[swift-evolution] [swift-evolution-announce] [Review] SE-0089: Replace protocol<P1, P2> syntax with Any<P1, P2>

[Austin Zheng]

(inline)

On Fri, May 27, 2016 at 10:09 AM, Thorsten Seitz <tseitz42 at icloud.com>
wrote:

>
> Am 27.05.2016 um 18:36 schrieb Austin Zheng <austinzheng at gmail.com>:
>
> I think the parentheses are the fundamental aspect of the suggestion :).
>
> Let me turn the question around. If tuples were declared like this:
>
> let myTuple : Int, String, Bool = (10, "hello", false)
>
> would the type be more or less readable? I find it a lot more difficult to
> immediately parse than:
>
> let myTuple : (Int, String, Bool) = (10, "hello", false)
>
> At the same time, nobody's complained about tuple type parentheses getting
> in the way.
>
>
> Parentheses are the hallmark of tuples, so its natural to expect them
> around the tuple type as well :-)
>

But this is a circular definition. Why can't parentheses be the hallmark of
structural types, not just tuples? After all, one of the big complaints
against the Any<> and protocol<> syntaxes has been that angle brackets
belong to generics already.


>
>
>
> We're trying to establish a syntax that will hopefully be used for things
> significantly more complicated than tuple definitions, which are just a
> list of types. I think readability is a major concern. Typealiases should
> be supported, but they shouldn't be required to make the feature useable.
>
> Finally, wouldn't we need some delimiter for nested existential
> definitions anyways? Now you have the confusing situation where the outside
> definition has no delimiters, but the inside ones do:
>
>
> Why is this confusing? The expression `2 * (3 + 1)` also has parentheses
> only around the part which needs them.
> Probably no one would require having to write `(2 * (3 + 1))` for
> consistency.
>

You can make the same argument about tuples.


>
>
>
> // Why does the inner existential look fundamentally different than the
> outer one?
> // Not to mention, visually parsing the boundaries of this type when you
> look at it in a function signature
> let x : Protocol1, Protocol2, (Protocol 3 where .Foo == Int) where
> Protocol2.Bar : Baz
>
>
> Using `&` instead of `,` will make it look better, too, IMHO:
>
> let x: Protocol1 & Protocol2 & (Protocol3 where .Foo == Int) where
> Protocol2.Bar: Baz
>

I agree that '&' is better - my mistake. I still submit my concerns about
legibility.


>
> or better (removing the asymmetry):
>
> let x: Protocol1 & (Protocol2 where .Bar : Baz) & (Protocol3 where .Foo ==
> Int)
>
> or putting all constraints in the where clause for the whole expression:
>
> let x: Protocol1 & Protocol2 & Protocol3 where Protocol2.Bar: Baz,
> Protocol3.Foo == Int
>

I would much rather have parentheses enclosing the outside type than force
people to structure their requirements a certain way.


