[swift-evolution] Should we rename "class" when referring to protocol conformance?
Tyler Cloutier
cloutiertyler at aol.com
Mon May 16 12:46:19 CDT 2016
> On May 16, 2016, at 9:21 AM, Matthew Johnson <matthew at anandabits.com> wrote:
>
>
>>> On May 16, 2016, at 1:17 AM, Tyler Fleming Cloutier <cloutiertyler at aol.com> wrote:
>>>
>>>
>>> On May 15, 2016, at 11:48 AM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>>>
>>>
>>> on Mon May 09 2016, Matthew Johnson <matthew-AT-anandabits.com> wrote:
>>>
>>>>> On May 8, 2016, at 1:51 AM, Dave Abrahams <dabrahams at apple.com> wrote:
>>>>>
>>>>>
>>>>> on Sat May 07 2016, Andrew Trick <atrick-AT-apple.com> wrote:
>>>>
>>>>>> On May 7, 2016, at 2:04 PM, Dave Abrahams <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 and
>>> 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.
>>
>> 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.
>
> I don't agree with this. I consider Array<Int> to be a pure value. If it exposes a capacity property that fact is incidental to its value. I agree with John here.
Yes, I revised my statement in a later email. I agree with you here.
>
>>
>> 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
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>
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