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

[Dave Abrahams]

> On Dec 16, 2015, at 4:04 PM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> With higher-kinded types, one could potentially [*] produce a collection of the same kind as Self but with the element type T. So, mapping a Set produces a Set, mapping an Array produces an Array, etc. 
> 
> I think structure preserving map is a great example and one I hope will eventually make it into the standard library someday.  

How, specifically, would you use it?  To which types would it apply, and how would the change benefit users?

-Dave