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

[Dave Abrahams]

on Fri Jun 10 2016, Thorsten Seitz <tseitz42-AT-icloud.com> wrote:

>> Am 09.06.2016 um 19:50 schrieb Thorsten Seitz via swift-evolution <swift-evolution at swift.org>:
>> 
>> 
>>> Am 09.06.2016 um 18:49 schrieb Dave Abrahams via swift-evolution <swift-evolution at swift.org>:
>>> 
>
>>> 
>>> on Wed Jun 08 2016, Jordan Rose <swift-evolution at swift.org> wrote:
>>> 
>>>>> On Jun 8, 2016, at 13:16, Dave Abrahams via swift-evolution
>>>>> <swift-evolution at swift.org> wrote:
>>>>> 
>>>>> 
>>>>> on Wed Jun 08 2016, Thorsten Seitz
>>>> 
>>>>> <swift-evolution at swift.org
>>>>> <mailto:swift-evolution at swift.org>>
>>>>> wrote:
>>>>> 
>>>>>> Ah, thanks, I forgot!  I still consider this a bug, though (will have
>>>>>> to read up again what the reasons are for that behavior).
>>>>> 
>>>>> Yes, but in the case of the issue we're discussing, the choices are:
>>>>> 
>>>>> 1. Omit from the existential's API any protocol requirements that depend
>>>>> on Self or associated types, in which case it *can't* conform to
>>>>> itself because it doesn't fulfill the requirements.
>>>>> 
>>>>> 2. Erase type relationships and trap at runtime when they don't line up.
>>>>> 
>>>>> Matthew has been arguing against #2, but you can't “fix the bug” without
>>>>> it.
>>>> 
>>>> #1 has been my preference for a while as well, at least as a starting
>>>> point. 
>>> 
>>> I should point out that with the resyntaxing of existentials to
>>> Any<Protocols...>, the idea that Collection's existential doesn't
>>> conform to Collection becomes far less absurd than it was, so maybe this
>>> is not so bad.
>> 
>> I think the problem is more that Any<Collection> does not conform to
>> a specific value for a type parameter T: Collection
>> 
>> What I mean by this is that `Collection` denotes a type family, a
>> generic parameter `T: Collection` denotes a specific (though
>> unknown) member of that type family and `Any<Collection>` denotes
>> the type family again, so there is really no point in writing
>> Any<Collection> IMO.
>> The type family cannot conform to T because T is just one fixed member of it.
>> It conforms to itself, though, as I can write
>> let c1: Any<Collection> = …
>> let c2: Any<Collection> = c1
>> 
>> That’s why I think that we could just drop Any<Collection> and simply write Collection.
>
> Let me expand that a bit:
>
> Actually all this talk about existentials vs. generics or protocols
> vs. classes has had me confused somewhat and I think there are still
> some misconceptions present on this list sometimes, so I’ll try to
> clear them up:

There are several objectively incorrect statements here, and several
others with which I disagree.  I was hoping someone else would write
this for me, but since the post has such a tone of authority I feel I
must respond.

> (1) misconception: protocols with associated types are somehow very
> different from generics
>
> I don’t think they are and I will explain why. The only difference is
> the way the type parameters are bound: generics use explicit parameter
> lists whereas protocols use inheritance. That has some advantages
> (think long parameter lists of generics) and some disadvantages.
> These ways are dual in a notation sense: generic types have to have
> all parameters bound whereas protocols cannot bind any of them.
> The „existential“ notation `Any<>` being discussed on this list is
> nothing more than adding the ability to protocols to bind the
> parameters to be used just like Java’s wildcards are adding the
> opposite feature to generics, namely not having to bind all
> parameters.

Protocols and generics fulfill completely different roles in Swift, and
so, **especially in a language design context like the one we're in
here**, must be thought of differently.  The former are an abstraction
mechanism for APIs, and the latter a mechanism for generalizing
implementations.  The only place you could argue that they intersect is
in generic non-final classes, because a class fills the dual role of
abstraction and implementation mechanism (and some might say that's a
weakness).  But even accounting for generic classes, protocols with
associated types are very different from generics.  Two utterly
different types (an enum and a struct, for example) can conform to any
given protocol P, but generic types always share a common basis
implementation.  There is no way to produce distinct instances of a
generic type with all its type parameters bound, but for any protocol P
I can make infinitely many instances of P with P.AssociatedType == Int.

Back to the my original point: while protocols and generic types have
some similarities, the idea that they are fundamentally the same thing
(I know you didn't say *exactly* that, but I think it will be read that
way) would be wrong and a very unproductive way to approach language
evolution.

