Steve, what's the reason that Rust doesn't have HKTs (higher-kinded types). Is there a technical barrier, e.g. to do with life-time inference, or is it a philosophical choice not to have them?
It's one of our most requested features (and would be really good for collections) but it should be backwards compatible and therefore was postponed until after 1.0.
Nobody has put in the work to actually make a formal RFC yet either, which is required.
Is anybody actively working on this? I suspect that doing this well is non-trivial. Neither Tofte/Talpin nor Cyclone, both of which heavily inspired Rust's lifetimes, have HKTs as far as I'm aware.
I understand that 1.0 was / is a priority. But I wonder what the state of discussions about HKTs in Rust is: is this addition believed to be an easy problem in the sense that it may be a lot of work, but no major roadblocks are expected? Or are there open questions that require substantial research?
The state is, "this should be backwards compatible, therefore, we don't need to think about it more until after 1.0." That's pretty much it. There isn't an active discussion, because we're actively discussing the things needed to ship 1.0.