We could think of type-level-abstracted return types as doing the same thing but at the type level; you give a function generic arguments as inputs, and it gives a certain return type back. ​ This roughly follows the progression of `impl Trait` in Rust, where it was first introduced only for return types, then was generalized to be able to appear structurally in both argument and return types. We think this is a reasonable first step because it directly addresses the biggest functionality gap in the generics model. After that first step, there are a few fairly orthogonal language change discussions we can have, some of which are already underway

it’s reassuring to know that the folks driving changes in Swift have a solid background in language design, and that they’re thinking about all manner of hard problems in order to make our lives easier. However, it makes me worry that people might be missing out on a truly exciting conversation about what might be coming in a future Swift version. The reason for the label “reverse generics” is that the flow of information is backwards from the existing system. Where right now, the caller binds generic types as it calls a function, the proposal would have the function itself bind the return types and pass concrete values back out.

Internet debate is very effective at changing minds— Joe Groff (@jckarter) May 1, 2019

Bonus content: here's how I'd shim it into a switch statement: pic.twitter.com/W4Wu7M0923— Ben Cohen (@AirspeedSwift) April 25, 2019

It's my fault, sorry. In the early days of Swift, we had a closure syntax that was very similar to traditional Javascript, func (arg: Type, arg: Type) -> Return { ... }. While this is nice and regular syntax, it is of course also very bulky and awkward if you're trying to support expressive functional APIs, such as map/filter on collections, or if you want libraries to be able to provide closure-based APIs that feel like extensions of the language. Our earliest adopters at Apple complained about...

Defining an addition and multiplication doesn’t make the thing a Ring—there are axioms that need to be satisfied. Protocols are not just bags of syntax. https://t.co/pLdUxdnxR3— 🎃 Spooky ABI Stability 🎃 (@stephentyrone) March 5, 2019

“@jckarter @Javi @mdiep The space of things that are typey enough to need labels but not typey enough to get a name and call a struct is pretty small.”

“@jckarter @lemire @cocoaphony @mpweiher @Ilseman @stephentyrone @aalonso128 @ericasadun Just kidding, works great! func asc(_ f: @escaping (T) -> (T) -> ComparisonResult) -> (T, T) -> Bool { return { f($0)($1) == .orderedAscending } } ["abc", "ABD"].sorted(by: asc(String.localizedCompare))”

“@jckarter @kylebshr I’d caution against judging any language’s teaching potential via toy examples of string manipulation that never happen in real life. More important is how the learning curve progresses into day 2, day 20, day 200.”