[racket] Contracts and submodules

From: Robby Findler (robby at eecs.northwestern.edu)
Date: Fri Nov 30 11:16:55 EST 2012

I think that we can do better in this case. The -> contract combinator
can be specialized when it sees that there really is a function with
the right type and it actually sees the function itself to insert the
checks into.

But I should point out that benchmarking programs like this one can be
very misleading. There are lots of optimizations that can kick in with
little programs like that (that contracts would inhibit) that lead to
benchmarking results that just don't generalize to real programs. I
think our numbers (which are still not great) in the OOPSLA paper I
pointed you to are more representative. The only reason I'd optimize
this particular case is to avoid people who do small benchmarks and
complain, not because I think it would really help anyone!


On Fri, Nov 30, 2012 at 9:15 AM, Greg Hendershott
<greghendershott at gmail.com> wrote:
>> This is a complete misunderstanding.
> Sometimes I feel like a kid in the room while the adults are talking.
> When it comes to contracts, I have to stipulate that most of you are
> smarter than me and have thought about this longer than me.
> Having stipulated that, it seems unfortunate and surprising that
> (define/contract (f x)
>   (integer? . -> . any)
>   x)
> is ~200 times slower than
> (define (f x)
>   (unless (integer? x) (raise-argument-error ....))
>   x)
> It seems like "pro" Racket code avoids the former and does the latter.
> One reason may be that such code predates contracts, and/or old habits
> die hard. But another reason seems to be that the former would be
> unacceptably slow. I wish it weren't.
> Are you saying that "contract" is a fundamentally different concept
> than "argument checking" -- it's not a superset but some whole
> different beast?  If so, then I guess I'm saying I want something with
> the declarative convenience of define/contract (or syntax-parse), but
> fast enough to be practical. I could refer to this other concept as
> "term sheets" (i.e. a light 2-page memo rather than a 100-page
> agreement) to avoid confusion. I've sketched that out and could just
> use that myself instead of disturbing you all with this feedback.
> On Fri, Nov 30, 2012 at 9:33 AM, Matthias Felleisen
> <matthias at ccs.neu.edu> wrote:
>> On Nov 29, 2012, at 10:29 PM, Greg Hendershott 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.
>> This is a complete misunderstanding. Contracts are specifications
>> that govern the flow of values across boundaries. We call this boundary
>> a 'module' but for several years now, module and #lang have been two
>> distinct terms. In this spirit,
>>  -- #lang with provide contract-out is a module
>>  -- define/contract is a module
>>  -- internal modules are modules but watch out for module+
>> In contrast,
>>  -- units and classes are values and we have contracts for them
>>         but they do NOT establish contract boundaries, they are
>>         NOT modules.
>> In your specific case, define/contract splits an existing 'module'
>> into two pieces.
>> ;; ---
>> As I have said many times, your use of contract is likely to be
>> much better satisfied with the use of Typed Racket. I doubt that
>> many of your contracts are checking more than constructors, which
>> roughly corresponds to type checking. For the remaining ones, Sam
>> will provide
>>   provide/typed with contract-out
>>   require/typed with contract-in
>>         ANY TIME SOON NOW.
>> ;; ---
>> As for performance of contracts:
>> On Nov 29, 2012, at 11:47 PM, Harry Spier wrote:
>>> And then to enable/disable contracts you can use the method Mathias
>>> pointed out here.
>>> http://www.mail-archive.com/users@racket-lang.org/msg12281.html
>> Thanks for reminding the list of this post. I should write this
>> out as a blog post or an entry into our Contract Guide.
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