[etoys-dev] Slots without watchers

Ricardo Moran richi.moran at gmail.com
Mon Sep 28 20:40:07 EDT 2009

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
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.


> Subbu
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