[racket-dev] Implementing contracts for async channels

From: Robby Findler (robby at eecs.northwestern.edu)
Date: Thu Jan 15 17:41:50 EST 2015

Just a small nit: why export that function from racket/contract and
not an async-channel library?


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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