>Personally, I view it as a stimulating thinking activity.  Note that it's
>>the same "mystery" as when you re-run a program and it does not work the
>>same as before.  I had a most pleasant experience teaching kids to use the
>>random function (with a "randomized" seed) and seeing their faces as they
>>learn how to intentionally make their program become unrepeatable.
>>
>>Chuck
>
>Mystery is great, but I think we should be playing with mysteries within a
>consistent and reliable software environment.  That's the case when using
>RANDOM.  It wasn't always the case with my TEACH program.
>
>-Michael
>
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--------------------------------------------------------
Michael Tempel                    tel: 212 579 8028
Logo Foundation                   fax: 212 579 8013
250 West 85th Street              michaelt@media.mit.edu
New York  NY 10024
http://el.www.media.mit.edu/logo-foundation/


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