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

Matthew Johnson matthew at anandabits.com
Mon May 9 18:07:09 CDT 2016

> 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.  We do not look at unobservable parts because we want flexibility to use things like CoW, shared immutable references, etc in our implementation.

Can you share your definition of value semantics?  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.

>> At any rate, we could add a PureValue magic protocol, and it would have
>> well-defined meaning. I'm not sure that it is worthwhile or even a good way to
>> approach the problem. But we don't need to argue about the definition.
> I don't want to argue about anything, really.  I just want a definition.
> -- 
> -Dave

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