[racket] I love TR

From: Arthur Nunes-Harwitt (anh at cs.rit.edu)
Date: Fri Sep 21 09:33:11 EDT 2012

Dear John,

   Although it's not a trivial change for other type systems, I believe 
that the natural numbers should be a base type.

   Thanks for the example.


Arthur Nunes-Harwitt
Computer Science Department, Rochester Institute of Technology
Room 70-3509

"I don't know what the language of the future will be
called, but it will look like LISP."

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On Thu, 20 Sep 2012, John Clements wrote:

> ? and I don't mean Teddy Roosevelt.
> TR just discovered a bug that other type systems totally wouldn't have. 
> As a side-benefit, it appears that TR should be able to generate 
> substantially faster code as a result.
> Short synopsis:
> I have inner-loop code that's using 'modulo'. As it turns out, modulo is 
> slow, because (among other things) it handles cases where the modulus 
> needs to be added or subtracted more than once.  So, I wrote my own. In 
> fact, I specialized my own to the situation where it wrapped down only, 
> because it was being applied to a counter that only got bumped up by 1.
> I found another use of modulo, and pointed it to the same function.
> OOPS! the program doesn't type-check any more. Why? because TR correctly 
> notes that in my other use of the function, it's entirely possible for 
> the index to be less than zero.
> In principle, any dependent type system should have been able to figure 
> this out. In practice, though, I don't know of any languages that 
> actually support dependent types in this way? er, agda?
> Anyhow, TR just saved me a bunch of debugging time.
> Of course, I just blew it all, writing this e-mail?.
> John

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