>
>
> -Thorsten
>
>
>
> I hope that explains my reasoning.
>
> Best,
> Austin
>
>
> On May 27, 2016, at 9:28 AM, Matthew Johnson <matthew at anandabits.com>
> wrote:
>
>
>
> Sent from my iPad
>
> On May 27, 2016, at 11:18 AM, Austin Zheng <austinzheng at gmail.com> wrote:
>
> Here's a strawman idea.
>
> What if we go with '&' and 'where', but we enclose the whole thing in
> parentheses?
>
> (class & Protocol1 & Protocol2 where .Foo == Int, .Bar : Baz)
>
> There are a couple of reasons I propose this syntax:
>
> - It makes it very clear where the definition of the type begins and ends.
> I understand people really despise angle brackets, but I really want some
> way to visually delineate the boundaries of the type. Plus, I imagine it
> makes syntax a little easier to parse and preemptively forbids some
> ambiguities.
>
> - It's a structural, not nominal, type, like a tuple, so it uses parens as
> well. This reserves "<" and ">" for generic types.
>
> - The '&' is easily understood - "Protocol1" *and* "Protocol2". It's also
> a signal that order doesn't matter - just like how order matters with
> things that use commas, like argument lists, tuples, and array members,
> order doesn't generally matter with bitwise or logical 'and' operators.
>
> - If we ever decide to have union types, we have a very elegant third form
> of nominal type syntax that naturally falls out: (MyClass1 | MyClass2 |
> MyClass3).
>
> Thoughts?
>
>
> Generally in favor.  But I would not require the parentheses.  I believe
> they would be allowed optionally automatically, just as (Int) is the same
> as Int (because single element tuples don't exist and the underlying type
> is used directly instead).  It seems better to leave parentheses up to a
> matter of style.
>
>
>
> Austin
>
>
> On May 27, 2016, at 9:07 AM, Thorsten Seitz via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>
> Am 27.05.2016 um 16:54 schrieb Matthew Johnson <matthew at anandabits.com>:
>
>
> On May 27, 2016, at 8:18 AM, Thorsten Seitz via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> Personally I think `&` is more lightweight (and it is established in other
> languages like Ceylon and Typescript) and `where` is more expressive (and
> established in Swift for introducing constraints), so I would stay with
> these.
>
>
> I agree.  If we can make `&` with `where` work syntactically it would be
> nice to go in this lighter weight direction.  If we decide to do that the
> question then becomes what to do with `protocol`.  Would it be feasible to
> replace it with `&` in Swift 3 if we decide on that direction?
>
>
> Yep. `protocol` should be replaced with `&` in that case.
>
> -Thorsten
>
>
>
>
> -Thorsten
>
>
> Am 27.05.2016 um 14:34 schrieb Vladimir.S <svabox at gmail.com>:
>
> Btw, in case we have `where` keyword in syntax related to types/protocols
> (when defining constrains. and not some symbol like '>>'.. don't know, for
> example), why we can't have 'and' keyword also when discuss the syntax of
> type/protocol conjunction?
> I.e.
>
> let x: P and Q
> let x: P and Q where P.T == Q.T
> let x: P and Q and R
>
> or, for consistency, as I understand it, we should have
> let x: P & Q >> P.T == Q.T
>
> On 27.05.2016 11:55, Thorsten Seitz via swift-evolution wrote:
>
> We could just write
>
> let x: P & Q
> instead of
> let x: Any<P, Q>
>
> let x: Collection where .Element: P
> instead of
> let x: Any<Collection where .Element: P>
>
> let x: P & Q where P.T == Q.T
> instead of
> let x: Any<P, Q where P.T == Q.T>
>
> let x: P & Q & R
> instead of
> let x: Any<P, Q, R>
>
> let x: Collection
> instead of
> let x: Any<Collection>
>
>
> This would avoid the confusion of Any<T1, T2> being something completely
> different than a generic type (i.e. order of T1, T2 does not matter whereas
> for generic types it is essential).
>
>
> -Thorsten
>
>
>
> Am 26.05.2016 um 20:11 schrieb Adrian Zubarev via swift-evolution
> <swift-evolution at swift.org <mailto:swift-evolution at swift.org
> <swift-evolution at swift.org>>>:
>
> Something like |type<…>| was considered at the very start of the whole
> discussion (in this thread
> <
> https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160502/016523.html
> >),
> but it does not solve the meaning of an existential type and also might
> lead to even more confusion.
>
> From my perspective I wouldn’t use parentheses here because it looks more
> like an init without any label |Type.init(…)| or |Type(…)|. I could live
> with |Any[…]| but this doesn’t look shiny and Swifty to me. Thats only my
> personal view. ;)
>
>
>
>
> --
> Adrian Zubarev
> Sent with Airmail
>
> Am 26. Mai 2016 bei 19:48:04, Vladimir.S via swift-evolution
> (swift-evolution at swift.org <mailto:swift-evolution at swift.org
> <swift-evolution at swift.org>>) schrieb:
>
> Don't think {} is better here, as they also have "established meaning in
> Swift today".
>
> How about just Type(P1 & P2 | P3) - as IMO we can think of such
> construction as "creation" of new type and `P1 & P2 | P3` could be treated
> as parameters to initializer.
>
> func f(t: Type(P1 & P2 | P3)) {..}
>
>
> On 26.05.2016 20:32, L. Mihalkovic via swift-evolution wrote:
> > How about something like Type{P1 & P2 | P3} the point being that "<...>"
> has an established meaning in Swift today which is not what is expressed in
> the "<P1,P2,P3>" contained inside Any<P1, P2,P3>.
> >
> >> On May 26, 2016, at 7:11 PM, Dave Abrahams via swift-evolution <
> swift-evolution at swift.org <mailto:swift-evolution at swift.org
> <swift-evolution at swift.org>>> wrote:
> >>
> >>
> >>> on Thu May 26 2016, Adrian Zubarev <swift-evolution at swift.org <
> mailto:swift-evolution at swift.org <swift-evolution at swift.org>>> wrote:
> >>>
> >>> There is great feedback going on here. I'd like to consider a few
> things here:
> >>>
> >>> * What if we name the whole thing `Existential<>` to sort out all
> >>> confusion?
> >>
> >> Some of us believe that “existential” is way too theoretical a word to
> >> force into the official lexicon of Swift. I think “Any<...>” is much
> >> more conceptually accessible.
> >>
> >>>
> >>> This would allow `typealias Any = Existential<>`. * Should
> >>> `protocol A: Any<class>` replace `protocol A: class`? Or at least
> >>> deprecate it. * Do we need `typealias AnyClass = Any<class>` or do we
> >>> want to use any class requirement existential directly? If second, we
> >>> will need to allow direct existential usage on protocols (right now we
> >>> only can use typealiases as a worksround).
> >>
> >> --
> >> Dave
> >>
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