[swift-evolution] [Proposal] Higher Kinded Types (Monads, Functors, etc.)

[Will Fancher]

> As long as the underlying theory is not widely adopted, I think HKTs should be provided by third-party libraries (like swiftz).

I wholeheartedly agree that protocols like Monad and Functor shouldn't be in the standard library. I really think they need their own external package.

But HKTs can't be provided by third party libraries! HKTs are a language level feature! HKTs aren't a collection of types we wish were implemented. HKTs are the language level construct that allows for generic types to be re-parameterized. An HKT proposal would NOT insist that Monad and co. be written into the standard library. It would simply insist that HKTs become physically possible, which they currently are not.