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

Matthew Johnson matthew at anandabits.com
Sat May 7 20:43:05 CDT 2016

> On May 7, 2016, at 3:03 PM, Dave Abrahams <dabrahams at apple.com> wrote:
> on Sat May 07 2016, Matthew Johnson <matthew-AT-anandabits.com> wrote:
>> This depends on the type. For types representing resources, etc it works just
>> fine. But for models it does not work unless the model subgraph is entirely
>> immutable and instances are unique. 
>> I agree that it isn't a good idea to provide a default that will
>> certainly be wrong in many cases.
> Please show an example of a mutable model where such an equality would
> be wrong.  

This is somewhat orthogonal to the main points I have been making in this thread.  I have been focused on discussion about reference types that have value semantics and the distinction between value semantics and pure values.  In any case, here you go:

let a: NSMutableArray = [1, 2, 3]
let other: NSMutableArray = [1, 2, 3]
let same = a === other // false
let equal = a == other // true

Reference equality does not match the behavior of many existing mutable model types.  You seem to be making a case that in Swift it should.  But that is a separate discussion from the one I am trying to engage in because mutable reference types *do not* have value semantics.  

>>    I assume what is meant by "PureValue", is any object A, whose own references
>>    form a subgraph, within which a change to any of the values would constitute
>>    a change in the value of A (thus impermissible if A is immutable). Thus
>>    structs would quality as “PureValues”.
>> As you noted in a followup, not all structs qualify. Structs that whose members
>> all qualify will qualify. References to a subgraph that doesn't allow for any
>> observable mutation (i.e. deeply immutable reference types) also qualify.
>> This means the following qualify:
>> * primitive structs and enums
>> * observable immutable object subgraphs
>> * any type composed from the previous
>> It follows that generic types often conditionally qualify depending on their
>> type arguments.
>>    I also assume that enforcing immutability on an object graph, via CoW or
>>    otherwise, would be unfeasible. You could enforce it on all values
>>    accessible by traversing a single reference for reference types, however.
>>    This is why I don’t really buy the argument that there is no such this as
>>    deep vs shallow copy. Deep copy means copying the whole “PureValue” or
>>    subgraph, shallow copy means traversing a single reference and copying all
>>    accessible values.
>>                I don’t mean to imply that it is the *only* valuable
>>            property. However, it I (and many others) do believe it is an
>>            extremely
>>            valuable
>>            property in many cases. Do you disagree?
>>            I think I do. What is valuable about such a protocol? What generic
>>            algorithms could you write that work on models of PureValue but
>>            don't
>>            work just as well on Array<Int>?
>>            Array<Int> provides the semantics I have in mind just fine so there
>>            wouldn’t be
>>            any. Array<AnyObject> is a completely different story. With
>>            Array<AnyObject> you cannot rely on a guarantee the objects
>>            contained
>>            in the array will not be mutated by code elsewhere that also happens
>>            to have a reference to the same objects.
>>        Okay then, what algorithms can you write that operate on PureValue that
>>        don't work equally well on Array<AnyObject>?
> You haven't answered this question.  How would you use this protocol?

I answered elsewhere but I’ll repeat that one use that immediately comes to mind is to constrain values received in the initializer of a (view) controller to ensure that the observable state will not change over time.  This is not an algorithmic use but is still perfectly valid IMO.

If I read Andrew’s post correctly it sounds like it may also be of use to the optimizer in some cases.

>>            let t = MyClass()
>>            foo.acceptWrapped(Wrap(t))
>>            t.mutate()
>>            In this example, foo had better not depend on the wrapped instance
>>            not
>>            getting
>>            mutated.
>>            foo has no way to get at the wrapped instance, so it can't depend on
>>            anything about it.
>>            Ok, but this is a toy example. What is the purpose of Wrap? Maybe
>>            foo
>>            passes the
>>            wrapped instance back to code that *does* have visibility to the
>>            instance. My
>>            point was that shared mutable state is still possible here. 
>>            And my point is that Wrap<T> encapsulates a T (almost—I should have
>>            let
>>            it construct the T in its init rather than accepting a T parameter)
>>            and
>>            the fact that it's *possible* to code something with the structure
>>            of
>>            Wrap so that it has shared mutable state is irrelevant.
>>            The point I am trying to make is that the semantic properties of
>>            Wrap<T> depend
>>            on the semantic properties of T (whether or not non-local mutation
>>            may be
>>            observed in this case). 
>>        No they do not; Wrap<T> was specifically designed *not* to depend on the
>>        semantic properties of T. This was in answer to what you said:
>>                A struct wrapping a mutable reference type certainly doesn’t
>>            “feel” value semantic to me and certainly doesn’t have the
>>            guarantees usually associated with value semantics (won’t
>>            mutate behind your back, thread safe, etc).
>>        I have been trying to get you to nail down what you mean by PureValue,
>>        and I was trying to illustrate that merely being “a struct wrapping a
>>        mutable reference type” is not enough to disqualify anything from being
>>        in the category you're trying to describe. What are the properties of
>>        types in that category, and what generic code would depend on those
>>        properties?
> Again, the key questions are above, asked a different way.
> -- 
> -Dave

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