[swift-evolution] Subclass Existentials

[Xiaodi Wu]

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.
On Sun, Jan 29, 2017 at 12:41 Austin Zheng <austinzheng at gmail.com> 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 <
> swift-evolution at swift.org> wrote:
> >
> > Thanks for writing this proposal David.
> >
> >> On Jan 29, 2017, at 10:13, Xiaodi Wu via swift-evolution <
> swift-evolution at swift.org> 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
> >
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> > swift-evolution at swift.org
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>
>
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