[swift-evolution] [Oversight] Reference types allow mutating methods through generics

[Matthew Johnson]

Sent from my iPad

> On May 5, 2016, at 10:59 AM, Dave Abrahams <dabrahams at apple.com> wrote:
> 
> 
> on Wed May 04 2016, Matthew Johnson <matthew-AT-anandabits.com> wrote:
> 
>>> On May 4, 2016, at 4:16 PM, David Sweeris <davesweeris at mac.com> wrote:
>>> 
>>> Having given it some more thought... Does "PureReference" make
>>> sense? What would it mean? At some point a reference has to, you
>>> know, actually refer to a concrete value. Otherwise it's just
>>> turtles all the way down.
>> 
>> In my thinking PureReference (or ImmutableObject) has semantics
>> indicating that the type is a value-semantic reference type.  I think
>> this could be quite useful in some cases.
> 
> Like what? To a first approximation, the only thing I can see this being
> useful for is Objective-C interop.  Aside from being an element of an
> NSArray, there's nothing a an immutable class can do that a struct
> can't.

Objective-C / cocoa interop is important enough to matter IMO.

> 
>> 
>> 
>>> 
>>> Sent from my iPhone
>>> 
>>>> On May 4, 2016, at 13:32, Matthew Johnson <matthew at anandabits.com> wrote:
>>>> 
>>>> 
>>>>> On May 4, 2016, at 1:21 PM, David Sweeris via swift-evolution <swift-evolution at swift.org> wrote:
>>>>> 
>>>>> 
>>>>>> On May 4, 2016, at 11:12, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
>>>>>> 
>>>>>> I can see value in there being some kind of PureValue protocol,
>>>>>> for types that represent fully self-contained values, but
>>>>>> conforming to that protocol requires a bit more thought than just
>>>>>> being a struct or enum, since there are structs that have
>>>>>> reference semantics (such as UnsafePointer), and there are hybrid
>>>>>> value types that contain references to data that isn't part of
>>>>>> the value (an Array<Class>, for instance).
>>>>>> 
>>>>>> -Joe
>>>>> 
>>>>> +1
>>>>> 
>>>>> I'd think that both "PureValue" and "PureReference" would be
>>>>> useful. Isn't it the mixed types that make for tricky mutation
>>>>> rules & serialization?
>>>> 
>>>> I also like Joe’s idea.  It fits with the direction in Swift of attaching semantics, not just syntax, to protocols.
>>>> 
>>>> I was thinking of something pretty similar to your PureReference
>>>> idea, but slightly different.  Pure / immutable references have
>>>> value semantics.  With that in mind I was thinking of an
>>>> ImmutableObject protocol which would inherit from both AnyObject
>>>> and PureValue.
>>>> 
>>>> 
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>>>>> swift-evolution at swift.org
>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
> 
> -- 
> Dave