[swift-evolution] [Manifesto] Completing Generics

Joe Groff jgroff at apple.com
Wed Mar 9 20:07:11 CST 2016

> On Mar 9, 2016, at 6:02 PM, Howard Lovatt <howard.lovatt at gmail.com> wrote:
> Wow, I would never have guessed that syntax :)
> It makes no sense to me to interpret a generic constraint as meaning all instead of any. How could anything either accept or return all possible implementations of something simultaneously, surely it only ever accepts or returns one of all the possible implementations at a time.

A type variable in angle brackets always means "all". It's like a function parameter, but at type level—it's in the caller's hands what it gets bound to. You couldn't write a function `func foo<Input, Output>(Input) -> Output` unless that function was able to generate a value of every possible type a caller might pass in for Output, just like you couldn't write e.g. 'absolute value' as taking its result as a second parameter.

> If the interpretation for output is that at time 1 it returns one of all the possible implementations at at time 2  returns another - then that is what I want. However I would describe that as returning one of the possible implementations, not all.
> But no doubt you are correct, since you probably wrote that bit of the compiler :(
> More practical points
>  1. My compiler, 7.3 beta (7D152p), rejects the syntax, it doesn't like where inside Any<> saying it expects > to complete generic argument list. When will this be available, so that I can try it out?
>  2. Will the declarations inside protocols also change to the Any<...> form or will all the generics remain following the function name rather in a returned Any<...>? Currently -> Any<...> doesn't work in protocols for me.
>  3. In the construct Any<Protocol where Type == Type>, i.e. same type name used in protocol and enclosing struct/class/extension, does the left Type refer to the protocol or the enclosing struct/class/extension?
>  4. Is there any documentation of all of this?

Sorry, this is all possible future syntax and features. It's not implemented today. You'd need to write your own equivalent of the "Any" wrapper by hand right now.


More information about the swift-evolution mailing list