[swift-evolution] [Planning][Request] "constexpr" for Swift 5

[Daniel Vollmer]

Hello,

> On 30. Jul 2017, at 08:52, Gor Gyolchanyan via swift-evolution <swift-evolution at swift.org> wrote:

[snip]

> This is a huge undertaking, but if we split it in two parts: "executing at compiletime" and "exposing swift’s frontend”, this could surely be done by Swift 5. From all the ideas that I’ve had or seen on this mailing list, in my opinion this is the most ground-braking, powerful and useful, so I’d be willing to work day and night to help implement this.

As you say, I agree that these are separate concepts (compile-time evaluation vs. meta-programming), but the first is surely going to be helpful for the second.

As far as I can tell, compile-time evaluation would probably lose most / all of the stdlib. I’d hate to replicate parts of the standard library in a restricted subset of the language so that they are available in `constexpr` mode (which is what 80% of C++’s template meta-programming libraries do). I’m also not quite sure how to make that feature work with cross-compilation when host and target differ in architecture.

As for the meta-programming / macro part, a recent C++ proposal tries to extend the language in that direction: https://herbsutter.files.wordpress.com/2017/07/p0707r1.pdf .
Ignoring the syntax, at the very least it might reduce the need for gyb in places. That said, because Swift’s and C++’s instantiation model for generic code is so different, there’s going to be some blockers there…

So while I think they’re both great features to have, I assume the core team will have higher priorities on their schedule, as I don’t see how these features wouldn’t eat up most resources for at least (!) one Swift release-cycle.

	Daniel.