[plt-scheme] Re: [plt-edu] the universe teachpack

From: Stephen Bloch (sbloch at adelphi.edu)
Date: Wed Mar 25 08:40:13 EDT 2009

Matthias Felleisen <matthias at ccs.neu.edu> wrote:
>> ... Could we please have end-of-time back, in addition to stop-when?
> No, it violates the idea that BSL should be just 8th grade school  
> mathematics. (I don't want to introduce effects this way.)


It seems to me that the state hidden inside a world animation, and the  
idea of "exiting" an otherwise-perpetual computation, have already  
left 8th-grade algebra behind.  But this isn't a big deal; it's just a  

>> 2) What do we gain (pedagogically) by insisting on the distinction  
>> between scenes and images?
> This will disappear when Robby revises images.ss.

Which I've been hearing about for several years.  Is there an ETA on  

>> It's not clear from the documentation what happens in universe if  
>> you call big-bang with no redraw handler;
> It doesn't draw :-) And it's a one-liner to test: (big-bang 1 (on- 
> tick add1)) is an infinite loop

.. indistinguishable from lots of other infinite loops that produce  
no visible feedback.  That's not much of a test.

>> I guess I could give absolute-beginning students the boilerplate  
>> code "(on-draw identity)"
> The state of the world should not be a scene so using identity would  
> not create a scene.

The state of the world can be any data type I choose.  In my textbook,  
the first data type students see is image, and it's very natural to  
write animations in which the model is an image.  Later on, I  
introduce animations with numbers, strings, structs, lists, etc as  


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