[plt-scheme] The perfect teaching language--Is this too much to ask for?

From: Matthias Felleisen (matthias at ccs.neu.edu)
Date: Mon Jun 15 10:07:31 EDT 2009

Thanks. This has been on my todo list. I considered adding

  (world? ...)

to big-bang descriptions and

  (universe? ...)

to universe descriptions. -- Matthias

On Jun 15, 2009, at 9:52 AM, Stephen Bloch wrote:

> On Jun 13, 2009, at 7:04 PM, Todd O'Bryan wrote:
>> Have you never had a student try to return a number instead of a  
>> posn?
>> Or (much more common and much more painful) in the chapters on binary
>> trees and family trees, students get confused about the kinds of
>> structures they're building and bury a mistake so deep in a structure
>> that by the time they get the error message it's not clear where they
>> painted themselves into a corner. These are the problems that they
>> spend ten or fifteen minutes staring at, and when I get there, I know
>> it's a type problem, but there are three or four functions, all with
>> working test cases that could have triggered the problem, and I have
>> to spend 5 minutes unwinding everything with them.
> One of the things I've run into frequently in student programs is  
> event handlers (for world.ss) that return a different type of  
> "world" than they took in.  This doesn't generally produce an error  
> message until the NEXT event, at which point the student gets an  
> error message from a different handler that may be perfectly correct.
> Here's a suggestion: add to world (now universe) an optional  
> "legal" handler (defaulting to (lambda (x) #t) ) that is applied to  
> the model returned by every handler, before doing anything else  
> with that model.  If it returns false, the animation ends with an  
> error message saying "Invalid world returned by ___ handler" (fill  
> in the blank with "mouse", "tick", or "key").
> For example,
> ; The model for this animation is a non-negative number.
> (legal-when (lambda (r) (and (number? r) (>= r 0)))
> ; End the animation when the model exceeds 300.
> (stop-when (lambda (num) (> num 300)))
> ; Tick handler: add1
> (on-tick add1)
> ; Redraw handler:
> (on-redraw (lambda (r) (circle r "solid" "blue")))
> This would not only make it easier for students to find their bugs,  
> but give them some actual code to write at the "choose a type for  
> the model" step of the animation recipe.  Students have a tendency  
> to skip steps that don't produce code.
> Of course, enforced contracts would take care of the issue too, but  
> I think we'll have to wait longer for them; this addition to  
> "universe" could be in the next release.
> Stephen Bloch
> sbloch at adelphi.edu
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