[etoys-dev] Slots without watchers

Rita Freudenberg rita at isg.cs.uni-magdeburg.de
Tue Sep 29 05:01:01 EDT 2009

Scott Wallace wrote:
> There's no deep reason why "slots with arguments" don't have readouts 
> in the Viewer, nor why stand-alone Watchers aren't available for such 
> slots.  Both would be desirable enhancements.
> The side-effect concern afaik would only apply to the "Car's copy" 
> tile; we can't be calling this repeatedly, since each call creates a 
> new copy.  But with that exception (already strange in other ways,) it 
> would be desirable to have readouts for all slot-like entities, 
> whether or not they involve parameters.
> Performance considerations may make us ultimately decide to refrain 
> from showing readouts in viewers for certain slots, but even for those 
> slots involving "expensive" computations, I think ideally we should 
> still allow the user to obtain stand-alone watchers for them.
I'm not sure if I understand how these things work, but I just had an 
idea. Would it be possible to write a description about how a user can 
add a watcher to a given tile? Using Squeak, class browser , writing 
Smalltalk code? Could that be one path to learn how to program for 
interested high school students?

>   -- Scott
> On Sep 28, 2009, at 5:40 PM, Ricardo Moran wrote:
>> On Mon, Sep 28, 2009 at 8:17 PM, K. K. Subramaniam 
>> <subbukk at gmail.com> wrote:
>> On Tuesday 29 Sep 2009 1:36:53 am Ricardo Moran wrote:
>> > I don't see why a function shouldn't have a readout. It returns a 
>> value,
>> > and IMHO all useful values should have readouts. This includes 
>> bearingTo,
>> > distanceTo, isOverColor, colorSees, overlaps, overlapsAny, and so 
>> on. I
>> > know some of them can't have readouts because of performance issues 
>> but if
>> > bearingTo is not the case I think it should have one.
>> I expect watchers are only for variables/properties.
>> I'm sorry but I disagree. I expect watchers for anything that will 
>> return a value.
>> When I ask for the distance to an object I care about the value, so I 
>> expect to have a watcher. When I ask an object to turn 90 degrees I 
>> only care about the movement of the object, so I don't expect to have 
>> a watcher.
>> With computations, one is
>> not sure about side effects.
>> I also don't think any of the functions I mentioned above should have 
>> side effects, so the only reason I found for not having readouts is 
>> performance.
>> One can always create a variable and use a script
>> to track the changes through this variable.
>>  I know it's easy to use a variable to track the changes but I don't 
>> think this is what I would expect.
>> There is a bigger issue that will confuse young kids. The viewer has 
>> three
>> types of slots - commands, variables and computations but they map to 
>> only two
>> types of menu buttons. The "observation" category also includes 
>> variables with
>> watchers, so not all relationships are computed :-(.
>> Maybe I'm wrong but I don't make the distinctions you made (and I 
>> don't think children do it either). When I work with Etoys I think 
>> about variables and commands. Nothing more. Clearly, "turn" is a 
>> command. And the distance to an object, at least for me, is a variable.
>> How about using round edges or changing color of computed tiles?
>> I believe a lot of different tiles would be more confusing for children.
>> Cheers
>> Subbu
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Rita Freudenberg
Otto-von-Guericke-Universität Magdeburg

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