[racket-dev] Implementing contracts for async channels

From: Alexis King (lexi.lambda at gmail.com)
Date: Thu Jan 15 17:50:13 EST 2015

Sorry, I wasn’t clear. The chaperone/impersonate-async-channel functions are exported from racket/async-channel. The async-channel contracts, however, are exported from racket/contract.

> On Jan 15, 2015, at 14:41, Robby Findler <robby at eecs.northwestern.edu> wrote:
> Just a small nit: why export that function from racket/contract and
> not an async-channel library?
> Robby
> On Thu, Jan 15, 2015 at 3:33 PM, Alexis King <lexi.lambda at gmail.com> wrote:
>> As an update, I’ve made a bit more progress on this. I’ve implemented an
>> impersonate-async-channel function, and I’ve actually included this in the
>> exports from racket/contract. I also realized the blame information is
>> correct, it works fine. Most of the other issues remain, as well as a few
>> new questions:
>> There is no impersonate-evt function, so I’m not sure that my implementation
>> will work. What should I do about this?
>> I’d assume this needs to be documented/tested as well. Where should those
>> things be located?
>> On Jan 14, 2015, at 23:44, Alexis King <lexi.lambda at gmail.com> wrote:
>> Currently, async channels do not have contracts to check their contents.
>> This is a problem for Typed Racket, and it prevents typed code from
>> interacting with code that produces async channels.
>> I started looking into how to add contracts to the language, and it seems to
>> use the chaperones/impersonator system, as I suspected. However, async
>> channels obviously cannot be impersonated, so I needed to implement that as
>> well.
>> I modified the async-channel struct to use generics to allow it to be
>> impersonated or chaperoned, which I have exposed by implementing
>> chaperone-async-channel. I then tried implementing async-channel/c. The
>> internals of the contract system are a little beyond me, but I got a working
>> solution by following the example of the box contracts.
>> My work thus far can be found here:
>> https://github.com/lexi-lambda/racket/commit/84b9f3604891f3f2061fb28ed4800af8afa4751b
>> First of all, is this a correct approach? Have I done anything wrong, or
>> should I have done anything differently? I didn’t find much documentation on
>> the internals of either of these systems, so I mostly went about things as I
>> found them.
>> Second, obviously, not everything is implemented here. Among the additional
>> necessary changes are:
>> I need to implement support for impersonators and impersonator contracts
>> (right now I’ve only bothered to do chaperones).
>> I need to implement async-channel/c-first-order and
>> async-channel/c-stronger. I can probably figure out the latter, but I’m not
>> even sure what the former is supposed to do.
>> I need to implement a wrap-async-channel/c macro for the export. I’m not
>> sure how this works, either. From looking at wrap-box/c, it seems to add
>> some syntax properties, but I’m not sure what they do or how they work.
>> Somehow, the blame information has to be correct. Is that what the wrap
>> function does? Or do the async-channel functions need to be updated to
>> assign blame?
>> I’d really like to get this working, and I think I’m close, but I’m a little
>> inexperienced. I’d appreciate any help, even if it’s just pointing me in the
>> right direction.
>> Thanks!
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