[racket] I love TR

On Sep 20, 2012, at 2:27 PM, Matthias Felleisen wrote:

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> 
> Are you sure that you blew your entire budget on this email? 
> 
> TR is a dependently typed language. While types don't entire values, they depend on those 'aspects' of values (is it a cons? is it a positive value?) that can be checked with (usually cheap) predicates. 

There's a key missing word in the second sentence of the second paragraph… I think I understand what you're saying.

Based on my tiny definition of dependent types ("types that depend on values"), TR doesn't look like it has dependent types (e.g. forall n . numbers less than n), but then again, staged compilation and modules may throw the definition of dependent types into a cocked hat, if I can extend the type system as part of an earlier phase. 

Tell me how confused I am, on a scale of 1-10 :).

John

PS: if TR really is dependently typed, then it should appear in the table on this page:

http://en.wikipedia.org/wiki/Dependent_type

I'm not quite sure what you'd put for "Program Extraction", though :).

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> On Sep 20, 2012, at 5:21 PM, 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
>> 
>> ____________________
>> Racket Users list:
>> http://lists.racket-lang.org/users
> 

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