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

[Dave Abrahams]

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.

>
>
>> 
>> 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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>>> 

-- 
Dave