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

L. Mihalkovic laurent.mihalkovic at gmail.com
Mon May 16 02:54:37 CDT 2016

(From mobile)

> On May 16, 2016, at 8:39 AM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>> on Sun May 15 2016, 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?
> “ethereal?”  Does he really use that term?  I don't know what it means.
>> 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*
> Yup, just like Array.  Thus the equality test for arrays ignores
> capacity.
>> 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.
> Yes.
>> 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.
> No, you just need documentation.
>> It’s not always the case that you have access to either the
>> documentation or the implementation.
> Without the documentation, you're lost.  We go a lot further with naming
> conventions in Swift than typical C++ does, but even in Swift you can't
> expect to fully understand semantics without documentation.
>> 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.
> Then Array<Int> is not a PureValue because it exposes capacity?!  That
> sounds crazy to me, since the Array's capacity in no sense has reference
> semantics.
>> 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.
> That's true for almost any value, provided we define equality for
> reference types properly.
>> I really apologize if this seems like rambling again, but I am very
>> interested in this problem.
> I'm glad you are! Few programmers dig far enough to understand value
> semantics at a deep level.
> All that said, I still think PureValue is a red herring.  


> Unless I'm
> forgetting something that happened in the thread two weeks ago, nobody
> has shown me code that relies on PureValue but could not equally well be
> written by using a Value constraint.
>> 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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>>    swift-evolution at swift.org
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> -- 
> -Dave
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