Hi, all.  A question arose recently on the Squeak-dev mailing list (a list mostly inhabited by software engineers) concerning "recursion in etoys."  I wrote a reply, which I'm herewith forwarding to the Squeakland list, on the chance that this information might be useful to some readers.  BTW my reply was suppressed by the moderators of the Squeak-dev list, who said it was too large, so it never did actually appear on that list  ;-)

  -- Scott

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Hi, Kevin,

Yes, etoys do support recursion.  But they don't have a good way of detecting and recovering from *runaway* recursion, which is what was befalling your script.

Here's a recursive etoy script which does what you want, and which *does* converge, provided that there is a patch of the appropriate blue somewhere ahead of the Star:


Two points to note here:

(1)  The critical difference between this and what you did is that after the *copy* is made, this script "includes" that copy in the container.  Without the "include," the copy isn't present in the "world", so neither it nor any future generation of "copy" will be "over blue" and hence terminate the recursion.  That's why your recursion ran away.

(2)  In the last line, it's *okay* simply to have "Star's nextGuy approachBlue".  The value of the "do" inserted here is that this defers the recursive #approachBlue script call until the next cycle of the simulation.  The result will be there will be a sense of "movement" since each generation will be born on a different tick, whereas without the "do" all the generations will be born within the same "tick" so all will appear instantaneously -- a much less nice effect.

Cheers,

Scott


PS:  Please check out Squeakland.org, and you may well want to subscribe to the Squeakland mailing list:
        http://www.squeakland.org/mailman/listinfo/squeakland


 
At 10:08 AM -0800 3/25/05, Kevin Lawrence wrote:
Is it appropriate to post questions about etoys to this list ?

[I'll risk it this time - let me know if there is somewhere else I should be asking, thanks.]


I tried making a simple etoys script with recursion and it hangs as soon as I click the run icon.

the script was essentially (excuse my Java-ish pseudocode) ...


spawn script
 test self.sees blue

 yes

 no
  self.newCopy = self.copy
  self.newCopy.forwardBy self.length
  self.newCopy.spawn


Assuming I haven't made a really silly *mistake, should this work ?

Is recursion supported ?

(I searched the bug database but couldn't find anything)


Thanks !

Kevin

* Please tell me if there is a silly mistake !