> Essentially `Any<Collection>` in Swift is just the same as
> `Collection<?>` in Java (assuming for comparability’s sake that
> Swift’s Collection had no additional associated types; otherwise I
> would just have to introduce a Collection<Element, Index> in Java).

I don't see how you can use an example that requires *assuming away*
assoociated types to justify an argument that protocols *with associated
types* are the same as generics.

> Likewise `Any<Collection where .Element: Number>` is just the same as
> `Collection<? extends Number>` in Java.
>
> And just like Collection<?> does not conform to a type parameter `T
> extends Collection<?>` because Collection<?> is the type `forall
> E. Collection<E>` whereas `T extends Collection<?>` is the type
> `T. Collection<T>` for a given T.
>
> In essence protocols with associated types are like generics with
> wildcards.

It is true that generics with wildcards in Java *are* (not just “like”)
existential types but I don't agree with the statement above.  Because
Java tries to create an “everything is a class” world, generic classes
with bound type parameters end up playing the role of existential type.
But protocols in Swift are not, fundamentally, just existential types,
and the resyntaxing of ProtocolName to Any<ProtocolName> for use in type
context is a huge leap forward in making that distinction clear... when
that's done (unless we leave Array<ProtocolName> around as a synonym for
Array<Any<ProtocolName>>—I really hope we won't!)  protocols indeed
*won't* be types at all, existential or otherwise.

> Coming back to the questions whether (a) allowing existentials to be
> used as types is useful 

That's the only use existentials have.  They *are* types.  Of course
they're useful, and I don't think anyone was arguing otherwise.

> and (b) whether sacrificing type safety would somehow be necessary for
> that, I think we can safely answer (a) yes, it *is* useful to be able
> to use existentials like Any<Collection> as types, because wildcards
> are quite often needed and very useful in Java (they haven’t been
> added without a reason) (b) no, sacrificing type safety does not make
> sense, as the experience with Java’s wildcards shows that this is not
> needed. 

I would call this “interesting information,” but hardly conclusive.
Java's generics are almost exactly the same thing as Objective-C
lightweight generics, which are less capable and less expressive in
many ways than Swift's generics.  

> Especially if something like path dependent types is used like
> proposed and some notation to open an existential’s type is added,
> which is both something that Java does not have.
>
> (2) misconception: POP is different from OOP
>
> It is not. Protocols are just interfaces using subtyping like OOP has
> always done. They just use associated types instead of explicit type
> parameters for generics (see above). 

They are not the same thing at all (see above ;->).  To add to the list
above, protocols can express fundamental relationships—like Self
requirements—that OOP simply can't handle.  There's a reason Java can't
express Comparable without losing static type-safety.  Finally, in a
language with first-class value types, taking a protocol-oriented
approach to abstraction leads to *fundamentally* different designs from
what you get using OOP.

> The more important distinction of Swift is emphasizing value types and
> making mutation safely available by enforcing copy semantics for value
> types.  

We don't, in fact, enforce copy semantics for value types.  That's
something I'd like to change.  But regardless, value types would be a
*lot* less useful if they couldn't conform to protocols, and so they
would be a lot less used.  Heck, before we got protocol extensions in
Swift 2, there was basically *no way* to share implementation among
value types.  So you can't take protocols out of the picture without
making value types, and the argument for value semantics, far weaker.

> But protocols are not really different from interfaces in Java. 
> I would have preferred a unified model using just classes with real
> multiple inheritance like Eiffel has and value types just being a part
> of that similar to Eiffel’s `expanded` classes. But that ship has
> probably sailed a long time ago :-/ So be it. But at least there
> should be no reasons for POP vs OOP wars ;-) (I’d like to add that I
> liked Dave’s talks at last WWDC very much, it’s just that I don’t
> think that POP is something new or different.)

Protocol-oriented programming is about the synergy of features and ideas
most of which not *individually* new, but that together create a new
world of possibilities.  I've already discussed the synergy of protocols
and first-class value semantics.  There's also the fact that in
protocols we have one construct with which to express dynamic
polymorphism (existentials) and static polymorphism (generic
constraints), both of which have important roles to play but that I
maintain are very different indeed.  One result is that you can “start
with a protocol” as your abstraction mechanism and know that you're not
going to design yourself into a corner where it becomes impossibly
awkward to express what you need.  Finally—and I'm certain this *is*
new—in protocol extensions we have a means to express both post-hoc
conformance and generic functions that is much more accessible to users
than in any previous language, to the point where generic programming
can become a natural part of everyday work.

-- 
Dave