[swift-evolution] [Manifesto] Ownership

[Dave Abrahams]

on Mon Mar 06 2017, John McCall <swift-evolution at swift.org> wrote:

>> On Mar 6, 2017, at 3:46 PM, Karl Wagner <razielim-Re5JQEeQqe8AvxtiuMwx3w at public.gane.org> wrote:
>> Thanks very much for writing this up, it’s very interesting.
>> 
>> The one thing which I think needs improvement is the Copyable protocol. I think that this is
> actually part of a larger problem in Swift, which is that we don’t expose enough information to
> generic code for it to operate safely. This goes back to people asking for a value-equivalent to the
> “class” or “AnyObject” magic protocols.
>> 
>
>> For example, often you would like to wrap a Collection and keep some information about what it contains. In order to do that generically, you need to ensure a client can hand you an exclusive view of a collection’s contents, so you know they will not mutate underneath your feet. 
>> 
>> Currently, that means you need a RangeReplaceableCollection because it includes an empty initialiser which allows you to create a unique copy of the Collection’s contents. It’s a workaround, but it means we have this unnecessary protocol requirement in the standard library, and in the implementation, which is making copies that may not be required. If my CollectionWrapper is initialised with something which has ValueSemantics, I don’t need to create a whole new instance with equal contents to ensure exclusivity of those contents. If this was an Array, for example, I could simply assign it and it would ensure the underlying contiguous buffer is never mutated.
>> 
>> struct CollectionWrapper<C: Collection> {
>>     var _underlying: C
>> 
>>     init(contentsOf other: C) where C: RangeReplaceableCollection { 
>>         _underlying = C();
>>         _underlying.replaceSubrange(_underlying.startIndex..<_underlying.endIndex, with: other) 
>>     }
>> }
>> 
>> // Would love to do this…
>> 
>> extension CollectionWrapper {
>>     init(contentsOf other: C) where C: ValueSemantics { 
>>         _underlying = other 
>>     }
>> }
>> 
>> As the proposal notes (with the File example), these are semantic
>> considerations which go beyond simple struct/class distinctions:
>> structs may have reference semantics, and classes may have value
>> semantics. Would it be possible to model Copyable/move-only in terms
>> of two new protocols, ValueSemantics and ReferenceSemantics, with
>> trivial types/non-trivial types given appropriate defaults
>> (overridable by the programmer, such as for “File”)?
>
> Class types have reference semantics and are still copyable; ownership
> is not going to change that.  More generally, I don't see how anything
> we can do in ownership could ever do something like eliminate the
> possibility of a reference-semantics collection from the language.

I think it might not be very related to ownership, but the use of the
word “Copyable” may be a problem.  For most programmers when applied to
reference types “Copyable” will connote “Clonable,” i.e. that the
referent (rather than the reference, which is what you're referring to
when you say class types are copyable) can be explicitly copied.  So I'm
betting Karl read “Copyable” and was led to this other topic which is of
concern to many people.  In that sense, claiming “Copyable” for
ownership without also addressing “Clonable” could be a problem.

The idea behind “Clonable” would be that it gives you a *generic* way to
create a logically *independent* copy of a value, that would work both
for value types and reference types.

-- 
-Dave