[racket] Contracts and submodules

From: Robby Findler (robby at eecs.northwestern.edu)
Date: Thu Nov 29 22:55:55 EST 2012

The main value of putting contracts on modules is, in my opinion, that
you can pinpoint blame. So when I fire up DrRacket and click in some
strange way and get a contract violation, it helps me pinpoint which
module. I don't generally need to pinpoint things at the definition
level inside a module (and so this helps with the performance problem
you're mentioning). Less slow in general would be better, but it is
also harder.

Does your performance experience not match what we wrote up in our
OOPSLA paper? (From this year; available from my webpage and
presumably others via google.) If not, we'd love to have more
real-world benchmarks!

(Chaperones are there no matter if you use define/contract,
provide/contract or any other contract attachment mechanism.)


On Thu, Nov 29, 2012 at 9:29 PM, Greg Hendershott
<greghendershott at gmail.com> wrote:
> AFIK these subtleties of contracts and modules arise with
> `provide/contract' -- by saying that the function should only use a
> contract outside the module (or submodule).
> But I prefer to use `define/contract'. For one thing, I think it's
> easier and more-maintainable to group the contract near/with its
> function in the source. But I also prefer it on the starting
> assumption that if a procedure is worth contracting at all, it's worth
> contracting all the time -- inside the module as well as outside.
> I understand there are two reasons why this might not be the case:
> - It might be desirable to violate the contract as an internal
> implementation detail. But in that case, which I've not yet
> encountered myself, I could also define a plain version plus
> define/contract a wrapper version (which IIUC is essentially what
> provide/contract is doing for you via a "chaperon"), or use
> provide/contract just in such a case.
> - It might be desirable for performance. But I think the preferable
> answer here would be to make contracts less slow. Which it seems like
> they could be in the simple cases, Which simple cases seem to be
> nearly all the cases in my own code to-date.
> This is my personal view/experience but I don't know if it's shared by
> others. For example I think some people use modules as a barrier of
> abstraction roughly like how people would use classes in C++ or Java.
> Although that idea hasn't really clicked for me, I understand and
> appreciate that.
> Regardless, I do wish contracts could be (much, much) faster, and
> lately I've doinked around with some experiments you could call "term
> sheets". The idea being to get the same declarative style of contracts
> but with speed comparable to handwritten checks. I wish I were smart
> enough to help do this as an optimization to the real contract system
> (if that's even possible).
> Maybe the long-term vision is, "use Typed Racket instead", but I think
> even in that case TR needs to interoperate safely with untyped code
> for some time to come ... which it does using contracts.
> On Thu, Nov 29, 2012 at 7:59 PM, Harry Spier <vasishtha.spier at gmail.com> wrote:
>> Dear list members,
>> Case 1 below doesn't give a contract violation but case 2 does.  Is
>> this desired behavour or is it a bug?
>> In DrRacket
>> Case 1
>> ---------
>> #lang racket
>> (provide (contract-out [ident-number (-> number? number?)]))
>> (define (ident-number x)  x)
>> (module+ main
>>   (ident-number 'a))
>>> 'a
>> Case 2
>> ------------
>> #lang racket
>> (provide (contract-out [ident-number (-> number? number?)]))
>> (define (ident-number x)  x)
>> (module+ main
>>   (require (submod ".."))
>>   (ident-number 'a))
>>  ident-number: contract violation
>>  expected: number?
>>  given: 'a
>>  in: the 1st argument of
>>       (-> number? number?)
>>  contract from:
>>       c:\users\harry\ocr_project\test2.rkt
>>  blaming:
>>       (c:\users\harry\ocr_project\test2.rkt main)
>>  at: c:\users\harry\ocr_project\test2.rkt:2.24
>> Thanks,
>> Harry Spier
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