> On Nov 2, 2016, at 9:05 AM, Joe Groff via swift-dev <swift-dev@swift.org>
> wrote:
>> On Nov 1, 2016, at 9:23 PM, Slava Pestov <spes...@apple.com> wrote:
>>
>>>
>>> On Nov 1, 2016, at 11:00 AM, Jordan Rose via swift-dev
>>> <swift-dev@swift.org> wrote:
>>>
>>> - Does this help us with the nested dictionary CoW problem?
>>> `foo["bar"]["baz"] += 1`
>>
>> My understanding is that this problem arises because we don’t have ‘optional
>> addressors’. A dictionary lookup might return nil. If addressors had a way
>> to return an optional wrapping a pointer, and optional evaluation supported
>> this, we could do in-place updates of this kind. For Apple people: I have a
>> radar that I think describes this problem (rdar://17509619).
>
> I think our long-term goal here is to let the property definition fully
> control the `inout` access with a coroutine-like definition, something like:
>
> var foo: T {
> get { return /*value for read-only access*/ }
> set { _foo = /*write-only access*/ }
> mutate {
> var tmp = prepareForMutation()
> yield &tmp // continue nested inout access
> reconcileMutation(tmp)
> }
> }
>
> This should let the dictionary implementation do whatever it needs to present
> a moved Optional value to the chained access. If Dictionary can't store
> Optionals directly inline in all cases, we ought to still be able to rely on
> enforced inout uniqueness to get away with moving in and out of a temporary.
It's tricky, because if Dictionary is going to support non-movable values, then
we also want the non-mutating accessor to be able to return a
Borrowed<Optional<V>>, which it can't do if that representation has to be a
pointer to an Optional<V>. It can returned an Optional<Borrowed<V>> just fine,
of course.
This relates to the earlier conversation we had about borrowed tuples. We may
have to make the representation of a Borrowed<T> potentially different from
just a T*, and that really stinks.
John.
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