[swift-evolution] Should we rename "class" when referring to protocol conformance?
Dave Abrahams
dabrahams at apple.com
Sat May 7 15:03:38 CDT 2016
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.
> 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?
> 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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