[swift-server-dev] Next HTTP API meeting

Brent Royal-Gordon brent at architechies.com
Mon Mar 27 05:40:23 CDT 2017

> On Mar 23, 2017, at 2:51 AM, Logan Wright via swift-server-dev <swift-server-dev at swift.org> wrote:
> Already we have multiple stakeholders in the group with very different ideas and use cases for the model, protocolizing this would allow us to focus on parsing and let frameworks choose how to model. By opting for generics constrained to our protocol internally, the compiler will inline the code and give us pretty good performance, in our generic constrained code, it was just as fast, or faster than it's model counterparts.
> Here's an example of a protocol required to parse a request
> ```
> protocol HTTPMessage {
>     init(status: String, version: String, method: String, headers: [String: String], etc...
> }
> ```
> It doesn't need to be anything particularly complex, and we could include easily the inverse as well for the serializer. This could be a distinct protocol even for use cases we might not consider.

Okay, but one of the major complaints I see in the GitHub thread is that some people want to use a reference type, and it seems like some people may want protocols so they can make that decision for themselves. I don't think that's going to work, though. If the protocol has any setters or mutating methods, then it will need to document whether it expects value or reference semantics; otherwise nobody will be able to know how to use those members properly.

(Also, if that really is the entire `HTTPMessage` protocol—just an initializer—why not have users pass in a closure with that signature instead? They could easily pass `MyFramework.HTTPMessage.init` if they want, but they would have other choices too.)

As for the performance aspect of value types, I don't quite see the concern. If we find that performance is a problem, we can switch to a copy-on-write implementation, where a public struct wraps a private class which actually contains the data; that should be exactly equivalent for ARC purposes. Choose value or reference based on the semantics you want; if the performance characteristics are wrong, you can then begin to optimize.

Brent Royal-Gordon

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