> On 27 Nov 2017, at 07:32, John McCall <rjmcc...@apple.com> wrote:
>
>
>>> On Nov 22, 2017, at 2:01 AM, David Hart via swift-evolution
>>> <swift-evolution@swift.org> wrote:
>>>
>>>
>>>
>>> On 22 Nov 2017, at 07:48, David Hart via swift-evolution
>>> <swift-evolution@swift.org> wrote:
>>>
>>>
>>>
>>> On 22 Nov 2017, at 07:41, Douglas Gregor via swift-evolution
>>> <swift-evolution@swift.org> wrote:
>>>
>>>>
>>>>
>>>>> On Nov 21, 2017, at 10:37 PM, Chris Lattner <clatt...@nondot.org> wrote:
>>>>>
>>>>> On Nov 21, 2017, at 9:25 PM, Douglas Gregor <dgre...@apple.com> wrote:
>>>>>>> Or alternatively, one could decide to make the generics system *only
>>>>>>> and forever* work on nominal types, and make the syntactic sugar just
>>>>>>> be sugar for named types like Swift.Tuple, Function, and Optional.
>>>>>>> Either design could work.
>>>>>>
>>>>>> We don’t have a way to make it work for function types, though, because
>>>>>> of parameter-passing conventions. Well, assuming we don’t invent
>>>>>> something that allows:
>>>>>>
>>>>>> Function<Double, inout String>
>>>>>>
>>>>>> to exist in the type system. Tuple labels have a similar problem.
>>>>>
>>>>> I’m totally aware of that and mentioned it upthread.
>>>>
>>>> Eh, sorry I missed it.
>>>>
>>>>> There are various encoding tricks that could make this work depending on
>>>>> how you want to stretch the current generics system…
>>>>
>>>> I think it’s straightforward and less ugly to make structural types allow
>>>> extensions and protocol conformances.
>>>
>>> Can somebody explain to me what is less ugly about that? I would have
>>> naturally thought that the language would be simpler as a whole if there
>>> only existed nominal types and all structural types were just sugar over
>>> them.
>>
>> What confuses me is that I always thought that T? was sugar for Optional<T>
>> by design, and found that to be quite elegant. But now you’re telling me
>> that its just a hack to allow conformance on Optionals until it can be made
>> structural. I would have thought that it would be cleaner to have specific
>> concepts (optionals, tuples, etc…) represented in terms of more general
>> concepts (enum, struct) so that the compiler had less to reason about. I’m
>> just trying to understand :-)
>
> Don't worry too much about it. The people in this thread are conflating a
> lot of different things, including some people who ought to know better.
> Your way of looking at it is perfectly accurate.
>
> A fairly standard formalization of types is that you have these type
> expressions, which, in a simple system, are either type constants or type
> applications.
>
> A type constant is something like Int, Float, or Array. struct/enum/class
> declarations in Swift all introduce new type constants, where each
> declaration has its own identity as a type. Since we don't allow type names
> to be redeclared in a scope, the identity of such declared types can be
> uniquely determined by (1) the type's name in its scope and (2) the path to
> that scope. Hence the identity is "by name", or "nominal".
>
> A type application takes a generic type and applies it at some number of
> arguments. The general syntax for this in Swift is <>. For example, Array
> is a generic type, and Array<Int> is a type application of the generic type
> Array with the argument Int. The arguments can be arbitrary type
> expressions, e.g. Dictionary<String, Array<Float>>. The identity of a type
> application is determined purely by its applicative structure: first, the
> identity of the generic type being applied, and second, the identity of the
> type arguments it is being applied to. That is, A<B> is the same type as
> C<D> as long as the type-expression A is the same generic type as C and B is
> the same type as D. Hence the identity is "structural".
>
> (Formal nitpick: we're making some assumptions about the nature of type
> expressions that generally work out until you get into really advanced type
> systems.)
>
> What is a tuple type in this formalization? Well, the rule we want is that
> (A, B) is the same type as (C, D) if A is identical to C and B is identical
> to D. We could add this as a special new rule to our definition of type
> expressions above, but we don't really need to, because it turns out that
> it's exactly the same as if we had some sort of tuple type constant — for
> sake of argument, let's call it (...) — and allowed it to be applied to an
> arbitrary number of types. Swift does not actually allow you to name this as
> an unapplied generic type — you cannot write (...)<Int, Float> instead of
> (Int, Float) — but that's just a matter of syntax. The fact that (...) is
> built into the language rather than being declared in the standard library as
> Tuple is not fundamentally significant. Nor is it fundamentally significant
> for Optional, which is currently declared in the standard library instead of
> being built-in.
>
> (Function types in Swift are weird because, as covered previously in this
> thread, there's more to their structure than just component type identity.
> You could still stretch the formalization to make this work, but, well,
> they'd still be weird.)
>
> Now, function, tuple, and optional types are special in the language in
> several ways: there are primitive declarations and expressions that introduce
> or manipulate these types, and there are special rules in the type system for
> them. (For example, optionals and tuples obey covariant subtyping.) But we
> could technically do the same for any type if we wanted to. We could even do
> it to arbitrary third-party types as long as we handled the possibility that
> they weren't imported into the current program.
>
> We could absolutely allow tuples to conform to protocols without first
> introducing variadic generics. We'd have to solve some straightforward
> internal representation problems, and then we'd just need some way of
> spelling them (if we didn't just hard-code a few conformances as special).
>
> John.
Thanks for the lengthy explanation! I really appreciate the time taken to
educate.
When you say that we could allow tuples to conform to protocols without first
introducing Variadic Generics, it would still require some new syntax though?
Is that a complexity cost that would be reason enough to wait for the general
solution of Variadic Generics? Here, when I mention complexity, I’m talking
about the complexity of the language, not it’s implementation.
_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution
