[racket-dev] Implementing contracts for async channels

From: Alexis King (lexi.lambda at gmail.com)
Date: Thu Jan 15 16:33:13 EST 2015

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 <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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