[swift-evolution] Should we rename "class" when referring to protocol conformance?

Mon May 16 01:38:33 CDT 2016

> On May 15, 2016, at 11:17 PM, Tyler Fleming Cloutier via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> 
>> On May 15, 2016, at 11:48 AM, Dave Abrahams via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> 
>> on Mon May 09 2016, Matthew Johnson <matthew-AT-anandabits.com <http://matthew-at-anandabits.com/>> wrote:
>> 
>>>> On May 8, 2016, at 1:51 AM, Dave Abrahams <dabrahams at apple.com <mailto:dabrahams at apple.com>> wrote:
>>>> 
>>>> 
>>>> on Sat May 07 2016, Andrew Trick <atrick-AT-apple.com <http://atrick-at-apple.com/>> wrote:
>>>> 
>>> 
>>>>>   On May 7, 2016, at 2:04 PM, Dave Abrahams <dabrahams at apple.com <mailto:dabrahams at apple.com>> wrote:
>>>>> 
>>>>>       2. Value types are not "pure" values if any part of the aggregate
>>>>>       contains a
>>>>>       reference whose type does not have value semantics. 
>>>>> 
>>>>>   Then Array<Int> is not a “pure” value (the buffer contained in an
>>>>>   Array<Int> is a mutable reference type that on its own, definitely does
>>>>>   *not* have value semantics). I don't think this is what you intend, and
>>>>>   it indicates that you need to keep working on your definition.
>>>>> 
>>>>> It sounds like you’re changing the definition of value semantics to make it
>>>>> impossible to define PureValue. 
>>>> 
>>>> Not on purpose.
>>>> 
>>>>> Does Array<T> have value semantics then only if T also has value
>>>>> semantics?
>>>> 
>>>> This is a great question; I had to rewrite my response four times.
>>>> 
>>>> In my world, an Array<T> always has value semantics if you respect the
>>>> boundaries of element values as defined by ==.  That means that if T is
>>>> a mutable reference type, you're not looking through references, because
>>>> == is equivalent to ===.
>>>> 
>>>> Therefore, for almost any interesting SomeConstraint that doesn't refine
>>>> ValueSemantics, then
>>>> 
>>>> Array<T: SomeConstraint>
>>>> 
>>>> only has value semantics if T has value semantics, since SomeConstraint
>>>> presumably uses aspects of T other than reference identity.  
>>>> 
>>>>> The claim has been made that Array always has value semantics,
>>>>> implying that the array value’s boundary ends at the boundary of it’s
>>>>> element values.
>>>> 
>>>> Yes, an array value ends at the boundary of its elements' values.
>>>> 
>>>>> That fact is what allows the compiler to ignore mutation of the
>>>>> buffer.
>>>> 
>>>> I don't know what you mean here.
>>>> 
>>>>> It's perfectly clear that Array<T> is a PureValue iff T is a PureValue.
>>>>> PureValue is nothing more than transitive value semantics.
>>>> 
>>>> You're almost there.  “Transitive” implies that you are going to look at
>>>> the parts of a type to see if they are also PureValue's.  So which parts
>>>> of the Array struct does one look at, and why?  Just tell me the
>>>> procedure for determining whether a type is a PureValue.
>>> 
>>> We look at the observable parts.  
>> 
>> That begs the question.  The “parts” of an Array are the observable
>> features that are considered by equality.
>> 
>>> We do not look at unobservable parts because we want flexibility to
>>> use things like CoW, shared immutable references, etc in our
>>> implementation.
>> 
>> IMO the important thing when it comes to functional purity is not what
>> you *can* observe, but what you *do* observe.
>> 
>>> Can you share your definition of value semantics?  
>> 
>> Explaining it well and in sufficient detail for this discussion takes
>> some doing, but I think John Lakos and I share an understanding of value
>> semantics and he has a really detailed explanation in
>> https://www.youtube.com/watch?v=W3xI1HJUy7Q <https://www.youtube.com/watch?v=W3xI1HJUy7Q> and
>> https://www.youtube.com/watch?v=0EvSxHxFknM <https://www.youtube.com/watch?v=0EvSxHxFknM>.  He uses C++ in places,
>> but it's not particularly advanced, and the fundamental ideas apply just
>> as well to Swift.
>> 
> 
> Super interesting talk! 
> 
> But consider: isn't a single value type able to represent *multiple* ethereal types?
> 
> std::vector is a good example. What are the salient attributes of this type? In the talk John says that
> 
> 1. the size is
> 2. the values in the vector are
> 3. the capacity, however *is not*
> 
> in which case std::vector would be an approximation of an ethereal type which has a list of values, and the capacity is just an artifact of the approximation. But you could also imagine an ethereal type which *does* depend of the capacity of the object, and std::vector unwittingly approximates that type too! In this case someone, unfamiliar with the implementation might use it under the assumption that capacity *is* part of the ethereal type and by extension the equality of std::vector. 
> 
> John avoids the problem by saying that this must specified in the documentation.
> 
> I tend to see this as breaking encapsulation since you need to know the implementation of the equality operator to be able to determine if a public property, the capacity, is part of the ethereal type. It’s not always the case that you have access to either the documentation or the implementation.
> 
> This implies, therefore, that if salient attributes *define* the immutability of the value type, then the public interface is not guaranteed to be immutable, since it is allowed to include non-salient attributes. For example, a vector’s capacity could change at any time, by virtue of it being stored via a reference.

John refers to this at 52:26 in the first video and mentions that this is not full value semantics. I assume, then, that PureValues would have full value semantics.

> 
> What I am saying is that a PureValue is a value type whose public interface comprises *only* salient attributes. And I also claim that this is a useful distinction amongst value types. 
> 
> John also says that a salient attribute must derive *only* from the state of a particular instance of a type. This by extension implies that a salient attribute must derive exclusively from pure values. However, this also means that without some “indirect” keyword, PureValues are restricted to acyclic and non-recursive structures. 
> 
> I also claim that equality can be automatically generated for PureValues by equating each of there salient attributes.
> 
> I really apologize if this seems like rambling again, but I am very interested in this problem.
> 
> Tyler
> 
> 
> 
> 
>>> It may be helpful
>>> if we start there and refine your definition to exclude impure value
>>> types like Array<UIView>.
>>> 
>>> In the meantime I’ll take another shot:
>>> 
>>> 1. Scalars are pure values.
>>> 
>>> 2. Any aggregate type with value semantics is a pure value iff all
>>>   observable parts of the aggregate are pure values.
>> 
>> 
>> -- 
>> -Dave
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