[racket-dev] Implementing contracts for async channels

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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