> On Jan 29, 2017, at 2:01 PM, Xiaodi Wu <[email protected]> wrote: > > On Sun, Jan 29, 2017 at 1:37 PM, Matthew Johnson <[email protected] > <mailto:[email protected]>> wrote: > > > Sent from my iPad > > On Jan 29, 2017, at 12:58 PM, Xiaodi Wu via swift-evolution > <[email protected] <mailto:[email protected]>> wrote: > >> Cool. Another avenue of improvement here is relaxing the single-class >> spelling rule for the sake of composing typealiases. >> >> As Matthew mentioned, if I have class Base and typealiases Foo = Base & >> Protocol1 and Bar = Base & Protocol2, it'd be nice to allow Foo & Bar. >> >> It'd be nice to go one step further: given class Derived : Base, if I have >> typealiases Foo2 = Base & Protocol1 and Bar2 = Derived & Protocol2, then it >> could be permitted to write Foo2 & Bar2, since there is effectively only one >> subclass requirement (Derived). >> >> As I understand it, the rationale for allowing only one subclass requirement >> is that Swift supports only single inheritance. Thus, two disparate subclass >> requirements Base1 & Base2 would make your existential type essentially >> equivalent to Never. But Base1 & Base1 & Base1 is fine for the type system, >> the implementation burden (though greater) shouldn't be too awful, and you >> would measurably improve composition of typealiases. > > Yes, this is what I was indicating in my post as well. > > Are you suggesting that Base1 & Base2 compose to a type that is treated > identically to Never do you think it should be an immediate compiler error? > I remember having some discussion about this last year and think somebody > came up with a very interesting example of where the former might be useful. > > Last year's discussion totally eludes me for some reason. But sure, if > deferring the error until runtime is actually useful then why not? In the > absence of an interesting use case, though, I think it'd be nice for the > compiler to warn you that Base1 & Base2 is not going to be what you want.
Deferring to runtime isn’t what I mean. If you try to actually *do* anything that requires an instance of `Base1 & Based` (which you almost always would) you would still get a compile time error. I managed to dig up the example from last year’s thread and it is definitely a good one: func intersection<T, U>(ts; Set<T>, us: Set<U>) -> Set<T & U> The desire is that we are always able to produce a result set. When T & U is uninhabitable it will simply be an empty set just like Set<Never> has a single value which is the empty set. This example points even more strongly in the direction of allowing *any* concrete type to be used, not just classes - even today we could produce uninhabitable existentials like this using value types. Here’s the link to the thread: https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160523/019463.html > > >> On Sun, Jan 29, 2017 at 12:41 Austin Zheng <[email protected] >> <mailto:[email protected]>> wrote: >> The "class comes first" requirement made more sense when the proposed syntax >> was still "Any<T, U, V>", intentionally mirroring how the superclass and >> conformances are declared on a class declaration (the archives contain more >> detailed arguments, both pro and con). Now that the syntax is "T & U & V", I >> agree that privileging the class requirement is counterintuitive and >> probably unhelpful. >> >> Austin >> >> > On Jan 29, 2017, at 10:37 AM, Matt Whiteside via swift-evolution >> > <[email protected] <mailto:[email protected]>> wrote: >> > >> > Thanks for writing this proposal David. >> > >> >> On Jan 29, 2017, at 10:13, Xiaodi Wu via swift-evolution >> >> <[email protected] <mailto:[email protected]>> wrote: >> >> >> >> As Matthew mentioned, the rules can certainly later be relaxed, but given >> >> that this proposal has the compiler generating fix-its for subclasses in >> >> second position, is there a reason other than stylistic for demanding >> >> MyClass & MyProtocol instead of MyProtocol & MyClass? >> >> >> >> From a naive perspective, it seems that if the compiler understands my >> >> meaning perfectly, it should just accept that spelling rather than >> >> complain. >> > >> > I had that thought too. Since ‘and’ is a symmetric operation, requiring >> > the class to be in the first position seems counter-intuitive. >> > >> > -Matt >> > >> > _______________________________________________ >> > swift-evolution mailing list >> > [email protected] <mailto:[email protected]> >> > https://lists.swift.org/mailman/listinfo/swift-evolution >> > <https://lists.swift.org/mailman/listinfo/swift-evolution> >> >> _______________________________________________ >> swift-evolution mailing list >> [email protected] <mailto:[email protected]> >> https://lists.swift.org/mailman/listinfo/swift-evolution >> <https://lists.swift.org/mailman/listinfo/swift-evolution>
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